Multifoil insulation — when 40mm beats 200mm

Both have been true at different times in the product's history, and the honest answer matters. The early multifoil products in the late 1990s and early 2000s did claim U-values they could not deliver in independent testing — the trade is right to remember that. The BBA-certification regime tightened from around 2010 onwards and the certified products on the market today have published, independently-tested lambda values that are checkable against the cert number. We only fit BBA-certified systems. The other reason multifoil under-performs in the field is bad installation: the reflective mechanism only works with the manufacturer-specified air gap above and below the foil, and a lot of trade installers skip that step because they cannot see the difference once the plasterboard is up. We measure and sign off the air gap before any board goes on. If yours is a job we cannot do that on, we will not fit multifoil at all and we will explain why on the survey.

Different products for different jobs. PIR rigid board (Celotex, Kingspan, EcoTherm) wins where you have full cavity depth available — between rafters in a new build, on a flat-roof warm-roof build-up over the deck, internal wall insulation on a solid wall. Multifoil wins where head-height or build-up depth are the binding constraint — room-in-roof, dormer ceilings, bungalow eaves, internal stud walls that need to keep their service void. We use both, often on the same job. The decision is on the survey, in writing, before you sign anything.

Mineral wool (Knauf Earthwool, Rockwool, Isover) is the cheapest insulation per kWh saved when you have the depth for it — typically 270mm in a ventilated loft. The trade installs it because it is forgiving, well-understood, and the product cost per square metre is low. It is not the answer when you need a thinner build-up to preserve usable space. A 30–40mm multifoil and a 270mm Earthwool quilt deliver similar U-values; the choice between them is whether you can afford to lose 270mm of head room.

Multifoil reflects radiant heat: infrared energy travelling as waves rather than through contact. That mechanism needs a continuous air space (typically 25mm) above and below the foil layer for the reflected energy to return to the warm side without being conducted straight through the next material. Squash the air gap by stapling the foil flush to plasterboard, and the radiant-reflection mechanism becomes a conduction path: the foil now performs like a thin layer of plastic, which is to say barely at all. Every BBA certificate names the required air gap; every reputable installer measures and signs off the gap before plasterboarding. We do.

Yes, but ventilation is critical and the build-up has to be designed by someone who has done it before. The standard solution we fit is a BBA-certified warm-roof build-up where joist depth is too shallow for 270mm of bulk insulation — most commonly on dormers and rear extensions. Ventilation gaps above the foil are set to the manufacturer's spec, the ridge and eaves details are continuous, and the job is signed off by a PAS2035 retrofit designer if it is grant-funded. We do not fit multifoil into a cold-roof build-up where the ventilation pathway is broken — the trapped moisture risk is real, and the right answer there is a different product.

Two reasons, both honest. First, the early-2000s history we mentioned above — a generation of installers got burned by products that did not perform and they have not revisited the certified market since. Second, the labour profile is different from rolling a quilt: it takes longer, the spec is more detailed, the air-gap discipline is the difference between a great install and a bad one, and a lot of trade firms would rather quote the job they can do quickly. We fit it because for the cases above (room-in-roof, bungalow eaves, dormer ceilings) it is the only honest answer, and because we have built the install protocol around the things that go wrong.

Yes. Room-in-roof multifoil upgrades are a named eligible measure under ECO4 for households on a qualifying benefit, and under ECO4 LA Flex for lower-income households below the £31,000 threshold who are not on a benefit. Eligibility depends on EPC band (D–G), council-tax band (LA Flex), and household composition. We check both routes on the same survey visit and where the scheme covers it the install is free; where the measure is part-funded we quote a transparent top-up price before any work starts. Not every multifoil job is grant-funded — a private retrofit on a 1990s loft conversion in a band-E council-tax property typically pays self-funded — and we will tell you which case applies on the survey.

No. Multifoil installs happen room by room from inside, usually finishing each room within a day. We cover floors and furniture, take all waste out at the end of every day, and leave the space dust-vacuumed. The room is sleepable that night. A typical room-in-roof job is 1–2 working days; a multi-room dormer retrofit can run to 3 days.