What is multifoil insulation?
Multifoil insulation is a thin insulating quilt built up from alternating layers of bubble film and aluminium foil. The bubbles trap still air; the aluminium layers reflect radiant heat. In the Netherlands the product type is sold as PIF — poly isolatie folie.
The product name usually carries the built-up thickness: a type with eight air layers of 10 mm bubble film comes to roughly 80 mm of total build-up. The material is light, rolls out easily and cuts with a knife or scissors.
How it insulates
Two mechanisms are at work:
- Still air. Air is a poor conductor of heat as long as it cannot circulate. The bubbles divide the layer into small cells and slow that circulation.
- Reflection of radiation. Bright aluminium has a low emissivity and bounces back a large part of the incident radiant heat.
That second mechanism is also the biggest trap: reflection only works towards an air space. If the foil lies tight against a board, most of the radiant benefit is gone. Virtually every manufacturer therefore specifies an unventilated cavity on both faces — in the order of 20 to 25 mm. Leave that out and you will not achieve the declared performance.
Points to watch
- Interrogate the R-value. Multifoils are assessed on the complete build-up, not on the foil alone. Ask for an independent assessment — in the UK typically a BBA certificate or a European Technical Assessment — and ask which build-up that figure applies to. Headline values without that context mean very little.
- Tape it airtight. Laps and junctions at reveals and service penetrations must be sealed with the matching tape. A multifoil is also a vapour control layer; a gap in it invites interstitial condensation.
- Dust and dirt. An aluminium surface that gets dirty or oxidised loses its reflectivity. In dusty or damp spaces that is a real risk.
- Compare fairly. Where the build-up is tight — a dormer, a low-headroom extension — multifoil can be a workable answer. If you need a high insulation value in minimal thickness, PIR or a similar rigid foam usually makes the stronger case.
Related terms
- Thermal conductivity
- Vapour control layer
- Airtightness
- PIR
- Cavity
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