Bubble foil insulation: what it does — and what it doesn't

What it actually does

• Reflects radiant heat across an adjacent air space — the reflective face facing a 20mm+ gap emits and absorbs a fraction of what plaster or timber would.
• Adds a small conductive resistance from the bubble layer: a few millimetres of trapped air.
• Stops air movement when joints are taped, which is much of the felt benefit in a draughty garage.

What it does not do

• It does not achieve a meaningful core resistance. Compressed between surfaces with no air space, it is a few millimetres of plastic.
• It is not certified as a primary insulation for a habitable room's roof or wall in any UK third-party assessment we are aware of; check for a BBA/LABC certificate before believing an “equivalent to” claim.
• It does not replace mineral wool on a loft floor or multifoil on a rafter slope.

Bubble foil vs multifoil

Where bubble foil is a good buy

• Sheds, workshops, garages and summer houses: a big comfort gain for little money.
• Behind radiators on cold external walls.
• Under a metal roof to cut summer radiant gain and drip condensation.
• As an extra reflective layer over proper insulation under boarding.

Where it is the wrong buy

• As the only insulation in a loft room, bedroom ceiling or converted garage used all year.
• Stapled flat to rafters with plasterboard directly over it.
• Anywhere a target U-value has to be demonstrated.

Marketing claims to read carefully

“Equivalent to 100mm of mineral wool”, “reflects 97% of radiant heat”, “thermal barrier” — the first only holds, if at all, for a specific tested arrangement that the packaging rarely describes; the second is a surface property that says nothing about the roof's U-value; the third is not a defined term. Ask for the certificate. If there is none, treat it as a radiant barrier and price it accordingly.

Questions homeowners ask