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Ventilation: PIV in a warm loft?

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  1.  
    • CommentAuthorMr_W
    • CommentTimeMay 1st 2025
     
    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.
  2.  
    I'M a bit confused about the terminology,

    Posted By: Mr_Wrockwool 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).

    I will assume that (100 mm PIR between joists + 25 mm PIR below joists). actually refers to (100 mm PIR between RAFTERS + 25 mm PIR below RAFTERS). to make the warm roof. That said

    You say that you have a warm loft but it is not sealed from drafts. The essence of a warm loft is that it is part of the heated envelope and as such should be as draft free as the rest of the house, also with a warm loft you would not normally put insulation on the loft joists since the insulation layer is at the rafters. If the loft has drafts then the insulation between the rafters will be nullified and the only effective insulation will be between the loft joists. How much insulation is between the loft joists?

    Before fitting PIV it might be worth fitting humidity sensors to the existing extract fans to keep them running until the humidity drops to an acceptable level. This should have the same effect as PIV without the cost and disruption of PIV, although noise may be more than with PIV but perhaps worth a cheap try to see if this solves the problem.

    If you want to install PIV in a warm loft (that ordinarily should be draft free = air tight) then an input to the PIV from the outside will be needed.
    •  
      CommentAuthordjh
    • CommentTimeMay 2nd 2025
     
    +1 for using a few humidity sensors to measure the actual humidity and get an idea what the situation is. Opening windows wide for a short period is a good way to improve ventilation quickly. As well as PIV it might be worth considering one or two single room MVHR systems or a MEV system.
  4.  
    As well as what P-in-H and djh have said, you need *intentional* ventilation, and something to 'distribute' it. It sound like all the 'in' ventilation you have up to now is 'accidental' - draughts.

    Yes, I agree that trickle vents can seem like a pain, but I use them as part of a decentralised mechanical extract ventilation 'system' -dMEV- (too grand a words as at its simplest it is *really* simple). Tricckle vents in the majority of the 'dry' rooms, and dMEV fans in the wet rooms. These run 24/7 on a very low wattage, and serve to pulll the incoming air from the trickle vents through the house via undercuts on doors. V simple. The Greenwood CV2GIP (I think) is widely praised. Cost me less than £60/unit supply only IIRC. Needs a 100mm core hole and elecs.
  5.  
    PS: For clarity I would use dMEV *instead of* not as well as PIV, which I feel in some circumstances is the possibly right answer to a different question!
    • CommentAuthorMike1
    • CommentTimeMay 2nd 2025
     
    Posted By: Mr_WPositive Input Ventilation
    Avoid PIV. It was fine a few decades ago when homes leaked air like a sieve, but not in a renovated home that should be at least somewhat less leaky. The PIV will tend to push moisture-laden air from the interior of your home through gaps in the structure, where the air may well condense and cause further mould, or worse. And, of course, you're loosing the warmth.

    The gold standard would be MVHR (mechanical Ventilation with heat recovery), though as a retrofit it's unlikely to be easy to hide the ducts. It's not impossible though, and as you're already thinking of installing PIV, you're presumably also prepared to accept a degree of disruption (and the availability of flat ducts may help). dMEV would be an easier-to-install alternative.
    • CommentAuthorMr_W
    • CommentTimeMay 13th 2025 edited
     
    Thanks for all the replies and info. Much appreciated and its given me things to consider.

    Firstly, you're correct, I meant 100 mm PIR between rafters not joists. There is also 100 mm of rockwool RWA45 accoustic battons between the joists of the loft, but that is there primarily to help with reducing sound transfer between loft and bedrooms below. (I have no idea how much it actually helps, the rockwool was leftover from other areas of the house).
    The loft is warm, generally a couple of degrees warmer with a more stable temp compared to the bedrooms below. much the same temp. However, I thought i could feel a tiny bit of air movement in there when it was very windy outside. There are a couple of small holes for cables running outside, so I wouldn't say the loft is completely draft free.

    dMEV in the bathroom sounds like it could be a good alternative solution.
    The current bathroom extraction is via an inline extractor (I can't remember the spec. unfortunately), which is housed in cupboard in room next to the loft (the room used to be part of the loft but we added a mezanine floor so the bedroom below could expand into the loft space). The extractor fan has ducts running from the bathroom and to the soffits to extract the humid air.
    What I'm now thinking is that an MEV or small MVHR unit could replace the inline extractor fan. If it's just an MEV with a switched live, it would be a straight swap, which i could do.
    However, I wonder if it would be worth going for MVHR and adding in ducting and vents to the bedrooms.
    Its the bedrooms that suffer from the mould. There is no mould in the bathroom, so i think the current extractor must be doing an OK job.

    But my wife is concerned I'm over-engineering the solution. She'd rather we put in trickle vents into the windows and leave it at that. She's also very concerned about droning noise from an MEV or MVHR unit running all the time, especially in the bedrooms.
    • CommentAuthorMr_W
    • CommentTimeMay 13th 2025
     
    I should add, because of the location of the bedrooms, bathroom, loft, mezzanine floor, and position for the extractor, it would be possible to hide all ducting.
    • CommentAuthorMike1
    • CommentTimeMay 13th 2025
     
    Posted By: Mr_WShe'd rather we put in trickle vents into the windows and leave it at that. She's also very concerned about droning noise from an MEV or MVHR unit running all the time, especially in the bedrooms.
    MVR or MVHR shouldn't create a noticeable noise, except perhaps on boost, if suitably sited and fixed. Whereas trickle vents will let in any outside noise - but maybe you're in peaceful countryside?

    Trickle vents will certainly help with the ventilation, but there's a tension going on - they'll also admit cool air, which could cause local condensation unless the rooms have enough heating to prevent that. And you need to keep them open or choose a humidy-controlled model. They're undoubtedly the cheapest option though.
    • CommentAuthorMr_W
    • CommentTimeMay 15th 2025
     
    I'm swaying towards putting in an MEV, to replace the existing standard bathroom extractor. I'm thinking to get an MEV with humidity sensor, plus switched live so that it boosts when the bathroom light is on. Plus some new extraction vents in the bedrooms too.
    MVRH would be great but m it's too expensive and complicated to install for the benefit we'd get in our house.
    I think that with an MEV pulling air out of the bathroom and bedrooms, it will draw air upstairs from our main entrance. Fresh air will pulled from outside. There's also a wood burning stove in that room, so when that's running (most days during mid winter), it will be warmed air being pulled upstairs. Should reduce humidity and reduce mould.
    • CommentAuthorMr_W
    • CommentTimeMay 15th 2025
     
    Anyone got any suggestions for reliable MEVs. Any that can link to home assistant?
  12.  
    Mr_W said "Anyone got any suggestions for reliable MEVs. Any that can link to home assistant?"

    The former, yes: Greenwood CV2GiP. The latter, way beyond my area of comprehension and I have no idea, sorry!
    •  
      CommentAuthordjh
    • CommentTimeMay 17th 2025
     
    A quick google threw up these threads with some ideas about HA:

    https://www.facebook.com/groups/HomeAssistant/posts/3925640474373960/

    https://www.avforums.com/threads/smart-relays-for-extractor-fans-lightwaverf-something-else.2287668/

    The basic idea seems to be to use a separate humidistat and relay to control the fan if required.

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