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How to fix condensation/damp in bathroom?

Discussion in 'Plastering and Rendering' started by ThomasAnderson, 5 Oct 2021.

  1. ThomasAnderson

    ThomasAnderson

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    Hi,

    I had no joy with this on another forum so I thought I'd share it here. I wouldn't call it a plastering or rendering issue but this forum seems to contain all the posts about it so here I am.

    I live in a terraced house, two up, two down, with a galley kitchen. Years ago this house had a bathroom extension added to it. When it's cold outside, condensation tends to form inside the bathroom. This creates black patches which do wipe off but are still a pain (this shows itself primarily on the inside of the two exterior walls, and the ceiling) - also, the grout and silicon turns black. Years ago we had an extractor fan fitted that runs when the light turns on, but that didn't fix the problem. We've been running a dehumidifier but that's not a solution. I wish to renovate the bathroom - put in a new suite and tile all the walls, but I don't want to if the damp keeps coming back.

    I believe the extension is a brick and block cavity wall (this can be seen by the fact that sometimes when it's really damp, you can see the outline of the blocks formed by the moisture on the far wall over the bath). However, measuring the width of the wall from external brick to the skim coat of plaster is 230mm.

    So, I expect that gives us:

    102.5mm brick width
    100mm 4" block width
    10mm backing plaster/skim coat
    -----
    212.5mm

    This leaves a measly 17.5mm for a cavity. Now, I've been told before that the cavity wasn't filled but saw the system used on certain building sites where small pieces of insulation have been pumped in. I was hoping to do the same but would it really make any difference? My wife tells me that some tradesman told her years ago that the cavity was too small to insulate. Is there any other solution? I could bash all the plaster off, tank it, and affix tiles on waterproof/insulated backing board, but I still don't think that would solve the primary cause of the problem - the disparity between the cold surfaces and the warm, moist air that cause the condensation in the first place.

    Also, I doubt there's insulation in the roof, either.

    Thanks for any and all help!
     
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  3. JohnD

    JohnD

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    tell us about this extractor fan you have. Perhaps it can be improved. post some pics

    when does it come on and when does it go off.

    I have a feeling the walls are not insulated. Drill a hole and see how deep the cavity is. Or maybe you can see it in the extractor duct hole. You could however use an insulating board on the inside (instead of plaster) and tile it. This would mean the internal surfaces are not cold, and will reduce heat loss from the room.

    how is the room heated

    are you able to get above the ceiling to insulate it?

    is the floor concrete?
     
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  4. lostinthelight

    lostinthelight

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    Does the fan have a run on timer so keeps running for a set time after the lights are turned off? Do you dry towels or washing on the radiator ?.
    Do you wipe the bath/ shower and walls after use?
     
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  5. RandomGrinch

    RandomGrinch

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    Forgive us for a barrage of questions, but it will help us get a grip of your situation.
    Providing some photos may also help.

    The causes of condensation are simple and a good summary can be found here:

    https://www.diydata.com/problem/condensation/condensation.php

    The solutions are also simple, ventilate, insulate and reduce the source of water vapour. However it might not be so simple in practice! :)

    So my questions:
    You say the bathroom is in an extension - is this G/F next to the kitchen, or F/F over the kitchen?
    Are there many of you in the household, including pets?
    Do you let washed clothes dry indoors?
    Do you have an open gas fire or gas hob?
    Does your kitchen have an extractor?
    Have you had issues with damp elsewhere in the house?

    Personally, I have an 1890's stone terrace that sounds similar to yours. The walls are solid stone with no insulation.
    My bathroom sits above the galley kitchen.
    There was a condensation and damp problem when I moved in. The damp was solved by letting the house breathe; removing cement render and reinstating lime plaster.
    I added an extractor to the kitchen and bathroom, and reinstated closed air bricks.
    I also ditched the open gas fire.
    In my case, I haven't insulated, but reducing the damp in the house and increasing the ventilation has completely solved my issues.

    Bathroom extractors are now available with run-on timers, humidistats and continuous background ventilation. However, to be effective, they need to have a source of make-up air and be placed in a sensible location.

    If you can try and reduce (at least temporarily) the other sources of water vapour in the house, and there is still a significant amount of condensation; you have an idea that there could be a damp issue elsewhere to deal with.

    If you are completely redoing the bathroom, and you can afford to lose some space, it may be worth dry lining. Use treated battens, insulation and a vapour barrier. If you can't afford the space, as JohnD says, insulated plasterboard (including a vapour barrier) would be a good idea.

    And of course insulate the roof space!

    I hope that gives you some food for thought and please forgive the essay! :)
     
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  6. ThomasAnderson

    ThomasAnderson

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    Thanks for the replies everyone. Sorry for the delay in getting back to you all but I've only just been able to find the time to get on my son's computer whilst he is off it!

    Hi, the extractor fan is just over 6"x6" and looks like this:

    [​IMG]

    It comes on when the light comes on, which doesn't really help the fact that the primary cause of the damp appears to be cold winter nights!

    Yep, the walls aren't insulated.

    Bashing the plaster off and affixing insulated board sounds great if it works. Would something like this be sufficient? https://www.wickes.co.uk/Knauf-XPS-...rd-Tapered-Edge---27mm-x-1-2m-x-2-4m/p/163699

    I've just taken the cover off the fan to check the cavity depth but it's dark now and I have to have the light off and a head torch doesn't quite do the job so I'll check again when it's light.

    The ceiling is boarded out and skimmed, I assume I'd have to create a hole to attempt to get up there and install some kind of insulation, not sure how feasible that would be but I wouldn't want to insulate the walls only for something to appear on the ceiling! But it is primarily the two walls facing outside.

    Heating is via a radiator.

    Sub floor is concrete, I think. I recently raised it with self level when I installed flooring running through the bathroom to the kitchen.

    Fan on timer? Nope, which I'm sure contributes. As does drying towels on the radiator and not wiping the walls after using the bath!

    Thanks for that and cheers for the link, very informative. I won't go into lots of detail responding because, even though you and others have drawn my attention to all of those factors, I know there's a damp problem elsewhere as the walls facing outside are soaking wet on a cold morning. But it is on the ground floor and there is five in the house and Yes! to the next four questions. The damp elsewhere in the house is just black though, and tends to form when something is propped up against certain walls - this is condensation that ends up turning black.

    Anyway, thanks all, lots of ideas there. Hopefully I can just board it out and find a way to chuck some rockwool above the ceiling, and get a fan that's actually on a timer.
     
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  8. JohnD

    JohnD

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    Is your fan actually a manrose?

    I have found them rather poor, and noisy. You can get a quieter and more powerful one that uses less electricity.

    A modern extractor will run for about 120 hours on 17pence of electricity.

    As you already have a damp problem, you might run it continuously until you can fix the other problems.
     
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  9. ThomasAnderson

    ThomasAnderson

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    Yes, it's a Manrose. I'm looking other ones up now, cheers!
     
  10. JohnD

    JohnD

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    Envirovent Silent 100T if you can find it around £35

    Soler & Palau Silent 100T which is the same thing (same company, different brand name) usually dearer
     
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  11. ThomasAnderson

    ThomasAnderson

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    Sound, thanks. One question though... if the existing fan gets its power from the light being turned on, how do I ensure this fan comes on on the timer? The only electric stuff I've ever done is like for like swaps of power point, light switches, and light bulb sockets.

    Edit: it's ok, I've just seen on the datasheet for the product you recommended how it works!
     
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