James Whitaker
What Insulated Plasterboard Actually Is
Insulated plasterboard — often called thermal laminate or backed board — is a factory-bonded sandwich of insulation and plasterboard. The insulation is usually PIR (polyisocyanurate), occasionally EPS or phenolic foam, and it is almost always foil-faced on the wall side. That foil facing does two jobs: it reflects heat back into the room and acts as a rudimentary vapour control layer (VCL) if you seal it properly.
It is a straightforward way to improve the thermal comfort of a cold internal wall, take the chill off a room, and reduce the risk of surface condensation on the wall face. It is not a magic cure for damp, and it is not always the right answer in older properties. Where it suits best is a solid brick or stone wall, a cavity wall that was never filled, or a single-skin garage conversion where you can afford to lose 60–100mm of floor space.
Thickness, U-values and the Space You Give Up
The number that matters is the U-value — how many watts escape per square metre per degree of temperature difference. Approved Document L suggests a target of around 0.30 W/m²K when you are upgrading a thermal element in an existing home, though always confirm the figure with your building control officer.
- 25mm PIR plus 9.5mm plasterboard — roughly 0.60 W/m²K on a solid brick wall. Modest gains, minimal lost space.
- 50mm PIR plus 12.5mm board — around 0.40 W/m²K. A sensible compromise for bedrooms and hallways.
- 75mm PIR plus 12.5mm board — close to 0.30 W/m²K. Worth it where the wall is genuinely cold or north-facing.
PIR performs at roughly 0.022 W/mK, which is why relatively thin boards work at all. But remember that adding insulation internally only helps if the whole wall is treated consistently. Insulating one elevation and leaving a neighbouring wall bare simply moves the cold spot to the junction, where condensation will appear instead.
Vapour Control and the Condensation Question
This is where most internal wall insulation goes wrong in older homes. Warm, moist indoor air will always try to migrate towards the cold outer wall. If you fit a foil-faced board with gaps behind it, that air can reach the cold masonry, condense inside the wall, and sit there quietly rotting timbers and staining plaster.
Practical steps that actually matter:
- Fix boards with continuous adhesive — a full perimeter bead plus solid dabs, not a few skimpy spots that leave large voids.
- Tape and seal every joint, corner and service penetration in the foil facing to make the VCL as continuous as you reasonably can.
- Do not fit insulation below a damp-proof course level or against a wall with active penetrating or rising damp. Solve the water first.
- In solid-walled, traditionally breathable buildings, consider whether a vapour-open insulation system would suit the wall better. Sealing a lime-plastered rubble wall with foil can trap moisture that used to escape freely.
- Keep ventilation in mind. Extract fans in kitchens and bathrooms, and trickle vents or background ventilation, do more for condensation than any board.
Fixing Methods: Adhesive, Battens or Mechanical
Insulated boards are heavy. A 2400mm × 1200mm × 62.5mm board can tip past 25kg, so two people and a decent support are essential.
Dot and dab is the quickest route on reasonably flat masonry. Use a plasterboard adhesive designed for insulated boards, applied as a continuous ribbon around the edges with dabs at 300mm centres in the field. The ribbon closes the air path at the perimeter, which matters for both thermal performance and vapour control.
Timber battens screwed to the wall give you a service void for cables and pipework and a flat surface to fix to, but the void must be sealed at the top and bottom. If air can circulate behind the insulation, you lose much of the benefit and increase condensation risk.
Mechanical fixings — screws with insulation washers into plugs or battens — are often specified where adhesion alone is not trusted, such as on uneven stone walls or in high-rise work. Follow the board manufacturer's fixing pattern rather than improvising.
Details That Decide Whether It Works
The junctions and edges are where performance quietly disappears. Return the insulation around window and door reveals, or you create a cold band exactly where curtains and furniture sit. Bring it down to the floor and up to the ceiling, and be careful at joist ends and party wall junctions.
Recessed socket boxes are a common weak point: they cut through the insulation and the VCL. Where possible, use surface-mounted accessories, or seal around back boxes with airtight tape and a bead of sealant. In kitchens and bathrooms, fit moisture-resistant plasterboard, and remember that foil-faced boards still need skimming or taping and filling for a proper finish.
Finally, choose a board with enough thickness to justify the disruption. If you are already emptying a room to redecorate, going from 25mm to 62.5mm costs little extra in labour and pays back in comfort for years.
