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.

[pajw]

[pajw]

Here are a couple of pictures to hopefully help explain what I am trying to do.

Bathroom is above porch and has three external walls.

Timber frame is made from 140x47mm timber, currently has 100mm of rockwool in the wall, Would like to improve insulation while I have the plasterboard off but not sure what would be best.

External surface seems to render on ply with a waterproof membrane between the ply and timber frame.

House was built in 1987  so not sure what the membrane (sorry if that is wrong word) is. It isn't roofing felt. Forgot to take a photo of it.

[pajw]

I've only lived there since last year so have only experienced one winter and the bathroom didn't feel particularly cold.

Had to take the plasterboard off as it was damaged removing the tiles.

Just wondering if i should improve the insulation while i have the opportunity.

OPtions I can think of

1) Do nothing, leave 100mm rockwool

2) Add 50mm rockwool to fill up gap without compressing rockwool.

3) Add 100mm rockwool  (compressed into 140mm gap with original 100mm rockwool)

4) Remove rockwool and install 100mm PIR insulation board (leave 40mm gap)

5) install 100mm PIR and fill 40mm gap with rockwool

 

I'm sure  there are other options. Any advice about what to do would be gratefully received. Don't want to do something and end up with condensation and a rotten frame. Frame has no sign of damp after 40 yrs so don't want to cock it up. As Iceverge said in an earlier post the outer wall is probably impermeable so any moisture would dry to the inside.

[Iceverge]

Firstly I'm not seeing any ventilation in the bathroom? 

 

I would make sure this is completed as part of the remodel. Get a Greenwood CV2GIP ( keep an eye on eBay for a cheap one). Get an appropriate hole in the wall/ceiling venting to the outside.

 

In any case I would maybe remove one bay of insulation to inspect the plywood sheathing behind it. If it's in good condition I would replace the glasswool. I would buy another pack of it and fill any voids in the current insulation. Don't use PIR or something impermeable. You'll only make trouble.

 

Then buy a variable SD airtightness membrane. Something like Gerband Vario, intello pro clima or Siga Marjrex. Diligently cover the walls with it, taping all joints and penetrations. Do the same on the ceiling and tape it to the floor also. The aim is to 100% ensure that any drafts cannot blow through the wall Via gaps and cracks in the structure. Tape the membrane to the doors and windows with airtighess tape.

 

Next run 50x50mm battens horizontally over the studs at 400mm centres to create an insulated service cavity. Add extra blocking for any heavy fixings needed in the bathroom. Insulate with 50mm mineral wool. Add some marmox board of jackoboard to the window reveals and head. 

 

Plasterboard and tile/skim as needed.

 

Wall u value will be about 0.25 but it's the Airtightness and lack of drafts will make it quite comfortable. ( Ensure the floor+ceiling is well airtight sealed and insulated underneath too). 

 

 

 

 

[saveasteading]

1 hour ago, pajw said:

Do nothing, leave 100mm rockwool

what you show was state of the art, but about 30 years ago. It's thin, irregular, and gappy. And there are old bridges from the room through the timbers to outside.

 

Its a matter of balancing cost and benefit. Youll find much more previous dicsussion on here.

 

I think, 1. keep what is there but straighten it out, and add an extra thickness... how much would it take? 20mm? PIR is best for insulation.

Or replace what is there with PIR. BUT it is tricky to fit tight so maybe you'd end up pushing in the fibreglass again, or a boarded version.

Then an insulating inner layer over the surface  would kill the cold bridging.