Condensation

Discussion in 'Getting Started FAQ' started by Glenda84, Dec 1, 2018.

  1. Glenda84

    Glenda84 New Member

    hi. I have recently bricked up a small first floor window. It's a 70s build with little if and cavity insulation.i have put celotex 80mm in between brickwork and internal plasterboard. But wen it came removing insulation I found it had formed condensation on outer side of celotex.any suggestions on how to cure this would be be great. Thanks
     
  2. rogerk101

    rogerk101 Well-Known Member

    Condensation forms at the dew point which occurs somewhere between the warm inside of the wall and the cold outside. That's just a law of physics, which can't be changed.
    In a well designed wall, the dew point will occur on the inside surface of the outside 'skin' of the double skin wall, and as long as the insulation in the cavity isn't touching that surface of the outside skin, the outside wall will just allow the condensation to dry off the outer surface of the wall.
    What can also help is reducing the amount of warm moist air that gets into the wall from the inside - obviously the lower the better.
    You can lower it during construction with a really good continuous vapour barrier just behind the plasterboard (assuming it has plasterboard walls).
    If you haven't got plasterboard walls, you can reduce the amount of warm moist air entering the walls by using a good vapour sealing paint on the internal walls. (You can only use such a paint if the outside wall finish is breathable, i.e. one side of the wall must be able to let moisture out.)
     
  3. Glenda84

    Glenda84 New Member

    Thank you for your reply. Basically it's a 9" brick wall with cavity. Iv removed the window and bricked up external wall and have battend internal and plasterboard inside. Was just surprised that the celotex was dripping wet. Pretty sure it wasn't touching external wall but can't be 100%. Have made sure it's not touching now.where the cil was there's a cement filet. Shall I remove it to allow moisture to go down cavity??
     
  4. rogerk101

    rogerk101 Well-Known Member

    It doesn't sound like you used a cavity tray when you blocked off the window. Cavity trays are intended to ensure any condensation dripping down the inside of the cavity is diverted to the outside, i.e. to keep the cavity insulation from getting wet. Modern building practices require a cavity weep vent above each door or window opening.
     

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