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Ventilation and damp problem

  • 20-11-2022 10:26pm
    Registered Users Posts: 5

    My Dad's house has a problem with high humidity and we've been told it's a ventilation problem but we're not sure how to solve it.

    It's a 1970's timber frame bungalow, 2000 square foot, no BER but it's cold and musty smelling. Has no air vents at all and no extractor fans in bathrooms. We're getting the latter put in now. Damp survey people told us to also get a PIV unit in the attic but my Dad doesn't understand why normal air vents put into the wall wouldn't do. The damp survey people aren't explaining why we need a PIV, they just say air vents are inadequate. The house is very exposed and west-facing so I think vents would be seriously draughty a lot of the time.

    I read that PIV in a house like ours with no vents at all is going to push the humidity out into the timber frame. Would DCV be better for us? Or is there something else?


  • Registered Users Posts: 2,810 ✭✭✭MicktheMan

    You're correct, a PIV unit runs the real risk of forcing moisture laden air into the timber structure where the vapour will condense into liquid water and rot the structure.

    Imo, a cMEV system (centralised Mechanical Extract Ventilation) extracting from the wet rooms would be superior to PIV and solve the current air quality issue.

    To improve the ventilation efficiency, the heating level will likely need to be improved also.

  • Registered Users Posts: 5 Dot2

    Thank you. Would a cMEV mean we wouldn't need separate extractor fans in the bathrooms then?

    I don't know what we're going to do about the heating levels. The damp people said we had to maintain a temperature of 18 degrees but that's completely impossible. We're getting attic insulation added, but the walls probably have 4 inches of fibreglass insulation and that's it. I suppose we should look at internal insulation next maybe.

  • Registered Users Posts: 2,810 ✭✭✭MicktheMan

    Yes, no need for separate extractors as the central unit takes care of this.

    At the moment there is excess moisture in the house. This means that there is also likely excess moisture in the structure including whatever insulation is present. Damp insulation is like damp clothes, useless. When the ventilation is working it should make a big difference to this and the bit of insulation that is there should start to perform much better, i.e. allow less heat through the structure and therefore it should be easier to heat.

    Also pay attention to air leakage (draughts) which can be quite significant in timber frame structures depending on how it was built originally.

  • Registered Users Posts: 5 Dot2

    Ok so we need to get the air leak situation looked at too thank you. If we made it less leaky, though, and had cMEV, would we then need to put in wall vents, or get windows with trickle vents? Otherwise how would the fresh air get in?

    Also, if you have cMEV, won't the negative pressure it creates pull air in from outside through the building fabric and wouldn't that air be even more humid outside than inside, like if it's raining?

  • Registered Users Posts: 2,810 ✭✭✭MicktheMan

    All very good questions.

    The cMEV system will draw external (cool but dry) air in through gaps and cracks in the building structure / fabric (there will always be routes for air to infiltrate - we're talking about background ventilation here so low levels of continuous air flow). This is completely safe in that the air coming in will be significantly dryer that the air being exhausted, (even if it is raining!) due to the temperature differential so no possibility of any condensation happening within the structure. Trickle vents can always be added to windows if deemed necessary (this would depend on what level of airtightness the house achieves).

    Bear in mind that this is a winter time (heating season) issue and ventilation is ineffective when there is little difference between indoor and outdoor temps.

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  • Registered Users Posts: 2,549 ✭✭✭Yellow_Fern

    General word of advice. Get the best ventilation system you can afford. Thank me later.

  • Registered Users Posts: 5 Dot2

    It's so hard to know what's the best though! I've just started looking at brands now. I presume like with most things it's a case of you get what you pay for?

  • Registered Users Posts: 2,549 ✭✭✭Yellow_Fern

    There is a hierarchy, see below. The worst is at the bottom.

    Centralised mechanical ventilation with heat recovery (MVHR / HRV)

    Centralised mechanical ventilation without heat recovery (can be very good now, but watch out for noise)

    Decentralised mechanical ventilation with heat recovery (arguable should be no. 2)

    Decentralised mechanical ventilation without heat recovery (often also demand controlled ventilation/ eMEV)

    Positive input ventilation

    Trickle vents and hole in the wall vents

  • Registered Users Posts: 5 Dot2

    Ok, thanks. We can't do heat recovery because the house is leaky like a sieve and barely insulated too. Why is centralized MEV higher up than decentralized MEV? Don't they do the same thing? Our attic is messy, so I suspect my Dad won't be too keen on having ducts going up there and a unit installed. He'd also prefer to have things where they're easy to access if there's a problem.

  • Registered Users Posts: 1,047 ✭✭✭TimHorton

    What Decentralised mechanical ventilation with heat recovery system would you recommend? Let's say it would be for 2 adjacent bedrooms at the north end of a bungalow.

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  • Registered Users Posts: 2,549 ✭✭✭Yellow_Fern

    I honestly would'nt have the expertise on the different makes to advise.

  • Registered Users Posts: 241 ✭✭Dutch Roll

    I have a similar issue although I don't think it's as severe. Bad condensation and very low air circulation it seems.

    Did you get an air tightness test done? Is that the damp survey you mentioned?

  • Registered Users Posts: 241 ✭✭Dutch Roll

    I don't know what my first step should be. Air tightness test? That will then inform if Im suitable to get a MVHR system, right?

    Attic insulation is quite good, installed between ceiling joists and the rafters. Walls were pumped years ago, but might need to be done again. How would I find this out?

  • Registered Users Posts: 1,737 ✭✭✭mp3ireland2

    What did you do in the end? I'm looking into ventilation at the moment as aside from the extension and bathroom fans the house has none.

    Hard to know which solution is the best value for money as the MVHR is so pricey from what I gather.

    Not much of a moulid problem, small bits around bedroom window at times, the other half doesn't like me opening windows in morning to freshen things up due to insects coming in.

  • Registered Users Posts: 102 ✭✭hydrus21

    If the OP has poor wall insulation then would IWI with vapour check and a PIV system be the solution?