Thermal plasterboard, wood fibre or PIR: which suits a solid wall?

What is the difference between thermal plasterboard, PIR and wood fibre?

Thermal plasterboard (also called insulated plasterboard) is ordinary plasterboard factory-bonded to a layer of foam insulation, usually PIR or phenolic foam, sometimes polystyrene. It goes up in one layer, then the joints are taped and skimmed.

PIR boards are rigid foam insulation boards fixed to the wall separately, with plasterboard or a plaster finish on top. Fitting them as a separate layer lets the insulation, the vapour control layer and the finish each be detailed properly. On my PIR jobs I bond the boards with PU adhesive and seal the edges.

Wood fibre boards are made from compressed softwood fibres. They are vapour-open and absorb and release moisture, and they are normally finished with a lime base coat on mesh and a lime finish, so the whole build-up stays breathable.

How do they compare side by side?

*Illustration only: the insulation needed to bring an uninsulated 225mm brick wall (U-value about 2.1 W/m²K) down to about 0.6, ignoring fixings and plaster. Real figures depend on the products and the wall. The 20 to 80mm I quote for finished wall depth includes the plaster.

What is interstitial condensation, and why does it matter?

Interstitial condensation is condensation inside the wall build-up rather than on the surface you can see. Warm, moist air from the room moves into the wall as vapour. Once the wall is insulated on the inside, the old brick behind the insulation is much colder than it used to be. If enough vapour reaches that cold layer, it turns to water where nobody can see it.

The results show up slowly: damp patches at the edges of the insulation, mould behind boards, rotting timber joist ends, and frost damage to brick that now stays wetter through winter. Solid walls also soak up driving rain, so they need a route to dry out, inwards or outwards.

The two approaches handle this differently. Vapour-closed systems such as PIR and thermal plasterboard stop vapour reaching the cold brick, so they only work if the vapour control layer is continuous and every joint, socket and edge is sealed. Vapour-open systems such as wood fibre let small amounts of vapour pass and dry back out, and the lime finish buffers humidity in the room.

BS 5250, the British Standard code of practice for managing moisture in buildings, recommends assessing this risk at design stage, and for solid walls that generally means hygrothermal modelling that accounts for rain and the wall's ability to store water. The government best practice guide to retrofit internal wall insulation sets out the same concerns.

When is thermal plasterboard or PIR the right choice?

When the wall is sound and dry, not badly exposed to driving rain, and space really matters. Examples are a narrow box room, a bay reveal where the window board cannot get deeper, or a wall where a thinner build-up saves moving a door frame. PIR's high insulation per millimetre is a genuine advantage there.

The detail is everything. Thermal plasterboard stuck on with dabs of adhesive leaves an air gap behind the board, and if cold air can move in that gap, the board's insulation is partly bypassed and condensation can form behind it. That is why I bond PIR with PU adhesive, seal the perimeter and tape the joints, and why I do not treat insulated board as a simple swap for standard plasterboard. My plasterboarding page explains the fixing methods.

When is wood fibre the better choice?

On most traditional solid brick homes built before about 1930, and on any wall that has had damp, salts or blown plaster before. Government guidance on internal wall insulation, and the Historic England guidance on insulating solid walls, both lean towards vapour-open approaches for traditional solid walls because they keep the drying routes open.

Wood fibre also has quieter advantages: it holds heat well, which keeps rooms more even, it softens sound, and the lime finish feels solid when you knock on it. It needs more thickness than PIR, and every layer, from adhesive to paint, has to be breathable for the system to work. You can see a full wood fibre and lime job in my 1930s bedroom case study.

Do I need building regulations approval?

Often, yes. Under Approved Document L, insulating or renovating more than half of an external wall usually counts as renovating a thermal element, and the wall is expected to reach a target U-value unless that is not technically or economically feasible, for example because it would take up too much of the room. I check this at the survey and tell you what applies, so it is built into the plan rather than discovered later.

When should I call a specialist?

Before you buy any boards. The choice depends on things you cannot see from a product page: how wet the wall gets, how exposed it is, what the old plaster is, where the joists bear onto the wall, and how the room is ventilated. Getting it wrong on a solid wall can be expensive to put right.

I am a Retrofit Assessor as well as an installer, and I fit both breathable and PIR systems, so I have no reason to push one. I survey the wall, run moisture modelling and recommend the system that keeps it dry. I work from Bloxwich across Walsall, Sutton Coldfield, Great Barr and Birmingham, mostly in 1930s semis with solid 225mm brick. Read more about internal wall insulation and what it costs, or get in touch for a free home visit and written quotation.

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