expanded polystyrene for wall insulation?

bobcube

I'm attempting to insulate the walls in my period house in Suffolk.
The walls are oak frame. Cladding is lath & plaster on both internal and external wall faces. External has also had wire mesh and some sort of render.
So far I've done 2 rooms.

On the first I stripped off the internal laths, installed Rockwool flexi between the frame gaps, then plasterboarded up and skimmed. That worked well!

On the second room, I replaced the lath & plaster with Celotex board. That also worked well!

The third room faces the wind! The frame is a bit chaotic, and I'm not comfortable whether Rockwool will stay in place, given the wind. I was wondering why I shouldn't use expanded polystyrene in the frame gaps, sealing it in with some sort of expanding gap filler.
I gather that polystyrene is a bit dodgy if it catches fire, but
a. is it actually illegal to use polystyrene as wall insulation?
b. is there a fireproof version?

I'd appreciate any views

Doozergirl

None of that is much good for an old timber frame building, I shouldn't think! :eek:

Furts

I second Doozergirl. This sounds like a recipe for ongoing problems.

That said, bear in mind that polystyrene is grossly inferior to Celotex/Kingspan/Xtratherm for insulation. It is also less rigid, and the edges inferior when cut. Hence for walls it would not be my choice.

Fire safety would concern me. I accept that there will be a plasterboard covering, but it would be prudent perhaps to double board, or Fireline. Regardless, the edge sealing should be meticulous - perhaps a substantial depth of fire rated expanding foam.

Davesnave

If it's period, your house wouldn't be Grade 2 listed, by any chance?

bobcube

No the house is not listed.
Insulating timber frame houses with either Celotex or Rockwool is both legitimate, legal and commonplace, provided a vapour barrier is included. Which in my case, it has been!
Both are widely used to insulate timber frame period houses.
My question is around why polystyrene sheet seems not to be generally used.

new_owner

bobcube wrote: »

No the house is not listed.
Insulating timber frame houses with either Celotex or Rockwool is both legitimate, legal and commonplace, provided a vapour barrier is included. Which in my case, it has been!
Both are widely used to insulate timber frame period houses.
My question is around why polystyrene sheet seems not to be generally used.

Why not use Celotex board? Its insulating properties are around twice that of rockwool. Not sure why you would want to use polystyrene sheet.

Reframe or fix directly and using expanding (high/low) foam would be fine. There are other materials if you were worried about breathability.

andyhop

Polystyrene used to be common , there is no difference in breathability between this or Kingspan/celotex/Quinn

Vapour barriers fitted, taped and sealed and should not have issues with sweating

You should be able to get a tight fit on the boards or foam fill/tape