Multifoil Loft Insulation

What multifoil is

Multifoil insulation is a thin, flexible product built from several reflective layers — foils and films sandwiched with wadding or foam — designed to reflect radiant heat as well as resist its conduction. It is a very different animal from the soft rolls laid on a loft floor, and it is used in a different place and a different way.

Rather than sitting on the floor of a cold loft, multifoil is generally fixed at rafter level, up against the slope of the roof, as part of a warm-roof approach. That distinction shapes everything about how it is specified and installed.

Where it is used

Its natural home is rooms in the roof and warm-roof build-ups — situations where the space under the slates is being kept warm, or where headroom and floor area are too tight for a thick layer of soft insulation on the floor.

For an ordinary cold loft that is simply being insulated at the floor, multifoil is rarely the straightforward choice. A conventional installation with soft material on the loft floor is usually simpler and better value. Multifoil comes into its own where the roof itself is the insulated plane.

Rafter-level installation and air gaps

Multifoil is fixed to the rafters, and its performance depends heavily on the air gaps left on either side of it. The reflective faces only do their job across an adjacent air space, so the build-up must be detailed to preserve those gaps — squash the foil tight against a surface and much of the benefit is lost.

This is why multifoil cannot be judged by thickness the way a roll of wool can. Its contribution comes from the layers and the air gaps working together, which makes correct installation and a proper thermal calculation essential.

Vapour control and joint taping

Because it is a warm-roof detail, moisture control matters a great deal. Multifoil systems act as a vapour-control layer, and that only works if the material is continuous and sealed. Joints and overlaps are taped with the manufacturer’s recommended tape, and penetrations are detailed carefully, so warm, moist air from inside cannot pass into the cold parts of the roof.

Poor taping or gaps in the layer can let moisture reach the timbers, which is why this is exacting work rather than something to improvise.

Roof structure, ventilation and headroom

Fixing at rafter level interacts with the whole roof. The existing structure has to be sound and suitable, and any ventilation the roof relies on — behind the underlay, at the eaves and ridge — must be understood and maintained. Getting this wrong risks trapping moisture, so multifoil and ventilation are always considered together.

Headroom is a practical constraint too. Even though the product is thin, the battens and air gaps it needs take up space, which can matter in a shallow roof.

Thermal performance and manufacturer specification

The performance of a multifoil build-up must come from the manufacturer’s tested figures and a thermal calculation for the specific roof, not from a rule of thumb. Independent test data and the intended construction — including the air gaps — are what determine whether it meets the target.

We work to the manufacturer’s specification for whichever product is used. Where a product is genuinely confirmed as SuperFOIL SF19, that is a multi-layer reflective product identified by its construction and markings; it is described by its make-up and tested performance, not treated as a simple insulation depth. And we will not label a visible shiny product as SF19, or any other named product, unless the packaging, markings or job records actually confirm it.

Limitations versus loft-floor insulation

It is worth being clear about the trade-offs. Multifoil is a specialist, detail-sensitive product for warm-roof and room-in-roof situations. For the common job of insulating a cold loft, it is generally more complex and more costly than laying soft insulation on the floor, and it demands precise installation to perform at all.

Which approach is right depends on the roof and how the loft is used — something a loft inspection establishes in person. The cost guide explains what drives the installed price, and an assessment gives you an accurate written quotation. Depending on the service and location, work may be completed by The Insulation Company or an approved installation partner.