Multifoil Insulation Calculator — Rolls, Battens & Staples

Yes — multifoil insulations rely on reflective air gaps on both sides to achieve their stated thermal performance. Without air gaps, the foil layers cannot reflect radiant heat and the insulation performs poorly. A minimum 25mm unventilated air gap is needed on each side, created by fixing battens before and after the multifoil. This is a critical installation requirement.

Thin multifoils have manufacturer headline R-values of about 1.4-2.8 m²K/W — but read the small print: those whole-up figures already include the two reflective air gaps, so you do not add the gaps again. That is roughly equivalent to 30-60mm of PIR board (at λ≈0.022 W/mK). Certified core resistances vary enormously between products — from around 0.9 m²K/W for thin foils up to over 4 m²K/W for thick quilted multifoils — so always work from the figure on the product's own BBA/UKAS-accredited certificate rather than a marketing headline.

Thin multifoils alone are unlikely to meet Approved Document L U-values for walls or roofs, so they are best used as part of a hybrid system — for example mineral wool between rafters plus a multifoil under the rafters with a reflective air gap. The multifoil adds reflective performance and acts as a vapour control layer. Since June 2022, BR443 (2019) sets out exactly how to include a multifoil in a U-value calculation: its certified core R-value goes in as a layer (in a roof, around 30% balloons up into the rafter cavity and 70% down) with the reflective air-cavity resistances added per BS EN ISO 6946.

As a trade rule of thumb, allow roughly 8 staples per m² to secure multifoil to timber battens — always follow the spacing in the manufacturer's installation guide. Typical guidance (e.g. SuperFOIL) is a fixing every 200-300mm along each batten for the body of the sheet, closing to about 50-100mm where layers overlap or for horizontal runs so the foil cannot sag. Use stainless steel staples (about 10-14mm) to avoid corrosion against the foil. Lap and tape joints per the maker's guide (commonly 50-100mm overlap) with aluminium foil tape, and keep the foil taut: sagging closes the air gap and can trap condensation.

Many multifoils carry a BBA (or other UKAS-accredited) certificate, but the certified core R-value can differ from a marketing headline, so quote the specific certificate number and its tested values. Multifoils do count towards Approved Document L: under BR443 (2019) the certified R-value is entered into a U-value calculation as a layer with the reflective air cavities. Note that an air-space emissivity below 0.2 may only be used where that figure appears on the accredited certificate. Products such as SuperFOIL and TLX Silver hold BBA certificates; some others (e.g. Actis Triso-Super) are certified through different UKAS schemes rather than the BBA — check which scheme applies before relying on a number.