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New Build Vapour Barrier - Condensation

Discussion in 'Building' started by aoused, 14 Feb 2021.

  1. aoused

    aoused

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    Hello,

    I live in a new-build timber-frame detached house. The construction is fairly simple, timber frame clad with OSB (which then has some kind foil on the outside). On the outside is a brick wall. On the inside there is insulation (some kind of rock wool) with a plastic sheet vapour barrier on top followed by plasterboard.

    We had some problems with mould in the ensuite. I've now scrubbed it all off and redecorated with some more suitable paints (Zinsser anti-mould matt on the ceiling, 'modern emultion' from F&B on the walls). What I can how see when I am showering is that the walls are dripping wet for hours - the fan in the wall obviously isn't up to the job so I'm adding an MF100S inline fan.

    When up in the loft, I've noticed that the vapour barrier is stapled to the top of the ceiling plasterboard. There is a condensation built up under it. Do I need to be particuarly worried about this? I am going to make sure the insulation covers over the top and get the ventilation sorted.
     
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  3. ^woody^

    ^woody^

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    There seems to be condensation problem and it may not be just the bathroom fan not being adequate. Are you producing excess condensation and not venting it?

    Consider a constant trickle fan with boost instead of the one you propose - which a new build should have been fitted with in any case. Or at least one controlled by a humidistat.
     
  4. aoused

    aoused

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    There is a trickle fan fitted in the wall which boosts when a switch is flicked (we always use it when showering) or humidity rises above a set level. However it's nowhere near the shower and doesn't clear the room of steam before it turns to condensation on the walls and ceiling. I'm going to leave this fan in place (in parallel from the isolator).

    Our windows have trickle vents but I understand that there's no point on having them open when it's this cold?

    I'm adding a new fan. Also cutting a hole in the bottom of the door to allow air flow.

    I am starting to wonder whether I need a more whole house air circulation system though.
     
    Last edited: 15 Feb 2021
  5. JohnD

    JohnD

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    do you know the throughput of your fan?
     
  6. aoused

    aoused

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    5-8 l/s on trickle, 8-13 l/s on boost according to https://www.greenwood.co.uk/product/153/unity-cv2gip

    It appears I could adjust the settings on it to get a few more l/s out of it but I think it's mounted in a stupid position (above the toilet on the wall, about 1.5m from the shower cubicle door).
     
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  8. JohnD

    JohnD

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    I work in cu.m/hr

    I think:

    8lps is 20cu.m/hr, which is very little

    13 lps is 47 cu.m/hr which is much less than usual.

    Ordinary extractors are in the region of 80-100 cu.m/hr, which I find sufficient for a WC or a warm bathroom, but not for a room for steamy showers. This sort of thing https://www.envirovent.com/products/extractor-fans/silent-100/

    Have I got the figures right?
     
  9. aoused

    aoused

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    I don't know if your maths are right but I am hoping addition of 68 l/s from the MF100s will help solve soaking wet walls after a 10 minute shower in the morning :).

    Perhaps I will find a better trickle fan eventually or explore a more 'whole house' extraction system (the bathroom has the same crap fan).
     
  10. JohnD

    JohnD

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    I must have misunderstood. You also mention a MF100S, which should be very adequate to ventilate your shower room.

    It is rated at 245m3/hr

    just the thing you need.

    though I'd recommend a half-hour run-on timer model
     
  11. aoused

    aoused

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    I'm going to use a Manrose 1361 controller which has both humidity sensing (in case someone forgets press the boost switch) and timer function; both configurable without going into the loft. This will drive the MF100s. I can set it to a half-hour overrun.

    Only reason I'm leaving the Greenwood fan is that I can't be bothered to repair the big hole in the wall!
     
  12. JohnD

    JohnD

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    That sounds like it will be just what you need.
     
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