Green Building Forum - Ventilation

MVHR and heating - costing - HELP please!

TonyinOxford — Sun, 01 Feb 2026 15:30:07 +0000

Hi Everyone,

Newbie here. First post. It is a big one, so I very much appreciate any advice given in advance. Thank you.

I live in Oxford and I have commissioned an architect to transform our 3 bed 1929 semi into a 4 bed EnerPhit home. Overall the house will end up at around 160m2. The architects have done many retrofits to EnerPhit standard.
The house is in a flood plain, with an immediate drop at the back of the house of 1.5m down to land that regularly floods. Side access is narrow. For this reason I would prefer not to have an external ASHP unit.

We have been back and forth on how to ventilate and heat the house, and supply hot water.
We have had multiple quotes but all seem alarmingly expensive.
Total Home Environment want £37K to design, supply at Pichler PKOM4 all in one unit, install and commision. VAT free but no BUS grant.
Williams Energy Design want £14 to design, supply a Zehnder ComfoAir Q600, install and commision. VAT payable but no heating / cooling or hotwater. To add the Comfoclime unit is an additional £6K plus VAT (but my understanding is that if an ASHP is attached then it can be zero rated). But still no hot water or cylinder, and would also need to additional heating through electric towel radiators and/or IR panels. The architect says Rod Williams is particularly good at the design so that there is no unnecessary boxing of ductwork.
Energy My Way want £26K for MVHR and separate external ASHP, hot water cylinder with a small amount of UFH and towel radiators. No VAT, and this has already taken into account BUS grant.
Heat Space and Light were cheaper but do not install and could not give me the name of a reliable installer in the Oxford area.
All of the above will also be subject to main contractor builder's 10% overhead and profit charge.

The first question is about approach. How would you knowledgeable guys go about heating and ventilating the place?

Secondly, it is feeling very, very expensive compared to the indicative prices I see online, but maybe I am missing something.
I am a competent DIYer, but I feel this might be too big a job, and as it is instrumental to meeting Enerphit and having a comfortable home it is not something I want to get wrong.

Am I being over-changed? Is there an architect's premium? Does anyone have any advice, or details of a reliable Passivhaus MVHR that covers Oxfordshire.

So many thanks in advance.

Tony

MVHR: Condensation on condensation pipe

morsing — Mon, 19 Jan 2026 21:07:50 +0000

'evening,

I installed a Vent-Axia Kinetic quite a few years ago, but this month I have suddenly had a new problem pop up.

We suddenly noticed a damp patch on the ceiling which turned out to be condensation on the outside of the condensation drain pipe. I originally fitted pipe insulation on the first two meters of pipe, can't easily go any further, and water is dripping out from the end of that.

It's very odd. I wasn't quite sure if removing or extending the pipe insulation would help, but I am more puzzled about why this is suddenly happening.

What should I look for? It's like the condensation water is suddenly colder than normal? Oddly it first happened when there was over-night frost but it is still happening now it is warmer.

Thanks

Sentinel Plus B no condensate

Dur — Sun, 07 Sep 2025 10:00:09 +0100

Hi

We have installed a Vent Axia Sentinel Plus B as part of our very long winded restoration. It is not balanced yet as we have yet to add a couple of rooms but with a crude manual set up seems to work fine.

It has been running since March but on summer bypass since mid May.
My concern is that it has never issued a drop of condensate. Being busy with other thins we just put a bowl under the drain intending to plumb to waste later.
The MVHR unit is installed in the end of a utility room downstairs.

Could condensate be backing up in the unit somewhere? How easy is it to get to the condesate "source" inside the unit?

Thanks for any thoughts!

PIV in a warm loft?

Mr_W — Thu, 01 May 2025 23:48:06 +0100

Hello,
Wish I'd found this website many years ago when we renovated our house. Better late than never!
During the winter we get quite a bit of condensation on our windows, which leads to black mould. I'm looking for solutions.
The house was renovated a few years ago with energy efficiency a major consideration. Prior to the refurb, the house was heated via an LPG boiler + electric heaters, which costs us £1000s / yr and was still cold. The renovation included interior wall insulation (12+38 mm PIR backed plasterboards), double glazing, rockwool battons insulation between floor joists on 1st floor and loft, a new and insulated roof (100 mm PIR between joists + 25 mm PIR below joists). Heating is supplied by an air source heat pump with UFH throughout. Plus PVs on the roof.
We have standard extractors in the bathrooms linked to the light switches and also in the kitchen for cooking. The windows are wooden (acoya) and traditional looking 'cottage style' casement with bars. They do not have trickle vents. The house is over 100 years old and the exterior walls are two skins of bricks without any cavity.

To solve the condensation and mould on the windows I'm thinking of either retro fitting trickle vents (not ideal due to cost, aesethics, and drafts), or Positive Input Ventilation. The PIV could be installed into the warm loft, which although is well insulated is not sealed from drafts.
However, all the info refers to installing PIV into a standard cold loft and I can't find any info on installing in a warm loft. I assume its fine to do this, but thought i'd ask on these forums to see what the experts on here think.
Any other suggestions to solve the condensation issue.

MVHR commissioned: master bedroom too cold - supply valve causing a draught towards the bed

stonecold — Mon, 07 Apr 2025 13:27:47 +0100

Well the Zehneder ComfoQ is all fitted and commissioned and has been running 3 months now, and unfortunately my initial concerns about having the master bedroom too cold and too draughty remain - it was warmer and more comfortable before so clearly not getting the full benefits here.

I'm generally sensitive to slight draughts (including warm ones) and feel really cold ??? compared to the average human so my people aren't going to notice it at all (spouse definitely doesn't). I wear a sweater or hoodie most of summer.

The master bedroom supply valve *feels* like it shoots air diagonally down towards the bed instead of (or as well as) across the ceiling. My arms need to be fully tucked up under blankets and duvet all night when they didn't before, which is a nuisance when you are trying to hold a book and it's no different now the weather is a bit warmer outside. I also notice the draught direction on walking into the room.

The supply air valve air temperature feels too low - but this might not so matter if it wasn't pointed at me.

The lounge at times also has the same thing happening towards the sofa although the temperature is OK and it's only noticeable some of the time.

Everything was well fitted and insulated and avoided the typical mistakes of trying to put the MVHR in the loft (it's downstairs and attic ducting is well insulated). It's a Zehneder so very efficient and nice and quiet, loft above is well insulated and there's EWI but no solar gain in bedroom or lounge and it's an old house. The airtight vapour membrane on the loft floor got taken out pre-MVHR (condensation was running down the walls because windows don't have air vents), and looking at the ducting up there I can't see how we can really get another put down. The supply valves have been carefully planned for addressing the areas we had conversation/ventilation issues ("dead space") while being far enough away from bed and sofa.

