Internal wall insulation is really three different products wearing one name — and choosing the wrong one for your wall type is the commonest IWI mistake we see. Our own quote book shows how the choice actually falls: breathable and wood-fibre systems appear on 103 of our quotes, PIR on 26, and plain insulated plasterboard on 16 — a split driven by London's huge stock of solid-walled homes. This guide compares all three honestly: performance, breathability, robustness and cost.
Three boards, three different jobs
- PIR insulated plasterboard (Kingspan and Celotex type) — rigid polyisocyanurate foam bonded to plasterboard, usually foil-faced. The performance champion per centimetre; vapour-closed.
- Wood-fibre boards (STEICO type) — dense boards of compressed wood fibre, usually finished in lime or breathable plaster. Vapour-open, robust, the heritage-friendly choice.
- Plain insulated plasterboard — thinner laminates of plasterboard with a modest insulation backing (often EPS or thin PIR). The budget option for taking the chill off a dry wall where space or money is tight.
All three fix to the wall by adhesive dabs and mechanical fixings, or onto battens where the wall is uneven or a service void is wanted. What differs is how they perform, how they handle moisture, and what they cost — and those differences map almost perfectly onto wall types.
Performance per centimetre
Thermal conductivity (lambda, W/mK — lower is better) is where PIR earns its reputation: around 0.022 W/mK, against roughly 0.038–0.043 W/mK for wood fibre. In plain terms, wood fibre needs getting on for twice the thickness to match PIR's insulation value.
| PIR insulated plasterboard | Wood fibre | Insulated plasterboard | |
|---|---|---|---|
| Typical conductivity | ≈ 0.022 W/mK | ≈ 0.038–0.043 W/mK | varies with backing |
| Thickness for a given U-value | Slimmest | Thickest | Thin, but modest performance |
| Vapour behaviour | Closed (foil-faced VCL) | Open — breathable | Mostly closed |
| Robustness / fixings | Soft core — plan fixings | Dense, forgiving | Soft core — plan fixings |
| Typical finish | Skim | Lime / breathable plaster | Skim |
| Cost position | Mid | Highest (+15–25% on the system) | Lowest |
If room size is the constraint — a narrow Victorian hallway, a small bedroom — PIR's slimness is a real advantage. But performance per centimetre is only the right yardstick when the wall behind is dry.
Breathability and moisture — where wall type takes over
Foil-faced PIR is deliberately vapour-closed: the foil is the vapour control layer, and with taped joints it stops indoor moisture reaching the cold masonry behind. On a dry modern wall that is exactly what you want. On an older solid-brick wall that handles moisture by absorbing and evaporating it, sealing the inside face changes how the whole wall behaves — which is why surveys matter more than datasheets.
Wood fibre takes the opposite approach: the whole build-up stays vapour-open, moisture moves through and dries out, and the board's density adds useful summer heat buffering. It is the approach we detail in our breathable wall insulation guide, and alongside SWIP systems (on 41 of our quotes) it is why breathable build-ups dominate our IWI work. Whichever camp you are in, the sealing details — joints, perimeters, sockets — decide whether the system works; our airtightness and vapour membranes guide covers that invisible layer.
Robustness, fixings and real life
Here is the detail brochures skip: an insulated wall is no longer a solid wall. Behind PIR insulated plasterboard sits soft foam, so a standard wall plug bites into nothing. Radiators, kitchen cupboards, TV brackets and stair handrails all need either long stand-off fixings that reach the masonry, or timber pattern battens fitted before the boards go on. On our jobs the surveyor marks radiator and heavy-fixing positions at survey stage, and skirtings and sockets are itemised as their own lines — skirting work appears on 51 of our quotes and electrical adjustments on 37, because doing IWI properly means rebuilding those details, not caulking over them.
Wood fibre is markedly more forgiving: the board is dense enough to take everyday fixings with appropriate plugs, and it shrugs off knocks that would dent a foam-backed board. In a busy hallway or a family kitchen, that robustness is worth real money over a decade.
