roof insulation problem

5 Jan 2005
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United Kingdom
I moved into a new house not long ago and one off the thing that came up in the survey was that there is not insulation in the roof. I had a look myself and there truly is no insulation at all!

I have read a number of post on the forum to find out what solutions are available: Glasswool, (knauff) for cold roof and Celotex/Kingspan for warm roof.

My problem is that my roof is sort of a mix of both! The top floor room are partially in the roof. i.e. on one side of the room the ceiling is pitched to follow the roof, in the rest of the room it’s flat. The only thing separating the room and the roof space is lath and plaster.

1-To insulate the pitched part I guess I can only go for Celotex/Kingspan and try to slide it down the gap between the lath and plaster and the roof tiles or is there any other alternatives?

2-If I use celotex/kinspan in the pitched part of the roof, should I just use it as well to insulate between rafters in the flat part? Or can I switch to Glasswool for that section?

3- How can I work out what thickness to use to ensure the insulation will be adequate/ meet building regs?


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you need 150mm of insulation with no gaps or thin spots. You will find this makes a tremendous improvement. However the latest standard is 250mm. But this saves you little extra, because you get diminishing returns from each layer.

Have a look at the depth of the timbers in your loft. Start by insulating to this full depth.

If you think you might want to board it later, you can cross-batten over the top with 50x50mm or 100x50 or similar, and lay an extra layer of insulation between the cross-battens. As it is going at right angles to the first layer, it will reduce the chance of gaps or thin spots. It is also possible to lay rigid foam over the timbers and put chipboard or ply on top of that for walking on.

If you lay a thick layer on top of the timbers, it will be difficult and unsafe to move about up there.

It is easier to pack fibreglass tightly between timbers than the rigid foams.

In your sloping space, you need to leave an air gap between the insulation and the tiles. It is possible to poke fibreglass into such a gap with sticks and string, but you must fasten it so it can't slip down.

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