New extension and ropey existing loft space... I need an insulation strategy!

21 Sep 2009
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United Kingdom

I've a new extension with 5in rafters and upstairs ceiling joists, connecting to an exist loft with 3.5in rafters and ceiling joists. I need to insulate all over really. 100mm of old stuff on most of the existing roof and want to progress towards having better (i.e. wife friendly) loft storage and possibly and the likes - possibly an informal, occasional den for the kids (not a conversion per se, just "storage" including a PS4 and a few bean bags...)

Clearly I don't need the entire space available, but need a strategy for loft insulation in general which would hopefully not reduce the available space too much, and allow for simple boarding.

For half of the main house I'll probably just go 270mm wool in the joists, but elsewhere I'm wondering if I can effectively split the insulation load between the joists and the rafters.

Naturally between this standard 270mm layer and what probable nonsense follows, I'd need an insulated partition wall too... in theory... right..?

Main loft... 100mm wool in existing joists, 100mm wool in new cross joists, 100mm in rafters? battens holding the wool in place, then plasterboard on top?

New loft... 170mm wool in joists (slightly compressed in 5in joists and boarded over?) and same in rafters?

When it comes to boarding a loft, celotex / kingspan insulation boards seem to be talked about as the only option instead of wool, but elsewhere I still see boarding projects covering wool..? In principle I'm happy to use celotex at the outset, but don't want to waste money when I have no plans for *formal* living space in the loft.

I've generally been hoping to find an article that says "for wool it's 270mm, which is equivalent to 100mm celotex" but I can't find it. I've come to think that this is really because it's a "whole house" thing, so becomes slightly meaningless to quote these things for an existing house, so it doesn't exist? Do I actually have ANY BR obligations WRT insulting the new extension given it'll be openly connected to the existing chilly, poorly insulated space?

Outside of wool / celotex, I also see references to membranes and reflective foil insulation. might these be key as well?
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