Green Building Forum - Can MHRV aleviate the need for a vapour barrier Thu, 25 Jun 2026 04:07:57 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226678#Comment_226678 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226678#Comment_226678 Tue, 07 Jul 2015 08:55:34 +0100 tony
There needs to be a vapour barrier to stop moisture moving out through the walls and causing problems in the structure.

Try this line in Canada and they would laugh.

Yet manufacturers of some modern building systems are saying that if there is a mechanical heat recovery ventilation systen a vapour barrier is not needed, how insane is this,

What if it was switched off, or brakes? Arent they concerned about going into print on this and leaving themselves open to claims, or about their customers having problems?]]> Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226681#Comment_226681 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226681#Comment_226681 Tue, 07 Jul 2015 10:57:03 +0100 DarylP ]]> Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226696#Comment_226696 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226696#Comment_226696 Tue, 07 Jul 2015 13:15:07 +0100 tony Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226704#Comment_226704 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226704#Comment_226704 Tue, 07 Jul 2015 19:15:40 +0100 renewablejohn Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226705#Comment_226705 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226705#Comment_226705 Tue, 07 Jul 2015 19:35:38 +0100 barney
Don't know if the average occupier would understand the importance of the repair, but I daresay they might if it was contained in a "hand-over pack" of information including the commissioning documents

Regards

Barney]]> Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226710#Comment_226710 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226710#Comment_226710 Tue, 07 Jul 2015 23:05:38 +0100 tony
Water vapour will find its way to the cooler places (where there is least insulation)

Water vapour moves from areas of higher partial vapour pressure to places where there is lower partial vapour pressure. When or where dew point is reached condensation will occur. However especially in wood, raised moisture content tends to lead to rot, woodworm, mould, silverfish, bad stuff.

Warm air in the house even with MHRV will have a higher partial vapour pressure generally than air in the walls or outside.]]> Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226711#Comment_226711 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226711#Comment_226711 Tue, 07 Jul 2015 23:32:53 +0100 Ed Davies Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226712#Comment_226712 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226712#Comment_226712 Tue, 07 Jul 2015 23:50:35 +0100 tony
Why haven't the insurers picked up on this yet

May be they are hoping for it to happen after the ten years are up

I can't see the manufactures giving a very long warranty then there is the installation, the middle Man and sheer difficulty of "catch me if you can" scenarios.]]> Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226717#Comment_226717 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226717#Comment_226717 Wed, 08 Jul 2015 09:35:36 +0100 fostertom Posted By: renewablejohnreferring to old buildings which breathe rather than have a vapour barrierThat's a false distinction - Old buildings may be retrofitted with vapour barriers, conversely new buildings can be designed to be safe without vapour barriers.
Posted By: tonyWater vapour moves from areas of higher partial vapour pressure to places where there is lower partial vapour pressure. When or where dew point is reached condensation will occur
True - but both new and old buildings can be configured so that dew point is never reached, or is safely transitory, even without vapour barrier - that's what WUFI modelling is for. And if that configuration is done right (i.e. the arrangement of layers/thicknesses of different materials, relative to local climate data) then a no-vapour-barrier system is far safer and more robust longterm than one that relies on vapour barrier which is hard to install faultlessly, fragile, deteriorating and disruptable.

Vapour barrier or not, and MHRV or not, are completely independent - neither has any significant bearing on the other.]]> Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226720#Comment_226720 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226720#Comment_226720 Wed, 08 Jul 2015 10:18:43 +0100 djh Posted By: fostertomVapour barrier or not, and MHRV or not, are completely independent - neither has any significant bearing on the other.
I agree with everything you said, Tom, well summarised.

I think the apparently spurious connection between the two that Tony is complaining about might well be explained by the warranty provider's lawyers :devil:

If you want to run a WUFI model, you need to set boundary conditions. If you set worst case conditions, you'll likely find that you can get condensation and consequent damage, for which there would be liability. If you set reasonably likely boundary conditions, you'll probably be able to get a no-damage result from WUFI. An MVHR designer will have been working to make sure the MVHR system is capable of preventing the worst case conditions (e.g. 100% humidity for more than a few minutes). So if the system build warranty provider makes the warranty conditional on a properly designed and functioning MVHR system, they can be fairly confident they can rely on the WUFI model, and if there is ever a claim, the first line of defence is to blame the MVHR design, installation or operation.

Obviously, there are other ways to keep the conditions sensible, such as opening windows, not drying too much washing indoors, using extractor fans etc etc but they're a lot more complicated to write into a contract.]]> Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226726#Comment_226726 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226726#Comment_226726 Wed, 08 Jul 2015 11:19:08 +0100 fostertom Posted By: djhto run a WUFI model, you need to set boundary conditionsNot a lot of choice in that? Weather file for the location has to be valid but can't be fudged. Choice of BS or other standard model for how internal RH follows external - several options but all goodish. What else? Lots of sweaty occupants constantly taking showers?]]> Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226728#Comment_226728 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226728#Comment_226728 Wed, 08 Jul 2015 12:00:25 +0100 tony
The problem is not with RH but with partial vapour pressure.

Even air with low RH indoors when moved to a cooler place can hit dew point.

Independent of cooking, drying washing or breathing, MHRV]]> Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226734#Comment_226734 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226734#Comment_226734 Wed, 08 Jul 2015 13:27:33 +0100 djh Posted By: fostertom
Posted By: djhto run a WUFI model, you need to set boundary conditions
Not a lot of choice in that? Weather file for the location has to be valid but can't be fudged. Choice of BS or other standard model for how internal RH follows external - several options but all goodish. What else? Lots of sweaty occupants constantly taking showers?
To write a warranty, you'd need to assume 100% internal humidity at elevated temperatures if you allow unconstrained behaviour by the denizens. Making warranty claims depend on people following rules set out in some BS would be difficult.]]> Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226735#Comment_226735 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226735#Comment_226735 Wed, 08 Jul 2015 13:33:49 +0100 djh Posted By: tonyDamage to wood based structure can occur if they get damp
No, only if they remain damp.

The problem is not with RH but with partial vapour pressure.

Even air with low RH indoors when moved to a cooler place can hit dew point.

Dewpoint is the point at which water condenses at the same rate it evaporates. RH is defined as a percentage of that. i.e dewpoint is 100% RH. i.e. the consequences of moisture problems are related to RH - it's RH that controls when wood rots. The way that water vapour moves is indeed related to vapour pressure but the damage it causes is not.]]> Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226742#Comment_226742 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226742#Comment_226742 Wed, 08 Jul 2015 14:51:42 +0100 fostertom Posted By: djhDewpoint is the point at which water condenses at the same rate it evaporatesI never knew that, as a definition - makes much sense.]]> Can MHRV aleviate the need for a vapour barrier https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226744#Comment_226744 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13495&Focus=226744#Comment_226744 Wed, 08 Jul 2015 17:21:08 +0100 Ed Davies Posted By: djhDewpoint is the point at which water condenses at the same rate it evaporates.Strictly, over a flat surface of pure water.

If the water's got salt or anything in it or if the surface is curved then the dew point will be different. The liquid water in wood is in little capillaries so the meniscus is concave (from the air side) so the dewpoint is higher. That's why the equilibrium water content of wood shoots up when the RH passes about 95%.]]>