What causes the mould at the highest point of the external wall?

Many residents here in Ireland are struggling with mould and dampness every winter. The most of them are pretty frustrated because they got new windows installed, the cavity wall pumped and the attic properly insulated. And now it seems, that the trouble with the black spots on top of the wall is getting even worse than before.


Was it the wrong way, to invest in the new insulation materials?


Definitely not (except you´re the proud owner of an old cottage) . But there is a weak spot in your building structure, that the most people do not realise. A traditional cavity wall structure is covered by a wall plate on top. Years ago, many builder were using a block-on-flat to close the cavity at the top end.

You got the new windows, the cavity wall and the attic insulation done. All these insulation upgrades are raising the surface temperatures for these parts of the building. But mostly nobody took notice about the non-insulated space behind the wallplate. This is one of the coldest spots in your home now! This space is completely blue in the thermal image above.


The following photo was taken during one of my testings regarding heat loss and shows the typical layout of the area between the wall and the roof. You might can see some beeds, coming from the cavity wall insulation. You can´t see the well insulated attic, packed with glass wool. But you definitely see the lack of insulation between the ends of the ceiling joists. That´s the reason for the blue area of the thermal imaging.



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Comments: 2
  • #1

    Brendan (Sunday, 18 February 2018 17:34)

    What solution would you recommend to counteract this cold spot?

    Extend the insulation all the way to soffit? Spray expanding foam? Use PIR insulation

  • #2

    prodomo Ireland (Sunday, 18 February 2018 17:43)

    There´s no universal solution available, that fits all purposes. It all depends to the layout of your roof & wall plate area. If you do not have cross battens and/ or the old bitumen felt, you need to insulate this area very carefully, to prevent direct access of the insulation material to the felt. Spray foam could be a solution for this case, when you can guarantee the installer a proper access to apply the cardboard and the foam.
    If you´re able to apply PIR insulation into this hard to reach area, why not?
    My honest advise for all new builds is, to stay away of the block on flat and to take attention to this particular area during the building stage, when you still get access.