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Green Building Bible, Fourth Edition
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Fundamentals: Interstitial condensation and MHRV

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    • CommentAuthortony
    • CommentTimeJun 30th 2011
     
    Does having MHRV likely reduce the adverse effects of interstitial condensation.

    This idea is a popular misconception and is untrue.

    The amounts of moisture in the air inside a building in winter are very much greater than they are in the air outside, therefore when house is cooled as it escapes or percolates into the structure is can condense and start to try cause problems.

    This is why in Canada they are very very careful when installing vapour barriers ditto in central Europe and Scandinavia.

    In Britain as our homes become more air tight and better insulated we must not think that MHRV will save us from the necessity of mitigating interstitial problems.
    • CommentAuthorTimber
    • CommentTimeJun 30th 2011
     
    Agreed!

    I have seen many people try and offset bad design with MVHR, claiming it will control internal RH levels and stop interstitial condensation formation.

    Load of old nonsense. Even if it were to work, after a few years the MVHR will fail, or occupants will turn them off or whatever.

    Good design is key, and bad design is just bad.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJun 30th 2011 edited
     
    Not sure I follow you there Tony.
    If the inside, high RH air is replaced by external low RH air, but heated to the same internal temperature on the way in (or close to it), surely you end up with a lower RH inside the building?
    Or do you mean letting a hot, high HR house cool to external temperature when you go out thus causing condensate problems?
    The Canadians have a very high proportion of timber housing and much larger temperature swings than the UK (well in some parts), this could the reason that they understand VC better.
    • CommentAuthortony
    • CommentTimeJun 30th 2011
     
    It is the amounts of water that is contained in the air inside being higher than it can hold when it cools that is the problem.

    eg resulting in condensation on windows. The same thing can happen if air from the building escapes into a wall or roof but we cant see it -- this is interstitial condensation.

    Yes the Canadians do ........and.........
    •  
      CommentAuthorSteamyTea
    • CommentTimeJun 30th 2011
     
    Posted By: tonyThe same thing can happen if air from the building escapes into a wall or roof

    Right I get you now. Yes possibly a problem in our leaky houses.

    Posted By: tony........and.........

    They probably have better building standards and better builders, though I suspect there is a very high proportion of British builders there.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJun 30th 2011
     
    •  
      CommentAuthordjh
    • CommentTimeJun 30th 2011
     
    Posted By: tonyDoes having MHRV likely reduce the adverse effects of interstitial condensation.

    This idea is a popular misconception and is untrue.

    No, it's not a misconception. The humidity inside a house with MVHR is likely to be better controlled than a house without it (i.e. likely to be lower). So interstitial condensation will occur less frequently and consequently there will be less adverse effects. Incidentally, the heat recovery is completely irrelevant. All that's relevant is whether the house is properly ventilated or not.

    In Britain as our homes become more air tight and better insulated we must not think that MHRV will save us from the necessity of mitigating interstitial problems.

    But this is also correct. When interstitial condensation does occur, it's effects are just the same whatever ventilation system is used.

    We shouldn't be looking in any case to mitigate interstitial condensation problems, we should eliminate the problems altogether by good design and careful building.
    • CommentAuthorandy500
    • CommentTimeJul 2nd 2011
     
    I'd say HRV helps - had ours nearly 5 years. But then again I haven't ripped any timber frame walls open to find out.
    Eg in winter - bathroom cleared of steam more rapidly with the system on boost than with a 4" fan in our case.
    We don't get condensation on windows apart from minimal buildups in the corners of the bedroom dg units on subzero mornings, which clears anyway when the curtains are opened. No mould or signs of condensation in bathrooms which have windows sealed shut nearly all year until the 'summer'.
    The interstitial/hidden side of things must be aided by permenantly running ventilation fans, I'm sure of it.
    No proof, but I really do think as long as a system is on and working correctly, it must help compared to bathroom fans and closed winter windows.
    • CommentAuthorTimber
    • CommentTimeJul 2nd 2011
     
    ^ yes it will help, but shouldn't be considered as the solution to an inherant problem.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 2nd 2011
     
    Seems to me that if we are going down the airtight route for housing then materials and design will have to change. Hygroscopic materials, especially untreated 'natural' ones may become a no no. Not sure of that is a good or bad thing in the scheme of things. Does not help the existing housing stock much and that is really the big problem.
    •  
      CommentAuthordjh
    • CommentTimeJul 4th 2011
     
    SteamyTea wrote: "Seems to me that if we are going down the airtight route for housing then materials and design will have to change. Hygroscopic materials, especially untreated 'natural' ones may become a no no."

    Not sure where you got that idea from. There've been repeated topics on here discussing condensation risks and humidity buffering etc. I'd suggest Tim Padfield as one source http://www.padfield.org/tim/ and it seems he lives near you. http://www.conservationphysics.org/ppubs/getty_tenerife_tp2007.pdf

    And if all else fails, a vapour barrier is probably the answer.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 4th 2011
     
    Thanks for the link Dave, lots to look up.
    Totness may be in the West Country, not that near me though :sad:

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