Basement Insulation Calculator

Approved Document L (2021) Volume 1, Table 4.2 sets a limiting U-value of 0.18 W/m²K for new thermal elements in existing dwellings — and that 0.18 applies to BOTH walls and floors (it is not 0.13 for floors; the 0.13 W/m²K floor figure is the new-build notional-dwelling SAP target and does not apply to basement conversions). In practice a pragmatic U-value of 0.15 W/m²K for both walls and floors is commonly targeted and readily accepted by Building Control. The ground temperature moderates heat loss, so basement elements lose less heat than exposed above-ground elements.

PIR (polyisocyanurate) boards such as Celotex or Kingspan are the most popular choice for basement walls because they provide excellent thermal performance in a thin profile (lambda 0.022 W/mK), are moisture-resistant, and act as their own vapour barrier when joints are taped. They are installed behind timber or metal studwork. Phenolic boards offer even better performance but at higher cost.

It depends on the insulation type. PIR and phenolic boards have foil facings that act as an integral vapour barrier — tape the joints and you have a continuous VCL. Mineral wool and EPS are permeable and require a separate vapour control layer (VCL) on the warm side of the insulation to prevent moisture from the room condensing within the insulation. This is critical in basements where ground moisture is always present.

For a target U-value of 0.15 W/m²K, PIR board thickness is typically around 100-135mm for walls and 105-120mm for floors, depending on the existing construction. An uninsulated masonry wall (~1.5 W/m²K) needs roughly 135mm PIR; a partly insulated wall needs less. A bare concrete ground slab (~0.7 W/m²K) needs about 120mm. Mineral wool (lambda 0.035) would need around 160-210mm for the same targets. Enter your existing U-values above for the exact figure — the calculator solves d = (1/U_target − 1/U_existing) × lambda.

A typical insulated basement floor buildup from bottom to top: DPM (damp-proof membrane), insulation boards (about 100-120mm PIR to hit 0.15 W/m²K over a bare slab), vapour barrier (if not integral), screed (65mm minimum for underfloor heating, 50mm without), and floor finish. Total buildup is typically 165-205mm. This must be factored into the available headroom calculation.