I have an extension on the back of my house which originally had a flat roof. At some point, a previous owner had a sloping roof put over the flat roof which covers it completely (i.e. it is not possible to see any part of the original flat roof).

The room of the extension gets incredibly cold in winter (so cold we get condensation on the ceiling) but taking the inner ceiling down to insulate between the ceiling and original flat roof would be a massive upheaval as the room is in daily use (it's our only toilet/bathroom).

So I am wondering if I can insulate in the cavity between the original flat roof and the sloping roof. It is watertight and there is a lot of space there. Could I use regular rockwool loft insulation? Solid insulation? Or is it a bad idea?

  • I'm curious how it is that you can get to the between-the-roofs gap, but can get into the ceiling. But I guess that's a different type of question...
    – FreeMan
    Commented Oct 3, 2023 at 15:22
  • @crip659 ah I see, I will check to see if there are some ventilation holes or anything
    – danwellman
    Commented Oct 3, 2023 at 15:23
  • @FreeMan we are having some wood replaced on the sides of the sloping roof, so for a short time only I can see into the space between the original roof and the sloping roof
    – danwellman
    Commented Oct 3, 2023 at 15:24
  • 2
    Ah! That makes sense. I would definitely get some insulation in there while you can!
    – FreeMan
    Commented Oct 3, 2023 at 15:26

1 Answer 1


If you can (evidently) get in there to place insulation, yes, you can certainly treat it as an attic needing to be insulated and place insulation on the "floor" (old roof surface.)

In many places the most cost effective insulation is cellulose fiber (it's treated to prevent fire and insect habitation) but whatever insulation you like will work, so rockwool or rigid foam are fine if you prefer those, or have some sitting around doing nothing.

  • 2
    I agree and would add that you need to maintain ventilation of the upper roof deck, to include baffles at the bottom. You don't want to create ice dams.
    – isherwood
    Commented Oct 3, 2023 at 15:21

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