Wall Insulation Calculator
Calculate insulation boards, stud framing, fixings and adhesive for internal wall insulation. Covers dot-and-dab, stud wall and direct fix methods with U-value thickness recommendation. 2026 Building Regs.
Solid walls: no cavity to fill — internal lining is the main option. Breathable materials recommended for pre-1920 properties.
0.30 W/m²K is the Approved Document L target for existing walls being upgraded in England (2026).
PIR: highest performance per mm. Best for dot-and-dab and direct fix.
Enter the combined length of all walls being insulated.
Standard door: 0.9×2.0m
Standard window: 1.2×1.0m
Installation Guide
Wall Insulation Methods
Dot & Dab — Insulated Plasterboard
The most popular DIY method. Insulated plasterboard (a PIR board factory-bonded to a plasterboard face) is adhered directly to the existing wall using dollops of EWI adhesive — no studwork required. Boards are typically 52.5mm (25mm PIR + 12.5mm PB) or 72.5mm (60mm PIR + 12.5mm PB).
Adhesive dabs are applied at approximately 5–6 per m² in a grid pattern. Mechanical fixings (typically 6 per board) are added once the adhesive has cured. The board is then taped, jointed and skimmed or plastered. Total build-up off the existing wall: approximately 60–85mm.
Not suitable for: pre-1920 solid walls where breathability is required, walls with significant surface irregularities, or where thick insulation is required for a high-performance target.
Stud Wall with Infill Insulation
Timber studs (typically 47×100mm C16 or C24) are fixed to the floor and ceiling, leaving a 50–100mm gap from the existing wall. Insulation slabs are cut to friction-fit between the studs. A vapour control layer is fitted on the warm side, then 12.5mm plasterboard is fixed to the studs.
This method allows greater insulation thickness, accommodates services (pipework, cables) within the stud cavity, and is better for uneven walls. Standard stud centres are 600mm for mineral wool batts or 400mm for cut PIR. Noggins at mid-height add rigidity.
Best for: solid brick walls needing thick insulation, where services need to be run, or where pre-1920 breathable insulation is being used.
Direct Fix PIR — Overboarded
Rigid PIR boards are bonded directly to the wall using full-coverage adhesive (such as Kingspan K-Bond or Sika EWI adhesive), then mechanical fixings are added. The PIR is then overboarded with a separate 12.5mm plasterboard layer fixed with adhesive or drywall screws.
Unlike dot-and-dab which uses insulated plasterboard as a combined product, direct fix separates the insulation and plasterboard layers — this allows thicker insulation boards (up to 150mm) without needing studwork. Joints between PIR boards must be taped with foil tape to prevent thermal bridging.
Best for: solid walls requiring higher insulation thickness than insulated plasterboard allows, or where the existing wall is too irregular for consistent dab adhesive contact.
Pre-1920 Solid Walls — Breathable Materials
Older solid brick and stone walls were built to manage moisture through vapour permeability — the wall absorbs and releases moisture as part of its normal behaviour. Sealing the inner face with non-breathable PIR can trap moisture within the wall, leading to interstitial condensation, frost damage and structural problems.
Wood fibre board (λ ≈ 0.040): vapour-permeable, high thermal mass, naturally manages moisture. Fixed between or over timber studs. Typical thickness 60–120mm.
Hemp / cork: carbon-negative and fully breathable. Higher cost but ideal for listed buildings or conservation areas where breathability is a condition of consent. Always use lime plaster over breathable insulation — never gypsum or impermeable paints.
Approved Document L & Building Regulations
Under Approved Document L (2026), upgrading a wall that separates a heated space from outside requires notification under Building Regulations. The target U-value for existing walls being upgraded is 0.30 W/m²K where technically and economically feasible. New build external walls must achieve 0.18 W/m²K.
Work carried out by a registered installer under a Competent Person Scheme (e.g. NAPIT, NICEIC for the associated electrical work, or a TrustMark registered insulation installer) can self-certify without a full Building Regulations application. For unlisted domestic properties, permitted development rights generally allow internal insulation without planning permission.
FAQ
Common Questions
To achieve the 2026 Part L renovation target of 0.30 W/m²K on a solid brick wall, approximately 60mm of PIR or 90mm of mineral wool is required. For a higher 0.18 W/m²K target, 90–100mm of PIR is needed. Our calculator determines the thickness for your wall construction and insulation type.
Dot-and-dab with 52.5mm insulated plasterboard reduces each wall by approximately 60–65mm. A stud wall with 100mm mineral wool plus plasterboard reduces each wall by 130–140mm. Applied to two walls in a 4×4m room, you lose roughly 0.5–1.0m² of floor area. The calculator shows you the total build-up depth for your chosen method and thickness.
It is technically possible but carries a significant risk of interstitial condensation. Pre-1920 solid brick walls manage moisture through vapour permeability — the wall is designed to breathe. Sealing the inner surface with impermeable PIR can trap moisture in the wall, leading to damp, mould and frost damage. A breathable alternative (wood fibre, hempcrete, cork) with a stud wall build-up is generally safer for solid walls.
Internal insulation generally does not require planning permission for unlisted properties under permitted development. However, it does require Building Regulations notification under Approved Document L. Listed buildings or properties in conservation areas may have additional restrictions on materials — always check with your local authority before proceeding.
Standard guidance is 5–6 adhesive dabs per m² for insulated plasterboard dot-and-dab. For a full 2.88m² board (2400×1200mm), this means approximately 15–17 dabs per board. Each dab is roughly the size of a tennis ball, flattened to about 25mm proud of the wall surface to allow for the boards to be pushed back level.