Solutions I can see now are:
1. Set MVHR temperature profile to warm, which I did this morning but won't fix the draught sensation

2. directional supply valves to avoid blowing air right at me (it would need to be adjustable so that air can still get to the corner of the room but maybe 30-40 degrees of the 360 degrees blocked from the outlet of the supply valve

3. adjusting the flow rate to lower it, which to keep things balanced means either increasing it in other bedrooms or reducing the extract rate in bathroom and kitchen (which I'm reluctant to do) .
This might not be ideal since both the problem rooms have only 1 supply valve and are the largest rooms, and that's the only downstairs supply valve (other beds are not currently used but also don't have the draught effect).

The master bed also has a raised ceiling so a large volume of fresh air is needed. Passivehaus good practice guide recommends starting at 30 m³/h for double rooms. I think I need to dig out my commissioning details for current settings.

What air flow rate counts as noticeable / slightly draughty?

stonecold — Mon, 07 Oct 2024 07:41:19 +0100

I'm not able to find numbers anywhere that indicate what air flow rate is comfortable vs a bit drafty/noticeable.

The situation is that the ground floor has no corridor and one huge lounge that you walk through to reach the other downstairs rooms - so all the air going into the 3 bedrooms upstairs is going to need to go through the lounge in order to be extracted via the kitchen and downstairs loo extract valves. Is the guidance somewhere suggesting a particular limit for flow rate?

Design is getting finalised soon but the lounge has 44m3/h coming through a single door undercut from upstairs, plus 34m3/h from the supply valve (meaning 78m3/h leaving the lounge via a single door). That sounds like a lot to me. The lounge is about 20sqm with fairly low ceilings. I'm super sensitive to air movement /draughts even if they are warm air, so is this flow rate too high? We could reduce flow through the lounge by adding an extract valve in the entrance hall/stairs area, before it reaches the lounge. Or we could increase the flow to the bathroom extract.

MVHR planning: maintenance elbow room and positioning

stonecold — Mon, 07 Oct 2024 07:22:08 +0100

Hi, we are renovating and getting a zehner comfoair Q 350 (725mm w, 570mm deep, 850mm high, weighing 50kg) and planning to wall mount it - what kind of elbow room is needed for servicing? The room we are planning to put it in is 900mm wide on the wall we hope to hang it (900x1300mm room) which we've been told could be tricky to fit the bends in, but what space do we need for elbow room for servicing? Even worse we may end up digging down under it to insulate the uninsulated floor below.

We could fit in on the other wall which is 400mm ish wider, but this means having 50kg MVHR plus ducting attached to a stud wall / dry lined solid stone wall. Will it be strong enough or could we run into issues during installation or later on?

Lastly, is it right that the condensate drain needs to run at a gradual slope (ie gravity fed) or does the water get pumped out, and can this drain into a toilet cistern or internal soil pipe - or will that be horribly unpleasant? I'm not looking forward to running the condensate drain around 3 walls to get to the plumbing.

Ventilation rates

tony — Fri, 27 Sep 2024 17:52:28 +0100

Ventilation rates were a mess in the guidance with ideas of rates needed per m2 of floor stea, volume, per person in the space/home, or depending on how many bedrooms

These used to be one bedroom = 13, 2 bedrooms 17, and 21 for 3 bedrooms

They now say 19, 25 and 31 respectively and figures are in l/s

My hobby is energy use reduction and this increase represents a 50% increase in energy use regardless of heat recovery or not !!

Nothing about occupancy

The good bit is that PIV and PSV are no longer allowed (except in Scotland) - I always felt PIV in particular was immoral (too expensive for the bill payer)

Thanks AECB - I will update my lecture notes now.

Wet room ventilation

do_discimus — Mon, 05 Aug 2024 08:09:10 +0100

House is typical 1970s end terrace. We're adding a single storey extension on the back, including a downstairs toilet. Plan is here -> https://drive.google.com/file/d/1IDHe2EnyqoR_DdUsw06ncXnF3iINyswi/view?usp=sharing
The party wall is on the left. Rear elevation faces east.
Our bathroom is above the new toilet. Obviously, the new toilet has to be ventilated but I'd like to improve the ventilation to the bathroom as well. Currently, we have the window almost permanently open and even doing that we get some mould on the artex ceiling each winter. Bathroom window is new. Bathroom is small, without much external wall available, see photos ->https://drive.google.com/drive/folders/1GAp-K4Xne4EMYHQX0LMeMWi7KSlujqBG?usp=drive_link
I have several thoughts and would appreciate any advice.
1. Simplest option is basic extraction fans for both rooms, through the wall. I understand single room MVHR isn't very effective at HR, and can produce a cold draught, so have dismissed that. For extraction, I've seen iCON 15 recommended for the iris shutter design. Anything else to recommend?
1a. Issue is finding enough wall to fit it in the bathroom. Any thoughts on how much wall is needed to put the hole through without tangling with the window frame? We've got 245 mm to the reveal edge on the right and 260 mm on the left next to the shower. Also, outside is currently tile hung on the right but we have planning permission to replace that with insulated weatherboard above the extension roof. Left side is brick cavity wall. Could possibly go through the ceiling and out the soffit. Very tight loft access, though.
1b. I think the new toilet extraction will need to go through the current rear elevation into either the extension, or the cavity in the extension wall, turn 90 degrees left, and out the extension side elevation. Are tight bends in very short duct runs okay? This will be right on our neighbours fence - bit concerned about ponging them!
2. Alternative is ducting both rooms out via a MVHR unit. The rooms are linked by the SVP. Issues are:
Cost is £1000s rather than £100s. Will save a bit of heating but is it anywhere near worth it? House is not draughty, but won't be anywhere near airtight, and is not that well insulated.
Are there MVHR units designed for this type of application. Would want extremely low background volume but decent boost triggered by bathroom humidity and toilet light switch.
Does it need sophisticated calculations or can you just have two extraction points from wet rooms and 1/2 inlets in other upstairs rooms (ducted through the loft)?
3. Are there other options. I've read about dMVHR with ducted extraction, like BluMartin, but I don't think our layout works well and, again, it's £££.
Thanks for any thoughts.

MVHR efficiency meamsurement reliability

wholaa — Wed, 17 Jul 2024 12:56:02 +0100

Hi all,

I have noticed there is a seeming discrepancy between MVHR efficiency listed on the passive house portal and what is described on manufacture specifications. For example, the Zehnder Q600 is regarded as high end and is passive approved and listed as having 87% heat recovery efficiency. The Vent-Axia Sentinels which are not passive approved are listed as 91% efficient. Has anyone explored this topic in detail?

Extractor fan or heat recovery?

randomRobin — Sun, 18 Feb 2024 12:34:02 +0000

Basic extractor fan or something with heat recovery best when you've got a heat pump?

Our downstairs shower room extractor fan died a few months ago but we now seeing a possible beginnings of a damp issue so it's time to put a new one in. Would people recommend a simple basic one or something more complex with heat recovery? The old one was a basic one, turned on when light switch to room was turned on and then stayed on for a few mins after light turned off.

