Insulated Plasterboard (Thermal Laminate) in London Extensions and Refurbishments: What It Is, When to Use It, and How to Fix It

**What is insulated plasterboard**:

• Insulated plasterboard (thermal laminate) consists of:
• •A plasterboard face (typically 12.5mm standard or moisture-resistant plasterboard)
• •A rigid insulation backing, factory-bonded to the rear of the plasterboard
• •The insulation material is typically PIR (polyisocyanurate foam; lambda ≈ 0.022 W/mK) or EPS (expanded polystyrene foam; lambda ≈ 0.038 W/mK)

The product dimensions are described as total panel thickness — for example, a '50mm PIR thermal laminate' typically consists of 37.5mm PIR + 12.5mm plasterboard = 50mm total.

**Common PIR thermal laminate thicknesses and their approximate U-values on London external walls**:

The U-value achieved by adding thermal laminate to an existing wall depends on the existing wall's thermal resistance. For a typical London Victorian solid 225mm brick wall (U-value approximately 1.9–2.1 W/m²K uninsulated):

| Thermal laminate thickness (PIR) | PIR only lambda | Additional R-value added | Resulting wall U-value (225mm brick + PIR lining) | |---|---|---|---| | 25mm PIR (12.5mm board) | 0.022 | +1.14 m²K/W | ≈ 0.60–0.65 W/m²K | | 37.5mm PIR (12.5mm board = 50mm total) | 0.022 | +1.70 m²K/W | ≈ 0.45–0.50 W/m²K | | 50mm PIR (12.5mm board = 62.5mm total) | 0.022 | +2.27 m²K/W | ≈ 0.35–0.40 W/m²K | | 72.5mm PIR (12.5mm board = 85mm total) | 0.022 | +3.30 m²K/W | ≈ 0.26–0.30 W/m²K | | 100mm PIR (12.5mm board = 112.5mm total) | 0.022 | +4.55 m²K/W | ≈ 0.20–0.22 W/m²K |

Building Regulations Part L (Conservation of fuel and power) target U-value for an existing wall being upgraded in England (from 2022): 0.30 W/m²K where achievable. This suggests a minimum 50mm PIR thermal laminate on London Victorian solid brick is needed to approach compliance — and 72.5mm PIR is needed to achieve 0.26–0.30 W/m²K.

**EPS-backed thermal laminate vs. PIR-backed**:

• •EPS-backed boards (e.g., Kingspan Kooltherm K118 uses phenolic not EPS; standard EPS products have lambda 0.038 W/mK): lower thermal performance per mm than PIR
• •PIR-backed (Kingspan Kooltherm K17; Celotex TB4000; Recticel Eurowall): the preferred specification for London residential thermal retrofits where floor-to-ceiling height is constrained — the higher lambda of PIR (0.022 vs. 0.038 W/mK for EPS) means a thinner board is needed to achieve the target U-value, losing less room depth
• •Phenolic foam-backed (e.g., Kingspan Kooltherm K17 uses phenolic foam; lambda ≈ 0.020 W/mK): the highest performance option — slightly thinner than PIR for equivalent U-value; higher cost

• **Products commonly used in London residential projects**:
• •Kingspan Kooltherm K17 (phenolic foam-backed plasterboard): lambda 0.020 W/mK; available in 25mm–100mm total thicknesses; the premium option for maximum performance in minimum space
• •Celotex TB4000 (PIR-backed plasterboard): lambda 0.022 W/mK; cost-effective; available in 37.5mm–112.5mm total thicknesses
• •British Gypsum Gyproc ThermLine (EPS-backed): lambda 0.038 W/mK; lower performance per mm but lower cost; available in thin sections (25mm–50mm total)
• •Rockwool Rockboard (mineral wool-backed): lambda 0.036 W/mK; vapour-permeable (no VCL risk); fire-resistant; suited where vapour control is a concern

**Fixing methods for thermal laminate boards in London residential projects**:

Thermal laminate boards can be fixed to existing masonry walls using two methods:

