Polystyrene sheet insulation for timber frame walls?

[bobcube]

I'm insulating the walls in my period house in Suffolk. It's not listed!
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

 

[Deleted member 174758]

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?

There is indeed a fire-rated version, called Fire Foam, but I doubt that any spray foam will offer the same insulation as Cellotex or Kingspan, etc as it seems less dense and is rather inconsistent in density. It's also going to be very costly to use in comparison. Don't know about and legal aspects - I work in the trade and we don't do what might be regarded as "bodge jobs" like the one you are proposing. Have you considered talking to Building Control at the council for an opinion?