Does foil insulation work? Yes — but only in the right build-up.

How heat leaves a roof, and what foil does about it

Heat crosses a building element three ways. Conduction moves through solid material — timber, plasterboard, compressed insulation. Convection moves with air. Radiation crosses open gaps as infrared, from any warm surface to any cooler one facing it. Mineral wool and PIR mainly fight conduction by trapping still air in a thick layer. A reflective surface fights radiation: a clean low-emissivity foil facing an air gap emits and absorbs a small fraction of the infrared a plaster or timber surface would.

That is why the gap is not optional. Radiation needs a space to cross. Press foil flat against plasterboard and the only path left is conduction through a film a fraction of a millimetre thick — which resists almost nothing.

Where foil insulation earns its place

• Roof slopes and loft rooms with limited depth. A certified multifoil, often with a 25–50mm PIR layer, reaches the Building Regulations target for an upgraded roof in less depth than board alone. This is the mainstream use and the one with certificates behind it.
• Warm-side vapour control with a thermal bonus. Reflective VCLs facing a service void add a low-emissivity air space to a PIR or wool build-up.
• Outbuildings. A garage or workshop lined with foil is noticeably more comfortable, cheaply.
• Summer overheating. Reflective layers under a roof reduce radiant gain into a loft room.

Where mineral wool may be better

Open loft floors, almost always. Wool to 270mm is cheap, breathable, tolerant of imperfect fitting and reaches the target on its own. Foil on a loft floor adds cost and a vapour-tight layer where you may not want one. Detail: multifoil vs mineral wool.

Where PIR may be better

Warm flat roofs, floors, and any roof where the target U-value must be reached in a fixed depth with a single product and a single certificate. PIR's performance does not depend on maintaining air gaps through the life of the building. Detail: multifoil vs PIR.

What an air gap changes

Manufacturers certify multifoil systems with defined air spaces, commonly around 20–25mm on each face, formed with battens. With the certified arrangement, the certified system resistance applies. Without it, only the quilt's core resistance remains — the air-space contribution and the core resistance are separate quantities, and for thin products the core is the smaller of the two. Air gap guide.

What happens when foil is fitted incorrectly

• Compressed against the underside of tiles or against plasterboard: the air-space contribution is lost, and a vapour-resistant layer sits on the cold side.
• Joints untaped or overlapped the wrong way: air movement and moisture bypass the layer.
• Eaves or ridge ventilation blocked: condensation on rafters within a season.
• Fitted over existing spray foam or damp timber: the problem is sealed in.

Condensation and moisture

Most internally fitted foils are highly vapour-resistant. Placed on the warm side of the insulation with the cold side ventilated or vapour-open, that is exactly what a roof needs and it reduces interstitial condensation risk. Placed on the cold side, or in a roof with no ventilation path, the same layer traps moisture. Condensation is a construction, moisture, temperature and ventilation question — foil neither causes nor prevents it on its own. The survey question is always: where does moisture go? Condensation guide.

Why generic claims are misleading

“Equivalent to 200mm of wool” and similar comparisons only hold for the certified build-up on the certificate — a specific quilt, specific gaps, specific adjacent layers. Take the certificate, not the packaging: check the certifying body, the build-up drawing and the stated thermal resistance. U-values explained. Certificates: BBA, LABC.

Real installation examples

Surveyed and photographed projects — property, existing build-up, what was specified and why — are published at case studies as they complete. We do not publish thermal images or before/after figures we have not collected ourselves.

Questions homeowners ask