Insulation advice

[pajw]

I'm after some advice about insulating a timber frame.

The timber frame is on the first floor and is three sides of a bathroom.

Timber frame is made from 140x50mm timber and was built in the 1980's. The outside of the frame there is cement render over a layer of ply and there is a waterproof membrane between the ply and frame. Everything is watertight and bone dry.

I recently removed the plasterboard inside the frame as some of it was damaged removing the ceramic tiles and i also needed to reroute pipework and electrics.

Previously there was about 100mm of rockwool inside the voids in the wall and i'm going to put 100mm of Kingspan insulation board in the voids while I have the plasterboard off.

My question is should i push the PIR board right into the void up to the  membrane/ply or leave an air gap. The gap wouldn't be ventilated.

My plan was to push the PIR right in and seal the edges with expanding foam, that would leave a 40mm void for pipes etc. I would then fit a vapour barrier and moisture resistant plasterboard on the inside of the frame. Does this sound OK. I know there will be thermal bridging through the timber frame but I'm hoping that will be negligible and am ignoring it.

Thanks

[pajw]

I hoped I'd get some feedback or advice about insulating my timberframe but it hasn't  happened. I'll try elsewhere.

 

 

[Mr Punter]

Sorry your post was missed.  You will be fine fully filling with the Kingspan.  No gap needed.  If you have room you could also use insulated plasterboard internally and this will help lower the bridging through the studs.

[pajw]

Thanks Mr Punter.

I already have the plasterboard which sadly isn't insulated.

Is there an insulating tape type product that I can attach to the studs before the plasterboard to help lower the bridging

[Big Jimbo]

Could you use some of that multi foil, which i believe is available in various thicknesses, over the studs, with taped joints, before you plasterboard. I bloody hate a cold bathroom.

[ADLIan]

But a multifoil will be compressed to next to nothing (a few mm?) over the stud giving little, if any, thermal resistance.

[Nickfromwales]

1 hour ago, pajw said:

Thanks Mr Punter.

I already have the plasterboard which sadly isn't insulated.

Is there an insulating tape type product that I can attach to the studs before the plasterboard to help lower the bridging

Cut some strips of XPS to give some thermal break?

 

https://gb-home.co.uk/products/insulation-boards-xps-under-floor-heating-thermal-6mm-10mm-20mm-30mm?variant=39814976995405&country=GB&currency=GBP&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gad_source=1&gad_campaignid=21193699419&gbraid=0AAAAApWtJjx38HziVW3HhIEbeXyiDe0yZ&gclid=CjwKCAjw_fnFBhB0EiwAH_MfZngO4IA5zvonhpp8uS3AgAxJJrg1r7kJT4xwhIwhEav0kHkFrMzqVhoCR-IQAvD_BwE

[saveasteading]

9 hours ago, ADLIan said:

little, if any, thermal resistance.

Agreed. Might as well fit clingfilm. No benefit.

 For something that would form a thermal break but be solid to work with and durable, I suggest tile backer such as Marmox. 

 

[Mr Punter]

10 hours ago, pajw said:

Thanks Mr Punter.

I already have the plasterboard which sadly isn't insulated.

Is there an insulating tape type product that I can attach to the studs before the plasterboard to help lower the bridging

 

You could just use 25mm Celotex type insulation over the studs before you fit the plasterboard with 50mm screws.

[Iceverge]

Woah woah woah........ 

 

 

On 04/09/2025 at 10:18, pajw said:

The outside of the frame there is cement render over a layer of ply and there is a waterproof membrane between the ply and frame.

 

This sounds like the wall has an impermeable layer outboard of the timer frame. 

 

Any sketches or pictures of the build up please to help us understand? I very much doubt your membrane is a fancy low permeability one. 

 

In any case I very much suspect that the wall was drying to the inside rather than the outside. Putting PIR between the studs would trap the sheathing between two layer of low permeability which really is a no no. 

 

Pics please...

 

 

[pajw]

I'll add some pictures at the weekend. I must admit, after doing some more reading about insulation,  i was beginning  to suspect that using PIR might not be the best idea. I think I need something abit more breathable.