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Housing - New Build: Can MHRV aleviate the need for a vapour barrier

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  1.  
    • CommentAuthortony
    • CommentTimeJul 7th 2015
     
    NO, No, no

    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?
    • CommentAuthorDarylP
    • CommentTimeJul 7th 2015
     
    Tony, who says this...?, and yes if it breaks, there would be problems!:devil:
    • CommentAuthortony
    • CommentTimeJul 7th 2015
     
    I can't really say but it is to do with SIPs
  4.  
    I presume as its in the new build section your not referring to old buildings which breathe rather than have a vapour barrier.
    • CommentAuthorbarney
    • CommentTimeJul 7th 2015
     
    Well it wouldn't be an instant problem, I suspect - if the space is designed for MVHR is that any difference to a space designed with a slate roof - a slipped slate will let in water, but it wouldn't be unreasonable to assume that it would be repaired - as would the MVHR to contro humidity

    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
    • CommentAuthortony
    • CommentTimeJul 7th 2015
     
    My point is that even with it running it still needs a vapour barrier to protect the structure from damage.

    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.
    • CommentAuthorEd Davies
    • CommentTimeJul 7th 2015
     
    Yep, it's conceivable that there could be a wall build up which, in some circumstances, would not work without MHRV but would work with it but if it's so marginal that MHRV makes a difference wouldn't it also be very dependent on occupant behaviour, weather conditions and so on. In other words wouldn't it be so marginal as to be realistically a real risk?
    • CommentAuthortony
    • CommentTimeJul 7th 2015
     
    I think it is a real problem that will show up after a few years and result in claims against the companies in question

    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.
    •  
      CommentAuthorfostertom
    • CommentTimeJul 8th 2015 edited
     
    Posted By: renewablejohnreferring to old buildings which breathe rather than have a vapour barrier
    That'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.
    •  
      CommentAuthordjh
    • CommentTimeJul 8th 2015
     
    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.
    •  
      CommentAuthorfostertom
    • CommentTimeJul 8th 2015
     
    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?
    • CommentAuthortony
    • CommentTimeJul 8th 2015
     
    Damage to wood based structure can occur if they get 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.

    Independent of cooking, drying washing or breathing, MHRV
    •  
      CommentAuthordjh
    • CommentTimeJul 8th 2015
     
    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.
    •  
      CommentAuthordjh
    • CommentTimeJul 8th 2015
     
    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.
    •  
      CommentAuthorfostertom
    • CommentTimeJul 8th 2015
     
    Posted By: djhDewpoint is the point at which water condenses at the same rate it evaporates
    I never knew that, as a definition - makes much sense.
    • CommentAuthorEd Davies
    • CommentTimeJul 8th 2015
     
    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%.

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