Situation: air source heat pump, ~1900 build solid wall house, radiators. South east Scotland. Wet room is approx 3 x 4m and has shower, sink, toilet and heated towel rail. Isn't actually used for showering that much since it's an electric shower and we prefer to use the mixer upstairs. The room is used for hanging wet swimming and watersports kit to dry.

We're not super at DIY so would prob get someone to install for us (in case that makes a difference to recommendation).

Thanks in advance for advice.

Mitsubishi Lossnay - Ebay

Victorianeco — Tue, 16 Jan 2024 08:40:58 +0000

I have advertised my Mitsubishi Lossnay unit on ebay starting at 99p, brand new and unused if it's of any use to anyone.

It was going to be used in my renovation project but never happened

I can post the link if I am allowed?

MVHR upstairs only - retrofit

enerk — Thu, 16 Nov 2023 16:37:04 +0000

Hi
My house is late 80s build, no cavity insulation and renovation upstairs is almost completed. I've recently decided (based on cold bathroom in winter and growing kids closing doors in small bedrooms) that I would want to install MVHR in my house. I like the idea of not needing to open windows during window, I also lack of fresh air quite often but don't like the noise from outside.

MVHR unit will be installed in cold loft
Ideally inlet and outlet through the roof
Needs to be very quiet!
Option to install better filters
Due to long process of renovation it may for at least a year or two be used upstairs only (2 bathrooms, 3 bedrooms, 1 office, so 3inlets 4 outlets).
As mentioned before the house has poor insulation and we are not planning to add anything apart from dealing with some bigger leaks (windows etc).

My main issues questions are:
1) Can I use a large unit (suitable for 120m2 house) use partially or would that cause some issues?
2) Does providing exhaust through the roof make sense? Im worried about positioning ducting upwards.
3) What unit to choose?
4) Ideally I would want a unit I can control over wifi but do I actually need it?
5) Either the unit would be supplied with good filters or have an option to upgrade it depending on needs.
6) Noise - needs to be quiet!

Filters for Sentinel Kinetic Plus B/BH

Simon Still — Sat, 18 Nov 2023 14:19:38 +0000

I bought a few years stock (!) of filters from FairAir.Eu

They offered the option of G4+M5 (pollen filter) filters rather than the usual G4/G4 which I went for.

I fitted a set today and (somewhat obviously) the denser filter means you need to increase the fan speed to compensate. given mine is already a little high I don't want to do that.

So
1) a warning for anyone thinking of using higher filtration filters in their MVHRs. You'll need to adjust the unit if you set it up with something else
2) does anyone want to buy these at a good discount?

MVHR maintenance

djh — Tue, 24 Oct 2023 12:26:36 +0100

Can anybody recommend any MVHR maintenance firms? I can do it myself but as I get older I can see that I won't be able/want to in the future so I'd like to establish a relationship with a firm now whilst I can still rectify any problems they leave myself. I'm thinking specifically of the occasional 'clean the heat exchanger' level of maintenance rather than the regular change/clean the filters (though I suppose the time will come for that as well, but any local person could do that). We're in Suffolk, if the firm's not national.

MVHR inlet and outlet positioning

blubb — Thu, 05 Oct 2023 09:25:32 +0100

Dear GBF experts,

I am retrofitting an MVHR into a pitched roof building. The idea was to have the inlet through the tiles at the bottom of the roof in the west-facing corner, and to have the exhaust at the top of the roof with a terminal going 1 m above the roof level. See sketch attached (sorry for the bad scan quality!).

Are there any issues with this layout?

An MVHR installer is telling me that the inlet needs to go through the wall of the building and never through the roof, which I find hard to believe.

Many thanks!

Using 6" flue liner for MVHR return ducts

blubb — Thu, 11 May 2023 22:56:49 +0100

Did anyone here on the forum use a flue liner as the main MVHR return duct?

I would be very curious to hear about any real-life user experiences with that.

Two specific questions:
- is grade 316 steel good enough, or should I go with grade 904?
- any advice how to join the 6" flue liner to the 160mm Lindab spiral duct?

Many thanks!

Ventaxia Plus B mvhr installation

Dur — Sat, 16 Sep 2023 13:28:13 +0100

Hi
I am finally getting close to fitting my mvhr unit.
It has this cryptic diagram on top
I presume it shows flow direction but apart from not knowing which is external and which internal, it does not seem to agree with the picture in the install guide.
Can anyone enlighten me?
Thanks very much!!

MVHR runs continually on boost on damp days

wholaa — Mon, 12 Jun 2023 11:39:41 +0100

Hi,

I have a Vent-Axia MVHR and although I have grown to love MVHR I have noticed it does have tendency to run continuously on boast on damp rainy days like today. has anyone else noticed this and thought of possible solutions given that they can be loud on boost? Do more expensive brands avoid this some how? I am not sure if the damp air is through open windows being summer time or via damp intake air from outside.

MVHR air quality, putting an air-purifier / filter inline

Swarm — Sun, 14 May 2023 21:50:46 +0100

I monitor the PM levels in our MVHR fed house and can see some high peaks when people are burning stuff.

I didn't have the sense to plan for this when the MVHR was installed and have been looking at ways to modify it cheaply and easily, to reduce PM levels.

My first attempt was using fairly large vacuum dust bags on the MVHR room outlets. This worked fairly well and after a couple of months the insides of the white bags have turned grey. Unfortunately, I share the house and can't live a hermit lifestyle and there's inevitable questions about why we have bags / pillows on our ceilings!

I'm thinking of changing the MVHR system to have a DIY box in the attic, which would contain an IKEA air purifier.

This would be on permanently and probably attach to the distribution box for all the room outlet. My reasoning for using a purifier is that the 14W fan could help maintain a good airflow from the main MVHR unit to the rooms. If I just put a filter there, the main fan may have to work hard.

Concerns are it could be fiddly to set up and the IKEA fan probably isn't designed for 24/7 use and could possibly be a fire risk.

Another option I suppose could be to build quite a large filter array of 2/3 filters horizontally and just have that inline without any additional fan, as the increased surface area would be less resistance for the main fan.

Any thoughts from folk wiser than me much appreciated.

MVHR: disused flue as return duct?

blubb — Wed, 10 May 2023 18:46:45 +0100

Hi all,

I am retrofitting an MVHR system (1927 two-storey property with Zehnder Q350 and Lindab ducts).

I was thinking of using the disused chimney flue (18 x 18 cm) from a former gas boiler as the return duct to bring the exhaust air from the ground floor to the MVHR system in the loft. Are there strong arguments speaking against this solution? Has anyone tried doing this?

I'd appreciate hearing additional thoughts about this!

Many thanks!

