Insulating plaster

[sirocosm]

Does anyone have any experience with insulating plaster? I have a 1930s house that is solid brick upstairs and the walls are cold. I redoing the upstairs and I was thinking to knock off the old on the outside walls and redo it with around 25mm of insulating plaster. Any recommendations on products/methods? Anyone else done this? and how did it work out?

 

[motorbiking]

25mm wont make a huge difference. 40-50mm will be worth the bother. Basically you take the whole lot back to brick with a spade SDS attachment (very messy and be careful about getting lime plaster/horse hair on your skin or in your lungs). Then you plaster board foam 40-50mm of celotox, then repeat with plasterboard. You can just use insulated plasterboard, but its more expensive.

Make sure all the gaps are foam filled as any cold areas will just bridge and cause condensation.

 

[Wayners]

Fix radio, last weeks podcast covered cork plaster internal and external render. Go listen

 

[sirocosm]

Then you plaster board foam 40-50mm of celotox

It is a lime mortar solid brick wall, so I am loath to put anything on it that might trap moisture, which is why I was thinking to use a lime based insulating plaster. There are some such as Diasen Diathonite Thermactive .037 that claim to have a thermal conductivity of .037W/mK, almost the same as Rockwool (.033W/mK). It is very expensive, around £29 per square meter for 25mm thick.

 

[motorbiking]

It wont trap moisture. Moisture does not penetrate from outside in, unless you have a leaky gutter or something. Moisture occurs because the warm air touches the cold wall. If its sealed with foam it wont trap moisture. Celotex has a foil back and 40-50mm will bring your U value from around 2, to 0.3

 

[sirocosm]

Perhaps you are right. The insulating lime plaster solution is really expensive to get the thickness required. It is a 1935 house with cavity walls on the main floor, and an externally rendered solid brick walls upstairs. I heard that moisture does come through bricks and lime mortar from the outside, but maybe I could seal it with some kind of paint on the outside, since it is rendered anyway. I assume that foam sticks well to brick, do you cover the whole wall/sheet with it when you stick the Cellotex on?

 

[motorbiking]

 

[sirocosm]

How much wiggle room does that stuff give you? Having taken the plaster off in one room, the brickwork is quite uneven, unlike in the video where the walls look nice and smooth.

 

[motorbiking]

about 15-20mm

 

[sirocosm]

It does not seem to stick so well to the foil on the Celotex. Seems to stick to brick and drywall OK.

 

[sirocosm]

I ended up putting strapping on the wall, that I made from the leftover floorboards I took up. They were 22mm, so I put 25mm Celotex in between, and another layer of 25mm Celotex over top to make 50mm. The Soudal does not stick to the foil on the Celotex at all, but it sticks great to the brick and the insulation in the Celtotex if you peel the foil off. I wound up peeling all the foil except on the inner most surface just under the drywall. I taped the joints with foil tape to make a vapor barrier, and then installed the drywall by screwing to the strapping.

I also found that a small multitool with a scraper blade took off the plaster with much less pounding, and you can use it to cleanly shave down the high spots on the brick. Mind you this is a 30s house with old brick, so YMMV with newer brick and plaster. It is also a bit slower, but does a nicer job IMHO.

 

[motorbiking]

Are you sure you purchased Plasterboard adhesive foam? not standard foam?

Soudal Genius Gun Plasterboard Adhesive Foam

Soudal Plasterboard Adhesive Foam is a fast curing replacement for traditional mortar adhesives. Use it to fill cavities and for bonding plasterboard to walls. You'll find that this foam is suitable for use on all common materials such as concrete, stone, plaster and wood.The adhesive comes...

www.toolstation.com

 

[sirocosm]

I used the plasterboard adhesive foam, not the filler foam. I did some tests. For all these tests I left them overnight.

1) Stuck two pieces of Celotex together, foil on foil.
2) Removed the foil and stuck two pieces together, bare insulation board on bare insulation board.
3) Removed the foil and stuck a piece of Celotex to the wall, bare insulation board on bare brick.

1 was easy to pull apart, 2 & 3 were bonded so strong that when I tried to remove them it would break the insulation. That adhesive is great stuff, but it doesn't seem to stick to foil well. Come to think of it, the foil is not stuck so well to the Cellotex insulation board from the factory either, it is pretty easy to peel it off. In any case, my drywall is held up with screws, but the foam adhesive made the job a lot easier.

To be fair, I doubt anything will stick well to foil. I tried coving adhesive (because I had some) and it also comes right off.