Insulated Plasterboard

You need some sort of moisture management when installing internal wall insulation as the wall will now be be on the cold side and condensation will occur between the insulation board and the wall. This can be done with a vapour control layer installed between the insulation and the wall, to avoid warm vapour from the inside of the space to travel through the insulation and condensate on the now cold internal face of the wall, mould and all that good stuff will follow. There are also insulation boards that come with an integrated vapour barrier. Care is needed to ensure continuity of the vapour barrier in board joints and in window reveals, etc.

I see now this was posted a few months ago and wondering if you went ahead with the job?

New account. 5 posts and 3 of them here. Would have to assume a troll with account set up for entertainment purposes rather than trying to help op

You can, but you shouldnt!

Its near on impossible to get a continuous VCL layer if you are just fixing boards to a wall. The edge of each sheet will not be sealed to anything so warm wet air will pass through these joints, hit the cold external wall and condense. The condensation will be trapped between the wall and the foil on the back of the board, run down and collect on the floor behind skirting and under the floor, potentially causing hidden mold and even rot.

External | Insulation | VCL | Batten for services void | Plasterboard

Insulation against the wall with taped seams/joints.

Then a VCL layer over this, also with taped seams and correct overlapping as per the spec. The VCL should be continuous in the house, i.e. it should be over the ceilings and joining the DPC on under/on the floor for ground level.

Then you just board over that. you could also batten over the insulation, assuming you are using PIR to leave yourself a service void for running electrics etc so that you dont punch a bunch of holes through the VCL.

You can get airtightness membranes that also act as VCL, so you are reducing condensation risk whilst also reducing/eliminating draughts around sockets/switches/skirting etc.

The problem with the foil backed board is the joints, you cant do anything to close these joints meaning you have tens of linear meters of space for vapour to pass through to the cold external wall.

Trying to turn a leaky old house into a modern warm/air-tighthouse is fraught with danger. The old houses were "designed" to have many air changes per hour and heated with open fires or radiators running at > 70*C.

If you change this design to reduce the breathability and air changes you need to ensure that you are following all the guidelines. Otherwise you can do bits to try to improve the house but introduce serious condensation/mould/rot issues.

Effectively ending up with a submarine that is 98% watertight…which is actually not a submarine at all :)

I found this the hard way when renovating an old house that had been internally insulated with warmboards. The external walls were damp and mouldy around the edges and at the joints in the boards. There was also evidence of condensation at the skirting level. Now this was in a solid brick house with zero insulation anywhere, but the point is that the house was fine for the first 70 years of its life, then changing the makeup of the walls and upgrading windows introduced serious condensation issues.

Your MHRV is likely the thing that is avoiding any issues, hence why its important that the OP (and others) dont just add insulation or change windows etc.

I haven't seen an airtight/VLC tape for jointing plasterboard? Regular jointing tape/compound is to stop cracks at joints, its not going to stop vapour.

Also I don't see how you effectively tape the boards to the interior walls and especially the ceiling/floor, most boards wont reach the floor unless you lay horizontally :)

The other issue is that most insulated boards that I have seen have damaged areas, especially the foil, so you would ideally need to be taping all of these up before fixing…

All that said, if the makeup of the rest of the room/house is such that humidity and vapour are not an issue, then just warmboards are likely fine. But if the reason humidity isnt a problem is becuase the house is old and leaky, then as you address each leaky room/window you are increasing the chances of a problem, especiallly behind furniture/wardrobes in seldom used rooms.

+1

Without MHRV or basically drilling a hole in each room, that final 10% could be the bit that causes black mould behind your walls.