Zehnder ComfoTube flat 51

wholaa — Fri, 21 Apr 2023 14:56:49 +0100

Hi,
has any one tried Zehnder ComfoTube flat 51? Are they prone to pressure loss and noise? They seem great for tight spaces but I heard some mixed reports so Id love to hear from people who tried it. Thanks a lot

MVHR design principals

adamsmith — Mon, 17 Apr 2023 18:51:35 +0100

Hello,

We are about to have our first airtight test done, if the results are good we would like to go ahead with MVHR self-installation.

Does anyone have any particular radial systems they would recommend? Due to us having limited space it does need to be a radial system.

Also are there any design principles that need to be followed Iâ€™m thinking based on my current understanding:

â€¢ Extraction only for (WC, ensuite, and bathroom)
â€¢ Supply only for small rooms (study, living room)
â€¢ X2 supply and extraction for 60m2 kitchen/ lounge/ diner
â€¢ X1 supply x1 extraction in the largest master bedroom
â€¢ X1 supply for medium size bedrooms

Attached is our construction drawing, any advice is really appreciated

retrofitting heat recovery

LittleRadRidingHood — Wed, 05 Apr 2023 09:46:52 +0100

Hello lovely people!

We bought our house (84m2, three bedroom, mid-terrace, 1950s build in Norwich) about two years ago, but have been experiencing damp problems ever since on cold days, with windows dripping with condensation in the mornings and mold starting up in cold corners of the walls and around windows.

The only ventilation seems to be one air brick each in the kitchen, the toilet and 2/3 bedrooms, but not in the bathroom. We have one dehumidifier unit, but it struggles to keep up and is too noisy to run at night.

To sort out these damp problems, improve our air quality and improve the energy efficiency of the house, I would really like to put in some kind of heat recovery ventilation.

Other options (extractor fans, PIV or PIV with heating) all seem like they run against every advice for energy efficiency, by swapping warm air from inside against cold air from outside (or even heating air with a resistance heater). Energy-wise none of those seem like a huge improvement compared to shock ventilating by opening all the windows briefly, which is what we currently do.

When I look for heat recovery ventilation online, I find lots of information on ducted, centralised MVHR and some on single-room/paired/decentralised MVHR, but weirdly, none of the companies I can find around Norwich do decentralised systems. I don't really understand this, as putting in and wiring a few of those seems so much easier and less costly than ripping through lots of walls and ceilings to fit all the ducts for a centralised system, all of which then need additional work to cover them up. I can find plenty of websites selling all sorts of single-room MVHR units, but no companies that seem to specialise in putting them in!? I don't have the skills or knowledge of building codes to design and install a proper ventilation system, so I would really like to find a company that can do it.

Hence my questions are:

- am I missing something about single-room/decentralised MVHR that makes it not the obviously better option for a retrofit compared to centralised MVHR?
- are there any better options we should consider?
- can anyone recommend companies that could design and install a suitable system?
- Given the size of the house, can we get away with just putting fairly powerful single-room MVHRs into the kitchen and bathroom (or, say, a single-room one in the kitchen and paired units in the bathroom and toilet, which are on opposite sides of the top floor) or do we need one in each room - I have seen differing advice on that?

For background information: We are generally happy with the fabric of the building. It has had cavity wall insulation and double-glazing put in at some point, but the seller could not tell us when (he was handling the sale for his mother who was moving to a care home, and she could not remember). Anyway, the house seems pretty tight - none of the windows or doors have drafts, but I don't have an air permeability measurement. There was a gas fireplace which we removed and blocked up, following the gas engineer's recommendation. We are keeping the heating very low to save money and GHGs, so room temperatures fluctuate between 15 and 17.5C in winter. I would imagine the previous owners used to have the heating on much higher, so probably had less damp because of that.

Thanks for all your help! :bigsmile:

MVHR drainage: icicles

wookey — Mon, 12 Dec 2022 16:57:29 +0000

Has anyone else got icicles growing from their MVHR vents in the current cold spell? It's been below freezing for about 4 days here and the size of the icicles shows how much condensation is dripping out of my vents (more than I had realised).

http://wookware.org/pics/online/greenbldg/iciclezoom.jpeg

I remember carefully sloping the vents through the wall to drain outwards if there was any water in them, but I'm wondering if that was the right answer now, because there is an internal drain in the machine, so having the condensation drain internally might be better than having it dripping down the wall/render outside. I can't remember if I just made that up or read it somewhere. I think it was recommended.

I had already adjusted the vent grills to be stepped out a few mm from the render surface so that any condensation dripped rather than ran down the wall making a stain, so I already knew that at least some of the time condensation occurs.

Need help! Still got condensation after all my efforts!!

deniance — Thu, 24 Dec 2020 13:10:04 +0000

Hiya guys! Me again!
Last year I posted here about insulating my attic!
I was getting lots of condensation on the felt which was dripping down into the insulation!

Well.... I thought Iâ€™d fixed it, sealed all holes and penetrations and was thinking it was good!

Just been up there this morning and itâ€™s exactly the same!! Just as bad as before!!

Any ideas what else I can do or even some method of monitoring where itâ€™s coming from?

Only 3 rooms upstairs, one is a bathroom, so, Iâ€™ve ...

Made an insulated hatch with a sealed lid
Sealed all cable penetrations
Checked the bathroom fan connection
Sealed all wall plates of internal walls
Foamed up gaps
Sealed any gaps in coving below

All of the above were massive sources of air from the house getting into the attic, used a fan to find all the leaks, I also bought a box of 50 lap vents that you slide in the felt, you can see daylight through some of them and there is very good airflow , you can feel it and see the cobwebs blowing about!! But there is still enough air condensing to overpower this! Just canâ€™t figure out where!

I thought all the work I did would make it less but itâ€™s just as bad as before! Like someoneâ€™s gone up there with a hosepipe and misted it all!!

All I can think of is, the chimney wall as it passes through the roof does get a bit soaked when itâ€™s raining, not dripping wet but just soaks up the moisture and the bricks turn dark, I wonder if the log burner flue dries it off and this then evaporates and floats across the attic and condenses again? Only had the burner on for two days so Iâ€™m doubting this theory!
Or maybe because itâ€™s a terraced house, I wonder if itâ€™s floating over the top of his attic and making it into my side??

Gotta try and sort it out, everything is going mouldy!!
Any ideas folks? Cheers

Mold in older MHRV System

quinnj3 — Tue, 20 Sep 2022 21:27:23 +0100

Hi All,

I hope someone here can help me. First some background on my system.

I DIY installed my MHRV about 13 years ago using a Vent Axia Air Minder plus ventilation unit. The pipes are all solid (apart from direct connections to HRV unit) and are either within the insulated layer of the building or wrapped with 40mm fibreglass foil wrap and taped up to avoid cold air pockets. There are a couple of condensate drains tied to the overflow of the house to carry away condensate that may build up in the extract pipework as it leaves the insulated layer to the uninsulated layer. In theory the unit is perfectly sized. In practice I believe it to be oversized as a draft can be felt at times when standing near the inlets. It's also quite noisy and I never need boost mode. It's permanently on low vent mode.

