Experience of / Advice about : Foam insulation backed plasterboard

[Sideways]

I need to update and redecorate a small bedroom in a late 70's house.
It's on the ground floor and one of the colder corners of the house.
I won't get into the debate here - just take it as a given - the house has a narrow cavity and I don't intend to install cavity insulation.

As the room's getting a full makeover, I think that lining the two exterior walls with foam backed plasterboard might warm it up a lot.

I'd love to learn from anyone who has experience of this.

• What thickness foam to go for ?
• Recommended brands ?
• Best way to install ? In a small room so don't want to lose more space than necessary ?
• Sequence of installation ? Thinking: take the ceiling out first, put underfloor heating down next, walls right up to the joists third and replace the ceiling 4th ? The ceilings are all done in artex so this is an opportunity to just replace it. The plain plasterboard ceiling will sit inside the new wall boards. Or is it better to replace the ceiling first and board up the walls to it ?

Thanks

 

[Doug B]

I used phenolic insulation back plasterboards on all external walls & the upstairs ceilings in our house 20 years ago when I refurbished the whole place, it was all dot & dabbed onto the walls.
Can’t remember the brand but think it was about 1&1/2” thick, worked a treat with only a couple of very small hairline cracks in all the time it’s been done which as we have practically no footings is remarkable really.

 

[baldkev]

I'd probably go for the 37.5mm too. Theres a diminishing return when you go up in thickness, so from 2" to 4" does not double the thermal efficiency..... in fact it's not a big gap at all..... theres instafix board foam which is great, or as suggested above, dot n dab..... but one thing to be wary of is the board tension / flatness. I recently fitted some which was bowed by maybe 10mm in the centre. Fine on the rafters as it gets screwed flat, but not so great with dot n dab

 

[deema]

I’m not sure which room @Sideways is thinking of doing, but part of his house is also rendered. Anyone know the pros and cons of adding insulation externally under render as an alternative? Just a thought.

 

[Woody Alan]

I have done this to my downstairs rooms and am now currently doing upstairs. Firstly yes the difference is very noticeable so worthwhile. I am using 40mm on the main wall and 25mm on the window wall just to keep the visual impact down. Talkng of visual impact I also am using instastik and mechanical fixing to pull the board to the wall whilst curing as the walls are flat and upright. I am not a fan of the dot and dab because of the micro climate that gets set up in the void, you have to be careful to seal the perimeter of the board or you could well get draughts circulating.
Downstairs has been done for several years some dot and dab some instastik, neither have fallen off or cracked.

 

[Peter Sefton]

We used insulated plasterboard within our barn conversion on the solid 9" walls, we used 52.5mm but I wished I had used thicker. When we had the cold spell over the winter and we were trying to keep the heating down it's this area that really lost it heat.

We used a spray foam and additional mechanical fixings as requested by the building regs inspector.

https://www.sealantsandtoolsdirect....fchcHxJzpfCZXqdrHp_MAw_UcCqE1JLBoCGo0QAvD_BwE
Cheers

Peter

 

[Sideways]

I’m not sure which room @Sideways is thinking of doing, but part of his house is also rendered. Anyone know the pros and cons of adding insulation externally under render as an alternative? Just a thought.

Yes, I have a ground floor high south facing wall that is rendered. I like the idea of updating this with the new K rend. Done well, that looks a very tidy system. There's a good opportunity to fit exterior insulation behind that, but I'm not sure that it will provide much benefit. I think that there may be a significant airflow through the cavity based on draughts I could feel while resealing around the window frames. If that's the case, it defeats any benefit from insulating on the outside of that airflow.

The small bedroom of this post is on the opposite side. The NE corner of the house.

 

[Sideways]

What is the recommended prep for putting up the board ?
This room is currently wallpapered.
From experience, when it's stripped the wallpaper tends to bring the surface skim coat of the plaster with it in patches. I would tap and flake off any plaster that sounded loose or hollow, but the bonding plaster onto the block wall is sound and I ASSUME the 2-3mm of unevenness left behind doesn't need to be made good before attaching the new boards with a foam adhesive.
It's not a big deal to run a sander over any painted plaster left behind to make a better key.

 

[baldkev]

I’m not sure which room @Sideways is thinking of doing, but part of his house is also rendered. Anyone know the pros and cons of adding insulation externally under render as an alternative? Just a thought.

Exterior house wrap is ok, but can be pricey. Interestingly building control allowed us to change from house wrap to cavity fill and the b.c.o said that recent info shows there isnt a huge gap between cavity fill and exterior wrap in therms of thermal gain. ( thats what he said, i dont have data to quantify that statement )

What is the recommended prep for putting up the board ?
This room is currently wallpapered.
From experience, when it's stripped the wallpaper tends to bring the surface skim coat of the plaster with it in patches. I would tap and flake off any plaster that sounded loose or hollow, but the bonding plaster onto the block wall is sound and I ASSUME the 2-3mm of unevenness left behind doesn't need to be made good before attaching the new boards with a foam adhesive.
It's not a big deal to run a sander over any painted plaster left behind to make a better key.

Remove the paper and sugar soap any bare plaster, make sure theres no loose paint etc... then, if you wanted to go belt and braces, paint it with febond bluegrit before you dot n dab. ( most people dont do that )
No, any uneveness wont matter at all

 

[Woody Alan]

I scraped the emulsion back to bare plaster with 4 inch scraper tedious but neccesary as adhesives will break that bond probably. I didn't have loose or flakey plaster or even dusty. I would use a bonding if it was, as kev suggested or an acrylic bond for instafoam. I used many mechanical plasterboard fixings as well almost as if going into studs. I have cavity walls but when the house was built they put inch polysyrene batts on the inner leaf without taping joints. With the microclimate in the cavity whistling around they might as well not be there but they have prevented me installing further cavity insulation.

 

[XTiffy]

On YouTube CharlieDIY covered this very well in his own(Old) house and is well worth a look. He is worth watching as he does't waffle but is honest in his approach and the results.

 

[Sideways]

On YouTube CharlieDIY covered this very well in his own(Old) house and is well worth a look. He is worth watching as he does't waffle but is honest in his approach and the results.

Thanks @XTiffy . I found Charlie's video. You're right, it is well done and contains a lot of useful info.

Thanks everyone for your replies to this one. It has convinced me that the job is well worth doing and perfectly doable as a DIY project. It will also be more energy efficient than cavity insulation.