insulating dormer walls

19 Jul 2008
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Tyne and Wear
United Kingdom

I have a dormer bungalow and after sleeping at freezer temperatures last Winter, it is a good time to insulate.
The stud walls to the voids are constructed from lath and plaster and I would prefer not to take them down (too messy I'd imagine).

The question is if I create an access to the voids, is it effective to stick celotex 100mm boards directly to the back of this wall.

Also I would like to keep the access as extra storage, so would it be better to insulate from the rafters instead?

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I'm in the process of doing something similar ... but I *did* take the walls down.

One issue with pushing insulation in from above is you might break off the plaster nibs, which is the bit that holds the plaster to the lathe.

You will not get a good air seal behind the L&P and rigid board, any tiny gaps will allow air to circulate and move your heat away.
I did this on a Georgian house some years ago, its a right pain, can't remember, but I did access the void. I think I put up a load of wire strings across the backs of the joists and then then stuffed in as much GF as possible. Back of your new door could use foam. Biggest suprise to me was that the big "box" where the dormer sat inside the house ridge had no insulation under it*. This mean't that that section of the lounge ceiling had a nice loft temperature above it. in retrospect, I wonder if I should have use hardboard strips (or panels for rich folk) to hold up the GF behind the vertical joists. Of course you try to hold your foam panels in with squirty foam, but it won't work, its expensive and messy.
* do the floor of your void?

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