I have to admit I've somewhat neglected the system as it's always just worked. I wash and clean the filters at least once a year but they are always filthy. I should be doing it 4 times per year. I've only previously opened the unit up and cleaned the heat exchanger once about 5 or 6 years ago. At that time there was no mold present except for a small bit around the condensate drain; the unit was very clean otherwise.

We dry our clothes in the house and due to the effectiveness of the MHRV a clothes horse dries in less than a day. Over the past year or two we have noticed it is taking a couple of days to dry clothes. Up until recently (the last few months) we have had zero mold in the house; now we have noticed a small amount developing in our main bathroom.

Over the past few weeks the unit has been getting quite noisy to the point I've switched it off a couple of times at night. This evening I split the unit to find the extract fan appears to have a failed / failing bearing. Of more of a concern was the dirt and mold on the H.E. The extract blower is caked in what appears to be dust and mold also. The inside of the unit is largely dry except for a small damp spot at the condensate drain. The unit is exceptionally dirty also. Fortunately the air inlet fan and ducting is pristine so the filtration appears to be working there.

Obviously I'm really concerned with the mold issue and would like to get to the bottom of it. I can't tell 100% just yet but I believe the HE may have blocked with dust (even though it is filtered), reduced the air flow through it allowing the damp extract air to mix with the dust and facilitate mold growth. Is this a likely scenario?

Comments and opinions welcome.

Retrofitting continuous ventilation after fitting new windows

Shevek — Thu, 05 Aug 2021 11:05:23 +0100

We've fitted new windows to our flat and exceeded expectations on airtightness. So next step is to add ventilation.

The most cost-effective method I think is to add trickle vents (easy retrofit in our case because there is a roller shutter above each window with an internal access panel they can be fitted into) and then swap the current intermittent extract fan in the bathroom for a continuous extract fan + humidity booster sensor. All internals doors have a 5-10 mm undercut.

Am I taking the right approach?

My main concern is noise. What is the quietest fan on the market and what can we do to make sure we minimise noise? A suspended ceiling in the bathroom to provide sound-proofing to a fan unit is definitely on the cards; it needs one anyway because it's currently a concrete soffit and attracts condensation because if it's surface temperature.

We may also fit a hot water cylinder with integrated heat pump of some description in the little room next to the kitchen. Could that factor into a ventilation strategy?

I did also wonder if we could fit a dMEV fan unit (e.g. Brookvent airstream) but fitting ductwork at a later date.

Rental with LPG - excessive ventilation due to Building Regs

zak99 — Sun, 20 Feb 2022 11:05:49 +0000

Hi,

My nephew has moved into a rental bungalow and it has big air vents in every room. Theres a good LPG boiler in the bathroom, the place has double glazing, loft and cavity insulation but he is cold. It's a private rent but he is challenged so rent is funded and he has very limited funds.

Landlord (who seems decent) is saying the vents are necessary because of Building Regs. Is this right? I saw there were some. revisions re natural ventilation for new builds that seem counter to low emissions but understand the need to prevent mould etc.

I believe the windows have closeable trickle vents. The room vents are large and the wind at the moment is hustling through them.

Whats your thoughts please?

MVHR/cooker hoods question

SimonD — Mon, 07 Feb 2022 17:34:41 +0000

As some of you may be aware, I've designed whole house natural ventilation for my house. It's been working fine and in the temporary kitchen, we've had no issues with excess moisture or smells. However, there's a distinct possibility building control is going to ask for some kind of mechanical extract for the new kitchen.

Now, I'm keeping things simple in the new kitchen and not installing any wall cabinets and I also have a curved vaulted ceiling. I've been looking at a variety of cooker hoods that might fit in with the design, but there's something that doesn't resonate very well in the designs for me. Historically I always been underwhelmed by domestic cooker hoods as I find them generally noisy and ineffective. Also, if I opt for an externally ducted version, I'll be chucking a high volume of air out of the house and will also need to open additional vents around the house to balance the extract. We do a lot of cooking and baking so our last cooker hood was on so much of the time and that gets irritating.

Instead, I've begun to consider a couple of different single room, wall installed and balanced MVHR units, both of which can provide the min. extract rate for kitchens. There are the potential units:

https://shopuk.partel.com/collections/ventilation-with-heat-recovery/products/e-go-inc-transformer-controller
https://ecostream.org.uk/d-mvhr/

The advantage with these is that they are significantly quieter than normal cooker hoods, less intrusive into the environment. Despite rubbing up againstmy own design principles, they could also provide a simple backup should the whole house natural ventilation ever need an extra helping hand. They also don't cost much more than some of the cookers hoods I've been looking at and by the time I've bought all the ducting/accessories etc. there's not much in it at all.

Thoughts and experiences. Is there anything I'm missing other than fit a cheap quiet extractor fan with 30 or 60l/s extract rate and be done with it?

Discussion about Natural and Mechanical domestic ventilation

SimonD — Sat, 31 Oct 2020 10:03:26 +0000

To set the context, I'm currently building my own house which will end up about 200sqm.

We originally had an architect who only viewed u-value as the measure of the function of the fabric of the house and thus despite my requests, designed our house to effectively be wrapped in a plastic bag and then insulated purely with lightweight insulation materials (e.g. PIR) and very little consideration given to ventilation other than to open the windows every day. I dismissed this approach and instead went on an 18 month journey learning about alternative methods of building a house that provide not just good levels of insulation but a comfortable living environment that works with the seasons.

As a result of this research, I gave the architect the necessary information to re-design the fabric of our house using natural materials. This was supposed to be entirely wood fibre, but due to problems I had with the supplier and manufacturer being rubbish at both sales and customer service, I modified the designs again. In the end I chose to use Thermafleece cosywool through our first floor timer frame construction and rendered woodfibre EWI of the ground floor existing masonry walls.

The main priorities for this is that I don't believe wrapping our houses in plastic so they don't interact with the outside environment in a healthy moisture exchange is a good way to build houses. In fact, I think it's a bit mad. From experience, I also know how a 'breathable' house fabric works and the living environment it provides thanks to my mother's house in the middle of Sweden which was built in the mid 1800s. It is insulated entirely using sawdust and it provides a lovely internal environment all year round, even when I was there a couple of years ago during the massive heatwave where temperatures exceeded 30 degrees c for much of the summer. I also have a garden office that is insulated with thermafleece and feels very much the same.

So I have this breathable fabric which I think is essential to deal with and buffer moisture variations within the house, but this is of course not enough as it's going to be fairly air-tight, so we obviously needed ventilation. This was totally overlooked by our architect and therefore something I've been grappling with since I started my build.

So after a long period of research, I've opted to reject mechanical ventilation in favour of natural ventilation. I'm aware this in going against the grain as it seems like the market is flush with MVHR solutions, yet when looking at the offering available for natural ventilation, they're quite limited, particularly when it comes to domestic products. Those that are available, often don't ustilise best design practise - e.g. with passive stack, the roof cowels aren't typically designed to work well with the wind unlike the commercial ones which are much larger and are designed accordingly. So there is obviously something missing here, unless I've totally missed it in my searches.

