Celotex without battens

[^woody^]

introducing timber to a solid wall sound like a time bomb.

The OP can replace it when he replaces the floor joists and roof.

I suspect the OP will use tanalised tile battens.

 

[JP_]

Sure I read somebody here put the battens on the inside to hold insulation boards in place, with screws going through to secure them, then plasterboard screwed onto battens.
Get a cavity behind the plasterboard for cables and pipes too.
But you lost an inch or so

 

[motorbiking]

Current wall is u2.5, with the 50mm I’m expecting it to increase to 0.5-0.3. It’s rendered on the outside. I’m not expecting much warm air to be able to contact the wall, so don’t think there is a massive risk of condensation. It might be different if I was putting 100mm plus and trying to hit 0.2u. The room is well ventilated as there is a log burner.

 

[catlad]

Log burner is a bonus I think its ok to use batons on a cavity wall but I don't think its a good idea generally on solid walls unless you are confident they are dry! anywhere there is a risk of moisture or water penetration I always use foil back plasterboard especially rooves.

 

[motorbiking]

No problem with damp. The outside is rendered. I’ll be using roof battens if I go down that route.

 

[padstar]

For what its worth I bonded 50mm celotex to my walls using drywall foam, tapped all joints then bonded PB to the insulation and skimmed. Its done the job, was easy to do and no apparent dramas to date.

Buying the insulation and board separately was about half the price of the insulated PB. The saving outweighed the extra bit of labour to install 2 layers instead of 1.

 

[motorbiking]

I think I'm leaning towards that and maybe putting a few 8mm frame fixers in each "unit".

 

[padstar]

With regards to mechanical fixings and what I have experienced, the foam adhesive is more than enough. The boards are not going anywhere. I would consider them if the board was maybe 1.5 floor heights like in a stairwell but I cant see there being any risk of the boards moving with just the adhesive.
I understand that mechanical fixings are advised to ensure no failure in the case of fire where the adhesive may fail under extreme temperatures. As far as I am concerned if my house is on fire the last thing i will be worried about is if a sheet of plasterboard comes away from the wall but I understand why the fire brigaded would think otherwise..

 

[^woody^]

As far as I am concerned if my house is on fire the last thing i will be worried about is if a sheet of plasterboard comes away from the wall

That's not the way to look at it.

What can happen, is that the boards become detached at a very early stage of a fire and allow the fire to spread (or hot gasses and smoke), and this means that rather than a fire being contained to one room or area giving warning and time to escape, it spreads rapidly or unexpectedly.

 

[padstar]

Fair point. In which case a few mechanical fixings would be a good idea and i will do so on the rest of the rooms I still have to do.