How to reinsulate my attic?

Tarcoman77

Hi newbie here,

Wondering if anyone with experience could help me. We have a Victorian house which had an attic bedroom originally insulated with straw, lathe and plaster. Due to damage, I stripped this all away to bare rafters/trusses (what a job!). Is it suitable to fit polystyrene insulation between rafters, then simply plasterboard beneath? I'm finding it hard to find good advice other than fit kingspan, I only have 75mm from under tile to fixing side of timbers.
Thanks in advance - Tarcoman

stator

The best insulation is PIR board which means kingspan/celotex or similar.
Polystyrene will fit but be less insulating.

Building regs would probably require PIR board but if it's DIY you can do what you like, as long as no-one finds out.

Tarcoman77

Thanks Stator. In either case would I need a membrane between insulation and plasterboard? I had the usual blokes in the pub frightening me about fireproofing. But as I said, it will be boarded over and the roof is made of highly flammable wood anyway

Niv

I have been putting insulation in the kitchen ceiling recently (single storey extension so above ceiling = roof space. This is only a 10cm gap. I originally thought about 5ch polystyrene but changed my mind due to a number of conflicting sources during internet searches. I decided to use 3cm PIR, this has a better insulation rating than the 5cm poly in any case.


I am not putting in a specific fire proofing layer, however the plasterboard, plastered over, is essentially doing this job.


I would think of it a bit this way - how fireproof was your straw? How concerned were you when that was the insulation?

stator

Does your roof have any sort of membrane underneath the tiles already?

Tarcoman77

Hi folks thanks again, I think I've decided on polystyrene and a foil backed plasterboard. No membrane under the tiles, and yes 117 year old straw might just set alight haha

Wookey

You cannot have polystyrene anywhere near electric cabling, you will need to use something else anywhere that you have cabling routed for ceiling lights or sockets. I'm sure a sparky will be along shortly to advise better but you should read the following.

https://www.esfrs.org/black-museum/all-the-black-museum-cases/pvc-cable-insulation-and-polystyrene-insulation/

Tarcoman77

Hi Wookey, thanks for the reply, I do not have cabling anywhere near to where the polstyrene insulation is going (under pitched roof rafters & plasterboard finish).
in fact thats my next problem, getting a spark in who wont charge me my weeks wages for 1/2 a days work lol

Furts

Tarcoman77 wrote: »

Hi folks thanks again, I think I've decided on polystyrene and a foil backed plasterboard. No membrane under the tiles, and yes 117 year old straw might just set alight haha

Bear in mind foil backed is not a substitute for a membrane. At all board joints there will be no membrane, so polythene is a wise move.