I'm aware of the proposed benefits of MVHR in terms of heat recovery and fresh air distribution around the house yet when I've looked and some research, despite these benefits, the balance still seems to lean towards natural solutions. ( I also have to say here that some of the most informative stuff I've read has come from books about ventilation published in the 1800s rather than more recent stuff).

I'm therefore interested in having a discussion about the selection of MVHR over natural ventilation, just because I'm really curious about this topic. This is especially because we're seing so many problems within houses most probably caused by poor ventilation design.

What are your thoughts?

Insulate MVHR metal intake/exhaust with sheep's wool

Jeremy S — Thu, 16 Dec 2021 19:26:37 +0000

Hi,
Regarding the intake and exhaust 150mm galvanised spiral metal duct for MVHR:

I understand that the metal duct is usually insulated with foil-faced mineral wool.
And, in the light of GreenPaddy's comment in Is loft insulation sufficient to insulate MVHR ducting? , I know to provide a vapour-barrier.

... these do need a vapour IMpermeable (polythene) wrap around them whilst 
 inside the thermal envelope, as regular glasswool will allow moisture through, 
to condense on the duct surface in winter.

So, is there any reason not to use sheep's wool insulation rather than mineral wool?

Also, taking account of GreenPaddy's follow-up comment

 ... they should have the same Uvalue of insulation around them again as per your house external envelope

does the exhaust need to be as heavily insulated as the intake?

Swegon Casa FreeAir 100

minisaurus — Sun, 31 Oct 2021 14:11:03 +0000

Has anyone tried these?
I thought they look quite nifty: https://www.swegon.com/products/air-handling/casa/freeair-100/

Kitchen with only MHRV extract duct

wholaa — Fri, 03 Sep 2021 21:49:55 +0100

Hi all,
Is it commonplace to have an MRHV system without any input duct? Just an extract duct? The house in particular has a small enough kitchen with low occupancy but it is in a three-story house and its MRHV is not beautiful so I wonder about pressure drop.

Energy and cost savings using heat pumps instead of MVHR

minisaurus — Mon, 19 Jul 2021 12:50:53 +0100

Something I've recently calculated in my job and that I wanted to share - newer heat pumps are now so effective that it is usually more cost and energy efficient to use the energy (and moisture) in exhaust air for heating water in an accumulator tank than to use MVHR.

With MVHR you recover max 70-85% of the heat energy in the air, and only for those times of the year when you want to recover that energy - i.e. when the outdoor temp is below 18 degrees or thereabouts.

With a heat pump you recover that 70-85% heat energy and the compressor will boost that so that e.g. with 20 degrees exhaust air in a house you will get 35-45 degrees water to your accumulator tank. Some of the newer pumps even recover energy from the moisture in the exhaust air - I have seen 50 degrees water in bigger buildings (higher airflow rates).

Connect solar energy and/or an air/water or groundsource heatpump to the same tank, and you'll have very cheap hot water year round, very cheap heating water in winter, plus good ventilation, less ducting, 1 less fan, no rotor exchanger to drive, 1 less filter.

The accumulator tank needs to have a coil, i.e. not one of those chamber things (or whatever they're called).

These tanks can also be heated with gas, so you can e.g. switch to an air/water pump later.

Both CTC and Nibe offer such solutions for houses - very reliable.

I can post some calculations if anyone wants to get into that level of detail :bigsmile:

how to modify flow in roof ventilation?

djh — Thu, 03 Jun 2021 22:22:56 +0100

On the east end of our house, there's a single storey extension, outside the thermal envelope. The larger southern half of the extension is a conservatory (well, actually a sun room with a big rooflight) and the northern part is the 'bin store'. The bin store was thus named because it suited the then relevant CSH assessment. It has since turned into a very useful pantry. There's a problem with it in that it overheats in summer; not something I thought about when designing a 'bin store' :cry:

Its construction is very crude. The external wall is built up with blocks from the hardcore base to the same level as the FFL of the house. The space between it and the main slab of the house on one side and the slab of the conservatory on another is filled with more hardcore, and topped by some paving slabs. The superstructure is just timber studding with OSB outside, followed by a breathable membrane and cedar cladding on battens. There's a flat roof, which is ply on timber joists with an EPDM membrane above.

The roof is shared with the conservatory, but the construction there is a bit more complicated. Starting from the outside this time, there's the EPDM membrane and ply below that with firring strips to provide a ventilation space underneath, followed by some mineral wool insulation between the joists and plasterboard on battens. There's a vapour barrier in there just under the joists too. The joists run east-west, being fixed to the wall of the house at their west ends.

The ventilation runs north-south through both the bin store and the conservatory. I think what is happening is that in summer the air under the roof over the conservatory heats up and flows out into the bin store, warming it up too much.

I'm wondering what, if anything, I can do to reduce or eliminate the overheating. The only thing I've thought of so far is to build an enclosure around the ventilation space where it emerges from the conservatory roof into the bin store, and fit an electric fan to blow air into that space, so hopefully reversing the flow above the conservatory and causing the warm air to emerge at the south end instead.

Does anybody have any better ideas? Or even think mine might work?

TIA, Dave

MVHR Valve Locations in Open-Plan Kitchen.

JCBGF — Fri, 14 May 2021 14:31:26 +0100

Hello,

We are installing a small, basic ground-floor MVHR (Blauberg D-105A) as part of a side-return open-plan kitchen extension to our 1910s terraced house. A full-blown MVHR is beyond the scope of the project.

I'd really appreciate any comments on the attached design. A few notes:

â€¢ North is at the top of the page. There is a steel beam running north-south as shown. We can just get some 125mm duct underneath where it meets the column to the south as shown, boxed off above the kitchen units. The beam is otherwise impassable (within the scope of the project).

â€¢ East of the beam, the south third of the kitchen has a pitched roof with Velux windows (usual London side-return extension). This also limits valve placement options to the area of the bench seating as shown (hence the proposed directional supply valve to avoid draughts).

â€¢ Valves can otherwise be placed anywhere in the kitchen ceiling west of the beam. It is not possible to place valves east of the beam other than close to the south wall as shown.

â€¢ The extension is being constructed with best efforts on the insulation and airtightness front but not to a certifiable standard. My main ventilation priority is the utility room, which will have a condenser tumble drier and be used to dry clothes on hanging rails. I have a CO2 meter and CO2 levels in the house are already poor in occupied rooms. The house does not "officially" need an MVHR, but I would like to ventilate in an energy efficient manner and improve air quality.