• *1. Dot-and-dab (adhesive fixing)*:
• The most common method in London residential retrofits:
• •A proprietary gypsum-based bonding compound (Gyproc ThistleBond-it; Lafarge Gyproc Dot and Dab adhesive) is applied in blobs ('dots') to the rear of the thermal laminate board and/or to the existing wall face
• •The board is pressed firmly against the wall, with the adhesive dots compressed to create a consistent face plane
• •A gap (air cavity) of approximately 10–25mm is left between the insulation backing and the existing wall — this gap is created by the adhesive dots and is maintained by the set adhesive
• •Boards are plumb-aligned during fixing with continuous screed or dabs at floor and ceiling perimeter

• *Critical limitation of dot-and-dab fixing*:
• The air gap between the insulation and the masonry wall is a significant weakness of the dot-and-dab method. In London Victorian solid brick houses where the solid brickwork is porous and can be penetrated by wind-driven rain in severe weather:
• •Air can convect within the gap between the insulation and the brickwork — cold external air circulation behind the board significantly reduces the effective thermal performance of the insulation layer
• •Moisture can condense within the gap on the cold surface of the brickwork
• •The gap also allows fire to spread behind the board in the event of a fire — this is addressed by specifying fire stops (mineral wool or intumescent strips sealed in the adhesive at floor and ceiling level) which close the cavity perimeter

For maximum thermal performance, the air gap should be eliminated by direct-bond fixing or mechanical fixing against a damp-proof membrane.

• *2. Mechanical fixing (direct to masonry)*:
• •The insulation layer is fixed directly to the wall using long mechanical anchors (typically nylon plug-and-screw systems with broad heads; e.g., Hilti HIU or Fischer DuoSeal fixings designed for thermal board)
• •The plasterboard face is then fixed over the insulation
• •This method is used where adhesive fixing is not appropriate (irregular surface; old painted walls where adhesive bond is poor; masonry in poor condition)
• •Thermal performance is improved because there is no air gap between the insulation and the wall

**Vapour control with thermal laminate boards — the critical detail**:

PIR and phenolic foam insulation have very low vapour permeability — PIR foam is approximately 100–200× less vapour-permeable than open-cell insulation. This means a PIR thermal laminate board on an existing wall acts as a vapour control layer (VCL) on the warm side of the insulation, which is the correct position for vapour control (warm side = high vapour pressure side).

• However, the board does NOT form a continuous vapour control layer unless:
• •All joints between adjacent boards are sealed with vapour-control tape (typically foil-faced tape pressed firmly over the joint at the rear of the boards before fixing — difficult to achieve in practice with dot-and-dab fixing)
• •The perimeter of the board is sealed at floor, ceiling, and window/door reveals to prevent warm moist air from bypassing the insulation and reaching the cold brickwork behind

In London Victorian terraces where there is significant moisture generation (cooking; bathing; drying laundry), inadequate vapour control behind thermal laminate boards can lead to interstitial condensation within the air gap between the board and the brickwork — causing mould growth and efflorescence on the brickwork surface behind the board. This typically remains hidden until the board is removed.

For high-moisture rooms (kitchens; bathrooms), vapour-permeable mineral wool-backed boards (Rockwool Rockboard) or external wall insulation (EWI) are safer alternatives to PIR-backed internal wall insulation.

**The cold bridge at perimeter reveals — the most significant thermal weakness**:

The single biggest weakness of internal wall insulation (including thermal laminate boards) is the cold bridge at the perimeter junction between the insulation and the uninsulated structural fabric:

• •At the floor/wall junction: the floor screed or slab connects directly to the external brickwork, bypassing the insulation — a significant cold bridge
• •At window and door reveals: the window/door reveal (the return face of the masonry at the window) is not insulated, creating a cold bridge between the external brickwork and the internal room temperature
• •At party wall junctions: where the party wall meets the external wall, a potential cold bridge path exists through the uninsulated party wall

In London Victorian houses where thermal laminate boards are applied to solid brick walls, the cold bridge at window reveals is typically visible as condensation or mould growth at the corner between the insulated wall face and the uninsulated plaster reveal — particularly in bedrooms with low ventilation rates. Insulating the reveals (with a thinner thermal board or with 25mm PIR cut-and-stuck into the reveal) reduces but does not eliminate the cold bridge.

