What is insulated plasterboard, and is it worth it?

Thermal laminate board: what it is, where it genuinely helps, the two cold bridges that undo it, and the knock-on work nobody warns you about.

Difficulty: intermediate
Time: 12 min
Read: 5 min
Safety: caution

Short answer

It is plasterboard with rigid insulation bonded to the back, sometimes with a vapour control layer between, sold as a single sheet. It is the standard way to insulate a solid wall internally without building a stud frame. It works, it costs room depth, it creates cold bridges at reveals and internal wall junctions, and it means every socket, radiator and fixing on that wall has to be moved out.

Thermal laminate board is a genuinely effective product with three practical consequences people underestimate: how much room it takes, how much second-fix work it creates, and how carefully the junctions have to be detailed. The last one is not cosmetic — the mould that appears round a window reveal after internal wall insulation is a direct result of getting it wrong.

Safety

Insulating a solid wall from the inside changes how the whole wall handles moisture and can cause condensation and decay within the structure if it is done without thought, particularly on older solid-walled buildings that need to breathe. Get advice specific to your wall before committing, and never fit a vapour-closed board to a wall that is already damp — find and fix the damp first.

Insulated plasterboard on a solid wall, and where it goes wrong

Step by step

Understand what the board is.A sheet of plasterboard with rigid insulation — commonly PIR, phenolic or expanded polystyrene — bonded to the back, in a range of overall thicknesses. Many versions include a vapour control layer between the two, which belongs on the warm room side of the insulation.
Establish whether internal insulation is right for this wall first.It is the usual answer for a solid wall where external insulation is impossible or unacceptable. On an older solid-walled building, the wall's ability to dry inwards matters and a vapour-closed board can trap moisture. Get advice on the specific building before choosing a system.
Deal with any existing damp before you start.Boarding over a damp wall hides the damp and makes it worse, because the wall can no longer dry into the room. Penetrating damp, a failed gutter, a bridged damp course — find the cause and fix it, then insulate.
Choose the thickness against the room you can afford to lose.Thicker board improves the wall more and takes more floor area. The whole thickness comes off the room on every wall you treat, so a small room can lose a surprising amount. Work out what the finished room measures before ordering.
Plan the reveals before anything else.There is rarely room for full-thickness board in a window or door reveal, so a thinner insulated board is run into the reveal to carry the insulation round. Without it, heat short-circuits round the edge of the insulation and that reveal becomes the coldest surface in the room, where condensation and mould appear first.
Plan the junctions with internal walls and floors.Where an internal wall or a floor meets the insulated external wall, the structure interrupts the insulation and the same cold bridge forms. The usual detail is to return the insulation a short distance along the internal wall to soften the junction.
Work out the second-fix implications before the boards go on.Every socket and switch on that wall has to be brought forward, which means extension boxes and, if any of it is more than moving an accessory forward, an electrician. Radiators come off and go back on longer brackets. Skirting, architrave, curtain poles and any wall fixings all now need to reach past the board.
Use fixings long enough to reach the masonry.Anything hung on the wall afterwards — a TV bracket, a shelf, a radiator, a kitchen unit — must have fixings that pass through the whole board and insulation and into the wall behind, or it is hanging on plasterboard and insulation foam. Long-reach fixings for insulated board are made for this.
Seal the perimeter and every penetration.Air moving behind the insulation destroys its performance and carries moisture to the cold masonry. Continuous adhesive or sealant at the perimeter, at floor and ceiling junctions, and round every socket and pipe penetration.

Tips

The alternative approach is a stud frame with insulation between and separate plasterboard over. It takes slightly more depth and more time, gives you a service void for cables, and lets you choose a breathable insulation for an older building.
Insulated board is heavy and unwieldy for its size and much less forgiving than ordinary plasterboard. It is a two-person job.
Insulating one wall of a room moves the coldest surface somewhere else. If you insulate the external wall and the reveals stay cold, the condensation moves to the reveals rather than going away.
Where the work is substantial, internal wall insulation is normally building-regulations work. Check with building control before starting rather than after.

Common mistakes

Not insulating the window reveals — It is the most common failure of internal wall insulation and it produces black mould round the windows within a season. The reveal becomes the coldest surface in a room that now has the same amount of moisture in it and fewer cold surfaces to put it on.
Boarding over damp masonry — The wall loses its ability to dry inwards, the moisture concentrates behind the board, and you have hidden a problem that is now decaying the wall and any timber in it. Fix the damp, let the wall dry, then insulate.
Fixing a TV or a radiator into the board afterwards — There is a plasterboard face and 50 or more millimetres of foam behind it. Nothing fixed into that will hold a real load. Fixings must reach the masonry, and heavy items are best planned before boarding so a timber pattress can be built in.
Forgetting the sockets until the boards are up — The accessories are now recessed 50mm behind the finished surface. Sorting it out afterwards means cutting the new boards about. Plan the electrical work first.

Questions people ask

How much difference does it actually make?

On an uninsulated solid wall, a substantial one — a solid brick wall is one of the worst-performing elements in a British house, and internal insulation transforms both the heat loss and, more noticeably to the occupant, the surface temperature of the wall. On a cavity wall that is already insulated, the gain is much smaller and rarely worth the disruption.

Can I fit it myself?

The boarding itself is within reach of a competent DIYer, and it is normally dot-and-dabbed or fixed to battens. The parts that need care are the moisture assessment beforehand, the reveal and junction detailing, and the electrical work — and it is often notifiable to building control, which is worth establishing at the outset.

What about the room feeling smaller?

It genuinely will, and on a small room the loss is noticeable. Measure the finished dimensions before committing, and consider insulating only the external walls rather than all four, since the internal ones are not losing heat anyway.