Internal insulated plasterboard

Did investigate this for a friend and ended up going with stud work, insulation and vapor barrier. Reason being that the insulated plasterboards mean a break in the vapor barrier at each board joint, this means you may still get moisture travelling through the plaster and into your insulation.

I'd recommend using stud work with separate insulation and vapor barrier.

Could be worth a watch -

The Best Internal Wall Insulation for Old Properties?

If you're in England note that building regs require a minimum U-value for interior insulation, best check this if you're doing it by the book 🙂

My own experience won't help much as I did wood fibre and lime plaster on a breathable wall... backed plasterboard sounds much easier!

Reason being that the insulated plasterboards mean a break in the vapor barrier at each board joint, this means you may still get moisture travelling through the plaster and into your insulation.

Aren't they offset so you don't get this problem? Seems a bit of an oversight. You could always cut them a bit yourself if this is an issue, so the joins of insulation and board don't line up.

insulated plasterboards mean a break in the vapor barrier at each board joint, this means you may still get moisture travelling through the plaster and into your insulation.

^^^Exactly this^^^

In my own 1850 stone built money pit I'm currently going around room by room ripping out the existing plasterboard, putting up new celotex, taping it, sealing socket box openings then adding plasterboard on top. The risk of getting moisture transmission across the board edges on insulated-backed boards was just too great. You essentially want to build an air-tight box and the insulated boards didn't appear to offer a satisfactory solution to that.

@ossify you need to tape the joins of the insulation to provide a continuous barrier layer. Offsetting the board and cleotex layer just makes it difficult to transmit vapour. you need to make it impossible.

After reading wildly mixed comments on the best way to fix it I just screwed it straight to the walls with masonry screws. Filler over the screws and taped the joins, then wallpaper. It's been up quite a few years now. No issues at all. The room is much warmer, well worth doing. The wall used to feel like a reverse radiator in winter, now it is warm to the touch. I only did one wall of a corner room. The other external wall has a bay window so less wall and a pain to fit.

I'm pretty sure you're meant to use the relevant sealant on the edges of the insulated boards.

Usually you'd use panel adhesive foam, but that doesn't necessarily count as a vapour barrier.

Celotex & Kingspan thermally insulated plasterboard recommend fixing directly to solid walls, via adhesive, or mechanical fixing. The board edges will be sealed with adhesive and taped, then skim plastered.  Can’t see any issues with moisture build up if installed correctly. This is why you also need a two pronged approach, the second being the heat recovery unit. The HRU will ensure a constant flow of warm dry air to all rooms. My walls are solid blockwork with external pebble dash, so not breathable currently.

PIR foam insulation is really not recommended on external solid walls. I can post some images of dry rot behind that kind of install if you want. Also dry rot in 18 months behind a swip system as promoted by that online diy bloke above. May be ok, but also may not and you won’t know about it until it’s too late.

Studded out and added Kingspan to an ancient pub with exterior brick walls and it made an astonishing difference. Would definitely do that over insulated plasterboard if I had the choice and was prepared to lose the interior space.

@ajc was that the supposedly breathable system with breathable studs?

@hot_fiat yes. The swip system is vapour open but cannot manage liquid water from interstitial condensation. It is pretty much impossible to ensure that no moisture reaches cold surfaces below dew point behind any insulation system, hence the importance of a truly Vapor open system that can also transit liquid water. Foam insulation is always going to be high risk.

This is something I'm looking to do at some point but will look into screwing studs to the existing walls putting kingspan or the likes in and then fitting plasterboard. Only problems I think will be the bedrooms are partly into the roof so each one has a sloping wall which might be a bit tricky.

“ Celotex & Kingspan thermally insulated plasterboard recommend fixing directly to solid walls, via adhesive, or mechanical fixing.”

they recommend both as you need the mechanical fixings for fire regs and the boards not falling off the walls and blocking an exit in a fire, if you dot/dab you need 6 (IIRC it may be 9?) but with foam adhesive it’s 3 per board.

Foam is quicker and easier plus you are creating barriers if you do a grid pattern, dot and dab is one big void along the entire wall unless you are meticulously applying it.

I did our flat in Kingspan K118 65mm (77inc the PB) with ceracit foam adhesive and InSoFast fixings. Walls were mostly cavity with one solid wall but that was tile faced on the exterior so I wasn’t worried about moisture transmission or breathability and we are six floors up so not worried about rising damp getting in.
Guidelines say to not under insulate but they also take the view of doing what is practical. 60mm was the sweet spot for making it worth the effort and cost, much beyond that and the gains started to drop off, has made a huge difference to our flat along with the secondary glazing, it’s a modernist block with huge crittal windows and we can’t change the exterior look.

Take pictures of the install and keep receipts as you will need these for a new EPC!