insulating sloping ceilings in attic flat

25 Jan 2007
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United Kingdom

i'm trying to figure how to insulate the bedroom and living rooms in my flat. the flat is on the top floor of the building in the roof space and a lot of the (sloping) ceilings of both rooms are made of uninsulated plasterboard which is attached to the roof rafters with no insulation in the gap between the plasterboard and the roofing felt.

the gap between the plasterboard and the roofing felt is just about accessible from around the edges of the floor of the (cramped) loft.

so far i've considered:

1. pouring loose fill polystyrene beads down into these gaps (this was advised to me by a chap british gas sent round to assess my insulation).


2. sliding insulation batts/boards (celotex, thermafleece (breathable apparently?), kingspan) between the rafters down the gap between plasterboard and roofing felt .

i've read on here that you need to leaving a 50mm ventilation gap between the insulation and the roofing felt which would seem to rule out the poly beads.

i've also read that sliding the insulation batts/boards down between the rafters might be tricky because the rafters are "unlikely to be parallel". The distance between the rafters varies (is that normal?) so i would have to cut the batts/boards to width accordingly.

The space between the plasterboard and the roofing felt is approx 100mm. So, if i need to leave a 50mm gap, the thickest insulation i could have would be 50mm.

I stripped, skimmed and decorated the whole flat before realising there is no insulation in the sloping parts of the ceilings so would really like to avoid taking out the plasterboards if at all possible.

i'd be grateful for any advice on what would be the best method of insulating these areas and what is the best (easiest/most effective) material to use.

all the best

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youve got all the info.loose beads is a no-no.sliding the celotex down is your best bet like you mention.could be a pain in the arse if the rafters narrow as youre sliding it in.also awkward to know if its where it should be when done.other than that,theres the taking the ceiling down,which you dont really want to do.installing celotex is a pain at the best of times and really gets in my throat,good luck.where a mask when cutting it.
Current reg's asks for 100mm celotex between the rafters (with 25-50mm air-gap above) and 40mm underdraw.

I know this information may be irrelevant but unless you do something like this correctly then you may cause problems elsewhere.

Do you know if the roofing membrane is breathable?
i work on the assumption that if theres nothing in there at the moment,adding anything is better than nothing,so building regs are irrelevant,as assumably this isnt "new works".

i could be wrong,i always am,lol
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In some instances, adding insulation may only be a token gesture. But as you have rightly said, a little is better than nowt.

This particular ceiling junction is often an overlooked part of a building and does warrant full insulation.
thanks for the responses so far. the chap from british gas said the roofing felt is the "old non-permable kind" so i guess that means it's non-breathable too.

as i've only got 100mm to play with and have to leave a 50mm gap. 50mm is all i'm going to be able to do. lower ceilings isn't a practicable option as the ceiling are already on the low side.
i'm not at home so i'm guessing but i'd say the one in the lounge is about 6 feet (on one side of the room only) and in the bedroom about 3-3.5 feet (on both sides of the room).

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