Quantities
Room floor area lost to internal wall insulation
- where
- floor area after insulation (m²)
- room length (m)
- lined walls across the length (end walls)
- total build-out (mm)
- room width (m)
- lined walls along the length (side walls)
- floor area lost (m²)
- floor area lost (%)
Explain this equation
Works out how much floor area a room loses to internal wall insulation. Useful for discussing IWI with customers and for Part L feasibility arguments.
Each lined end wall shortens the room by the build-out thickness (converted to metres), and each lined side wall narrows it. The new floor area is the reduced length times the reduced width. The loss is the original area minus the new one, also shown as a percentage.
Simple rectangle areas before and after.
Sense check: a 5 × 4 m room with 100 mm on both end walls and one side wall loses about 1.3 m², about 6%.
Watch out: also allow for moving radiators, sockets, skirtings and window boards, which often cost more than the floor area.
Shows how much floor space a room loses when one or more external walls are lined with internal wall insulation.
Enter your values to see the result.
Good to know
Count every layer that projects into the room: insulation, battens or adhesive dabs, plasterboard and skim. A 60 mm PIR laminate with board and skim is about 75 mm; a 100 mm wood-fibre system with lime plaster is often 120 mm or more. Remember the knock-on work: skirting, sockets and switches, radiators, window reveals and boards, door linings and coving all have to be moved or returned. The example lines both end walls and one side wall of a 5 × 4 m room with 100 mm.
Common mistake
Quoting the insulation thickness only. Adhesive dabs, battens and finish layers can add 20–40 mm to the real build-out.
Limits
Rectangular room only. Ignores chimney breasts, bays and the effect on furniture layout. Does not cover the thermal-bridge returns into reveals and at floor and ceiling junctions, which need their own detailing.
Sources
- Approved Document L Volume 1: Dwellings, 2026 edition (England): Table 3.2 new or replacement elements in existing dwellings (roof 0.15, wall 0.18, floor 0.18 W/m²K); Table 3.3 existing elements: threshold / improved U-values roof 0.35/0.16, cavity fill 0.70/0.55, internal or external wall insulation 0.70/0.30, floor 0.70/0.25; para 3.14 15-year payback and 0.7 backstop. In force 24 March 2027; the same values appear in Tables 4.2 and 4.3 of the 2021 edition, which applies until then
- BS 5250:2021 Management of moisture in buildings – Code of practice (uses BS EN ISO 13788 for surface and interstitial condensation risk)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS EN ISO 6946, BS 5250, PAS 2035, Building Regulations Part L and manufacturers’ data. Whole-house retrofit needs a qualified retrofit assessor and coordinator.