**Installation sequence for thermal laminate boards in a London Victorian terrace room**:

1. *Prepare the existing wall*: rake out cracked or failed mortar joints; repair any areas of damaged or hollow plaster; remove existing skirting and architraves (these will be refixed over the new board face); fix any damp issues at source (DPC; roof; window frame seals) — do NOT apply thermal laminate over a damp wall (the damp will be trapped behind the impermeable PIR and will cause ongoing deterioration)

2. *Mark the finished face line*: determine the finished face plane of the thermal laminate board, considering the floor-to-ceiling height loss (a 50mm PIR board takes 50mm off the room depth on each insulated wall — significant in London Victorian rooms of 3.5–4.5m depth). Mark the finished face vertical line on floor and ceiling.

3. *Apply perimeter adhesive strips*: apply a continuous bead of bonding compound at floor and ceiling level and around all perimeter reveals before fixing — this forms a continuous fire stop and also prevents air movement at the board perimeter

4. *Apply dot-and-dab adhesive*: apply adhesive in blobs at standard spacing (five blobs per standard board — four corners and one centre; additional blobs for heavier boards)

5. *Fix boards*: press each board to the wall; check plumb; align to the finished face line; use temporary props or wedges to hold while adhesive sets (typically 24 hours before adjacent boards can be fixed)

6. *Tape joints*: apply foil-faced vapour control tape to all board-to-board joints and board-to-reveal junctions. Fill any gaps at perimeter with fire-stopping material

7. *Fix skirting and architraves*: new skirting must be fixed through the board to the masonry behind — use longer fixings (nails or screws) than the board thickness plus the air gap depth (typically 90–100mm nail into masonry for a 50mm board with a 20mm air gap). Skirting fixed to thermal laminate board face alone will pull away from the wall over time.

8. *Plaster finish*: a thin skim coat or finishing gypsum plaster over the board face (typically 2–3mm skim plaster; applied after taped joints have dried)

**Costs for insulated plasterboard in London residential projects (2025, all-in supply and install)**:

| Thermal laminate type and thickness | Cost per m² (installed — boarding + adhesive; excludes skim/decoration) | |---|---| | 37.5mm PIR-backed (50mm total; Celotex TB4000) | £30–£50/m² | | 62.5mm PIR-backed (75mm total) | £38–£60/m² | | 85mm PIR-backed phenolic (Kingspan Kooltherm K17) | £50–£80/m² | | EPS-backed (25mm board; Gyproc ThermLine) | £18–£30/m² | | Skim plaster finish (add to above) | £8–£15/m² |

**Common defects and failures with thermal laminate boards in London properties**:

*1. Board fixed over existing damp*: The most serious failure mode. PIR backing is impermeable — applied over a damp wall it traps moisture, accelerating wall deterioration and causing structural problems within the wall cavity. Always fix the source of damp before applying thermal laminate.

*2. Skirting fixed to board face only*: Skirtings nailed to the face of thermal laminate boards fall away within months as the board flexes and the nail pullout resistance in the foam is minimal. All skirtings must be fixed through the board to the masonry behind with appropriate-length fixings.

*3. Cold bridges at reveals creating mould*: Window reveals left uninsulated while adjacent walls are insulated create cold bridges at the corner — visible as condensation and black mould within 1–2 winters. Insulate reveals with a thin thermal board (12.5mm PIR minimum) cut to fit the reveal profile.

*4. Air gap behind dot-and-dab not closed at perimeter*: Omitting the continuous perimeter adhesive seal allows warm moist air to enter the gap between the board and the brickwork and condense. Apply a continuous perimeter adhesive bead before fixing boards and ensure fire-stopping at all floor, ceiling, and junction positions.