What each costs
Our published IWI figures: a median project of £5,550 across 108 priced quotes, advertised rates of £100–£140/m², a single room at £1,800–£4,000 and a full 3-bed at £8,000–£15,000. Within that, plain insulated plasterboard sits at the bottom of the range, PIR in the middle, and breathable wood-fibre build-ups carry a 15–25% premium on the system — the boards cost more and the lime finishes take longer. The full breakdown, including how IWI compares with other measures, is in our insulation cost guide and the 2026 London price report.
Matching board to wall
- Modern cavity-wall home (roughly 1930s onwards), dry walls: PIR insulated plasterboard, or plain insulated plasterboard where budget leads. The wall is dry and vapour-closed build-ups are safe; take the slimness and performance.
- Victorian or Edwardian solid brick: wood fibre or another breathable system, finished in lime. The wall needs its moisture path kept open — the full reasoning is in our Victorian terrace guide.
- Solid wall with a damp history: fix the moisture source first, then a breathable system. Never seal a damp wall behind foil-faced foam.
- Small rooms where every centimetre matters (and the wall is dry): PIR — thinnest build-up for the target U-value.
- Rental or budget refresh of a dry wall: insulated plasterboard — honest, modest improvement at the lowest cost.
The honest caveat: sometimes IWI is the wrong answer
Not every wall should be boarded from the inside. If the wall is actively damp, insulating over it hides the problem while it worsens. If original cornices, panelling or joinery make the room, IWI means losing or remaking them. And if you are treating most of the house anyway, external wall insulation may serve you better — it keeps the masonry warm, adds no thickness inside, and side-steps the fixing and detailing compromises entirely. The trade-offs are set out in our EWI vs IWI guide.
If you want the right board for your actual wall rather than the one a brochure prefers, get a free, fixed, itemised quote — the system, thickness, finish and every making-good line will be on paper before you decide.
IWI board FAQs
What is the best insulation board for internal walls?
There is no single best board — there is a best board for your wall. PIR insulated plasterboard (Kingspan or Celotex type) gives the most insulation per centimetre and suits dry, modern walls. Wood-fibre systems such as STEICO are vapour-open and suit older solid-brick walls that need to breathe. Plain insulated plasterboard laminates are the budget middle ground for straightforward dry walls. On our own quotes, breathable and wood-fibre systems actually appear most often — 103 quotes, against 26 for PIR and 16 for insulated plasterboard — because so much of London’s housing is solid-walled.
How much thickness does internal wall insulation take off a room?
Typically 50–100mm per external wall once board and finish are on, depending on the system and target U-value. PIR takes the least because its conductivity is the lowest; wood fibre needs more thickness for the same performance. In a typical room you only treat one or two external walls, so the floor-area loss is smaller than people fear — but in a narrow room every centimetre counts, which is a genuine reason to choose PIR there.
Can I hang shelves or a TV on an insulated wall?
Yes, but plan it. Behind PIR insulated plasterboard there is soft foam between the plasterboard and the masonry, so heavy items need stand-off fixings that reach the wall behind, or timber pattern battens fitted before boarding. Wood-fibre systems are more forgiving because the board itself is dense. Tell your installer where radiators, cupboards and TVs will go before boarding starts — it is a five-minute conversation that saves a wrecked wall later.
Is wood-fibre insulation worth the extra cost?
On the right wall, yes. On a Victorian solid-brick wall, a vapour-open wood-fibre build-up manages moisture the way the wall was designed to, adds useful summer heat buffering and takes a lime plaster finish beautifully. On a dry modern cavity wall it is usually money you do not need to spend — PIR or insulated plasterboard will perform better per pound there.
Do you install all three systems?
Yes — we install PIR (Kingspan, Celotex), wood-fibre (STEICO), SWIP and insulated plasterboard systems with our own in-house team, no subcontractors. We are TrustMark registered (2259022) and PAS 2030-19 certified, with 500+ installations completed. The survey decides the board, not a brand allegiance.