â€¢ The kitchen will have a conventional extractor hood over the hob, vented to outside. Given this, I am questioning the need for an MVHR extract valve in the large open-plan kitchen space and wondering whether having an extract valve solely in the utility room would ensure it is well-ventilated without providing excessive airflow to the 2 supply valves. Would the supply valve location in the south east of the dining area, coupled with the utility room extract valve, set up a drift of fresh air towards the kitchen door to ventilate the kitchen, with the conventional extractor hood used for cooking as required? Such that an extract valve in the kitchen is not needed?

â€¢ If a kitchen extract valve is needed, where is the best location for it? I thought next to the door so as to be diagonally opposite the supply valve, although I'm concerned this might work against the utility room extract valve.

Thanks.

P.S. An image file doesn't seem to upload very clearly, hence the .pdf.

Can anyone recommend MVHR companies?

chrisduncan — Wed, 14 Apr 2021 14:20:26 +0100

Hi. We are finally on site in Monmouthshire. 300m2 new build with green oak frame. We are having whole house MVHR and I did get a couple of quotes 2 years ago from Rega and Envirovent.

Can you lovely people say whether these are good companies, and suggest another couple to send enquiries to?

Many thanks.

insulating a mvhr unit it the loft

jms452 — Sat, 27 Mar 2021 15:20:02 +0000

Our current loft mounted MVHR unit screws onto the ceiling joists and is buried under loft insulation keeping it nicely insulated.

http://www.nuaire.info/IandM/671438.pdf

However, it is 10 years old, the fans were replaced under warranty after about 4 years when the engineer told us that they no longer use that type of fan in the newer units due to poor lifetime. One of the fans is starting to go again so I'm looking at options. It is also rather primitive in that you need to take it apart to change the filters which is a bit of a pain.

Most of the current units (e.g. Vent-Axia 438222 Sentinel Kinetic B) have neat easy change filters and talk about loft mounting but they are effectively wall mount units and our loft is 'cold'. As the units are not particularly well insulated I'm contemplating building a PIR box around such a unit in the loft but this will make access a pain.

Any thoughts - common sense tells me there must be a decent accessible + insulated MVHR out there but I'm not seeing it.

bathroom extractor fan or single MVHR?

bardo — Wed, 20 Jan 2021 15:49:10 +0000

Just finished the build of a straw bale home. The external wall is 450cm + double board on board cladding making the duct length around 60cm. Ideally I would go for a single room MVHR extractor unit such as the Vent-Axia Tempra however with a max 45 com the in built ducting is too short. I have installed a 105 (ID) soil pipe though the bathroom wall to connect a standard extractor fan. Two questions. Do you know of a simple MVHR which would work with this set-up? Alternatively if I go for a simple extractor with humidistat/timer can you recommend a back draught unit which works well. If I can't exchange heat then the next best thing is to stop drafts. Cheers!

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

chrisduncan — Sat, 02 Jan 2021 15:58:03 +0000

Hello people. I recently finished converting a small barn into somewhere to live while our new house is being built. The barn is 60m2, single storey one main room with attached kitchenette and shower room, essentially one space. The walls are insulated on the outside with taped foiled PIR which is then clad in larch. Windows are thermally broken aluminium with 24mm IGUs.

I am feeling very depressed that after all my hard work, and with the recent cold snap, I have significant condensation on the inside of the frames. The place it really matters is in the living area where the bifold door frame butts up against engineered oak flooring. The condensation is enough to flow onto the floor and if I don't do something will start rotting or at least degrading the floor (see attached photo). Although a little condensation may be forming on the glass, most of it is forming on the frame.

After asking Mr. Google, there are plenty of websites which say it's not the window's fault, it's inadequate ventilation. I (perhaps wrongly) opted for windows which have a night vent setting rather than trickle vents as I couldn't bear spending lots of money on insulation only to throw money away on heating lots of cold air trickling in! I thought we could open windows a crack only when needed. Our problem is that we are not here all the time and I have to have an unattended solution to removing moisture from the air.

I suppose the humidity is coming from living in the barn without having the windows open, but a bit of me wonders if the two substantial stone walls are still drying out. The cladding was put on last spring so the barn has had 9 months in the dry and the walls had not been noticeably wet before anyway.

It seems there are two options unless someone can suggest something better. 1) I buy a dehumidifier and leave it running near the sink during the winter (eg Meacodry ABC 10L/day approx Â£140 and 160W) , and 2) I replace the shower extract fan with a single room MVHR unit (eg Blauberg Mini-air approx Â£370, 3.2W).

How can I tell if either of the above options will solve the problem? Has anyone had experience of dealing with condensation in this way? I would of course want to go for the most cost effective option, but I don't want to spend money on kit which doesn't in the end permanently remove the problem.

A couple more questions: both the dehumidifier and MVHR would be about 10m away from the bifold doors. I presume airflow in an empty (and fairly airtight, I think) building would be enough to ensure that the air next to the bifolds would be adequately dehumidified? Although the MVHR would have a far lower running cost, its efficiency would mean we would have to run the heating more to make up for this? Heating is underfloor from an LPG boiler. I have the temperature set to 20degrees when here and 15degrees when away.

What do I do? All comments gratefully received! Thanks

AIR TIGHT vs air tight - where does all the air come from in a standard house?

greenfinger — Wed, 06 Jan 2021 12:02:08 +0000

Hi

This is a bit of a thinking aloud type thread... Following my recent flurry of posts in the Stoves, Fires & Fireplaces forum, I was reading last night about direct air supplies for wood stoves, and it reminded me of stuff I've read in the past RE air tightness in houses, MVHR, etc. Our house is old so I think a lot of that doesn't apply. But being warm(ish) and reducing humidity are important factors which is why I guess I keep coming across these things.

So if we broadly say there are 1. passive houses, 2. modern new builds, 3. older houses from the last ??? years, 4. very old houses - I'm interested to know how they differ. For example, if in a very old house you replace the windows for double glazing, add some insulation and address a few draughts, does it move up a notch? Or two? I've read that it is very hard to make a house passive retrospectively, but what elements are hard to achieve if not implemented from the outset?

To put it another more awkward (but perhaps more relevant) way, I live in an old house with some extensions from the 1970s and some extensions from around 2000. Where there are old single glazed windows we'll be changing them for modern double glazing. There's lots of decorating to be done, so the caulk gun will be out and gaps around windows and doors (and pipes etc) will be sealed. Likewise where walls meet, ceilings, etc. And we'll be adding insulation to the loft (although not to walls). Once that's all done, I'm guessing the house would still be far from air tight, but where would the air then be coming from? Would there still be enough for the 5kW wood stove without an air brick? Does air come through the fabric of the house itself (solid walls)?

Sorry it's all a bit rambling, but I know people around here know their stuff and could shed light on at least some of my thoughts...

Cheers

COVID 19 and mechanical ventilation

wholaa — Sat, 01 Aug 2020 22:35:13 +0100

Given that ventilation is shown to have a major impact on the transmission of covid19, how is this seemingly never ending corona crisis going to impact domestic ventilation? Could the crisis undermine confidence in this near essential greentech? Can domestic MHRV filter covid 19?

MHRV purge and dust control

wholaa — Sun, 26 Jul 2020 14:42:41 +0100

Hi all,

Can purge mode on MHRV be used to reduce dust? Does it make sense to take it on during and after hooving your home?

MVHR intake & exhaust length/route

Dominic Cooney — Sat, 16 May 2020 09:40:45 +0100

Having installed the MVHR intake and exhaust ducts in insulated duct, I am now wondering if there is any advantage to having one route shorter or longer than the other? I used existing holes through the 450mm thick stone walls, they are more than 1.5m apart along the wall on the outside. As you can see, one route is longer and goes down and back up again to get around the joists (then falls to the outside through the wall). The other one falls continuously to outside.

I can arrange it at the unit end so that either of them is intake or exhaust, although one arrangement will be slightly neater than the other.

Does the hive mind have any thoughts or insights?
Or am I over thinking it and should just go for the neater arrangement?
My instinct says exhaust on the right with the shortest route and the constant fall, but the opposite is the slightly neater layout in the utility room.

Need basic help again please guys.. DIY MVHR

deniance — Sun, 09 Feb 2020 19:15:20 +0000

Hi all, me again!!

Now that my insulation work in attic is nearly complete I need to think about mvhr!
I know you can only have it in a really tight house but I thought sod it, Iâ€™ve got the unit, may as well give it a bash!

So... I was wondering if you could cast your eyes over the following drawings please!

The original plan was to fit the unit and the supply extract as far away from wood burner on the chimney as possible, hence itâ€™s position in the attic, it can really only go there.

Then I was going to supply bedroom one and two and then drop down the wall and supply living room.
Then I was going to extract from bathroom and drop through bathroom to extract kitchen.
Also drop the condensate pipe into bathroom.

Easy peasy.....but weâ€™ve now had another baby and we are thinking of changing bathroom to bedroom 3 and building a 3m x 3m flat roof cube extension stuck on the back of the kitchen , so if I change the old bathroom extract to a supply that means that upstairs only has supplies and no extract, and downstairs will only have extract in kitchen possibly linked over to new bathroom? Is this a problem with only supplying fresh air to upstairs? Will I still get the circulation ? Or am I over thinking it?

Iâ€™m planning on the vents being as far away from room doors so air travels across each room and Iâ€™m also hoping that I can use the ducting that is fully made of eps type stuff, just big eps tubes!

Pictures in the next post below
Cheers guys!

DIY dehumidifier - reality check

gyrogear — Sun, 26 Jan 2020 21:27:45 +0000

Mainland cousin says:

"Multi-unit dwelling, owner-occupier: I have a problem siting a bathroom extractor fan -- it can't be in the outside wall or in the window, as they are both over the bath, which isn't allowed. The only alternative is a ceiling-mounted one, which routes through the loft space, and vents in the eaves soffit. However, venting to eaves soffit is a NOK as water vapour could be drawn back into the loft. Also not my loft (even if I do have access to it...). Can't afford the angst of cutting a hole in the soffit viz. neighbour's ratting and hassle from managing agents etc.

Whence flash of inspiration: I have a condensing tumble dryer, which manages to condense the soup from my washing without releasing any moist air into my hall. Putting two and two together, I wondered about buying a condenser to install in the loft, to receive a duct from a bathroom extractor fan. I ended up deciding that it would be worth making one, to see if it works (don't see why it wouldn't!).

So I have bought two 2nd-hand drier condenser units off of ear-bay...

These will need hacking around somewhat, then mount one atop the other, inside a cabinet with a fan pulling air through the fins.

I have ordered an 80mm 240v fan, but think I might need two. Then the unit would be plumbed in so that the condensate drains down through the wall and into the waste pipe in the kitchen.

Sort-of replacement for a commercial dehumidifier, but Â« somewhat far cheaper... Â»

Project guidance appreciated".

via gg.

Condensation under loftboard

benawhile — Sun, 05 Jan 2020 21:39:55 +0000

I donâ€™t so far have the problem, but a general question with regard to boarding over a loft storage space.
I have found info relating to condensation forming on the underside of loft boards if an air space is not left between the top of the insulation and the underside of the board, but cannot find any definite statement about whether standard loft board is, or should be, water vapour permeable.

There definitely is information on the net to imply that â€building materials â€œ such as ceiling plasterboard, ARE permeable, and so on pre 1970s houses there is a risk of condensation on the underside of loft board.

Is it best to assume loft board is not permeable, or are there some board materials that have a certain amount of permeability?

Ventilation for New Build

bardo — Tue, 22 Oct 2019 19:57:09 +0100

Hi, I'm a first time house builder and looking to install moisture extractor fans from above a DIY kitchen cooker hood and in the bathroom (with bath and shower).

The cooker and bathroom share the same party wall.

Our walls are wide : 550mm (straw bales 450mm wide with another 100mm of lime / cladding/battens on the outside wall).

Can anyone recommend a simple effective way of doing this Do we really need MVHR or just a simple extractor fan? Any brand/models?

Thanks

Condensation: No ventilation in dormer cold roof - loft conversion

LoftDIY87 — Wed, 04 Dec 2019 14:17:59 +0000

Had a builder do the shell/structure of my loft conversion earlier this year and I'm DIYing the rest.

Whilst putting insulation in I've discovered he hasn't left ventilation at either end of the cold dormer roof. It's felt on top, with 150mm (145mm) ceiling joists, plus firrings. Am using 120mm Celotex in between joists, with 25mm Celotex underneath. Taking the firrings and 120mm Celotex into account, the ventilation gap (between top of insulation and bottom of roof OSB) spans from 63mm at one end to 33mm the other end - I know, a little less than the recommended 50mm ventilation gap, but that particular detail is the least of my worries at the moment! Because...

I've already discovered severe condensation when I removed one of the roof insulation boards and found the dormer roof OSB wet to the touch! The top of the ridge beam is also wet to the touch.

I know it will be (relatively) easy enough to vent at the far end of the dormer roof - using soffit or fascia vents. However I know I need the air to circulate i.e. vented at both the 'far' dormer soffit end, as well as the ridge end. Am toying with the idea of mushroom vents on top of the dormer roof, but there are 13 openings and obviously I don't want 13 mushroom vents on my dormer roof! (would need one for each opening as there's currently no way for the air to circulate between them)

I'm also thinking I could notch and drill holes in each and every joist to the maximum regulations permitted, that would create some sort of airflow in between joists, and possibly result in needing far less mushroom vents? (could I get away with just 3 - one in the middle and one at each end?) I've attached a diagram showing joist holes and notches I've calculated. I'd have to batten 44mm x 44mm timber to the bottoms of the joists and bring the 120mm Celotex down to that level to allow for 30mm holes to still have the required 50mm height clearance from the hole to the top of the joist.

Also concerned about the roof leaking if I get a roofer to put mushroom vents into a felt roof, whether it's 1, 3 or 14...

Any other solutions to provide ventilation?