Envelope and protection · Insulation
Multifoil Insulation (Reflective Multi-layer)
Explains how multifoil products are put together, why the air spaces around them are part of the arrangement rather than incidental gaps, and why their assessment belongs with a qualified professional.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What multifoil insulation is
Multifoil insulation is a thin, flexible, layered product. Reflective metallised films or foils are interleaved with wadding, foam or fibrous spacer layers and then quilted, welded or bonded together into a roll that can be cut with scissors or a knife and fixed with staples, tape and battens rather than being cut and dropped into place like a board.
Part of how these products are intended to work involves reflection across an air space, which means the cavities on either side of the material are part of the arrangement rather than incidental gaps. Fixing the material flat against a surface, or losing the intended cavity to a batten, a service run or a later alteration, changes the thing that was specified into something else.
They exist because depth is often the binding constraint rather than anything else: shallow rafters in a loft conversion, a floor that cannot rise, a reveal that cannot be lost. How the contribution of such a product is assessed, and whether that assessment is accepted for a given purpose, is a particular subject that varies by product and by location, so the manufacturer's documentation and the relevant authority are both part of the conversation.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Multi-foil insulation
- Reflective foil insulation
- Reflective multi-layer insulation
- Foil quilt insulation
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Fixed across rafters in a roof build-up designed by a qualified professional, with the cavities the product requires.
- Used in loft conversions and similar work where the depth available between structural members is genuinely limited.
- Applied within timber framed walls and internal linings where a thin layered product has been specified.
- Used in floor build-ups where finished levels cannot rise and the designer has planned the arrangement around that.
- Used as one component of a wider insulation arrangement rather than as the whole of a build-up.
Consideration
Where it is commonly considered
- Where depth is the binding constraint and the designer has compared thin products against one another deliberately.
- Where the cavities the product requires can genuinely be provided and will not be lost to battens or services.
- Where the manufacturer's own installation documentation can be followed closely rather than adapted on site.
- Where a qualified professional has assessed how the product sits within the whole build-up and its junctions.
- Where the relevant authority has been consulted about how the contribution of the product will be treated.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where the required air spaces cannot be provided, or will be filled by battens, cables or later work.
- Where the product is being treated as a straight substitute for a thicker layer without the build-up being reassessed.
- Where the laps, tapes and fixings the manufacturer sets out cannot be achieved in the actual construction.
- Where the manufacturer describes the product as doing more than one job and nobody has checked what that means here.
- Where the assessment method behind the product's documentation has not been discussed with the relevant authority.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Thermal behaviour
- The contribution of a multifoil is tied to the whole arrangement including the air spaces, not to the roll on its own, and it is assessed by particular methods that vary by product. Figures and the basis for them belong to the manufacturer's documentation and to a professional's calculation for the actual build-up.
- Installation dependency
- This is a product where the installation detail is the product. Laps, taped joints, staple patterns, batten positions and the cavities on either side are all set out by the manufacturer, and departing from any of them changes what has been installed.
- Moisture behaviour
- Some products are described by their manufacturer as performing more than one function within an assembly, which makes the manufacturer's own documentation essential reading. Where any layer sits within a build-up is assembly design for a qualified professional.
- Fire-related questions
- No fire characteristic or comparison is given here. Fire behaviour differs between insulation families and between products and depends on the assembly, so it is a matter for the manufacturer's documentation, a qualified professional and the relevant authority.
- Substrate dependency
- The material is fixed rather than laid, so what it is fixed to matters: the spacing and straightness of rafters or studs, the battens that form the cavities, and whether the structure can take the counter battens the arrangement needs.
- Documentation
- Ask for the product's own installation instructions in full, the technical documentation setting out how its contribution is assessed, and a professional's view on how the relevant authority treats that assessment locally.
Exposure
Exposure and environmental conditions
- Whether the cavities the product requires exist in the real construction or only on the drawing.
- How the roof space, wall or floor void is ventilated, and whether the proposal changes that arrangement.
- Whether water could reach the layer from the covering, a gutter or a plumbing run above it.
- Whether later services, cables or fixings are likely to be run through the cavity the product depends on.
- How humid the rooms below or behind the layer become in use, which a professional has to factor into the assessment.
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The battens and counter battens that create and hold open the cavities, which are part of the arrangement rather than fixings.
- The actual depth available, since these products are usually chosen precisely because that depth is limited.
- Where the control layers sit within the build-up, which is assembly design for a qualified professional.
- How the material is terminated and sealed at eaves, ridges, openings and the perimeter of each area.
- Whether the structure can carry the additional battens and the finish that will be fixed over the material.
Appearance
Appearance and finish considerations
- The material is concealed in service, so what is seen is the plasterboard, lining or covering fixed over the battens.
- The reflective face is briefly visible during construction and is not intended as any kind of finished surface.
- Battens fixed over the material set out the lining, so their spacing shows up in how the finish is fixed.
- Taped laps and terminations are visible while the work is open, and a professional inspecting them at that stage is the only opportunity.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- Whether the cavities will still be there in years to come, or whether later work will fill or bridge them.
- How the tapes and laps behave over time in the temperatures and humidity of the position they sit in.
- Whether the material can be punctured by fixings for lighting, shelving or services fitted afterwards.
- How anyone would know if the layer had been damaged or bridged, given that it is enclosed.
- Whether the fixings holding the material and the battens will remain sound in this particular structure.
Upkeep
Maintenance and repair considerations
- What would need removing to reach the layer if a fault appeared behind the lining or the covering.
- Whether the same product and tapes would still be available if part of the layer had to be reinstated.
- How a puncture or a torn lap would be made good so that the installation still matches the documentation.
- What record exists of what was installed, so a future owner or trade knows what the build-up contains.
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- The manufacturer's own installation instructions, followed in full rather than adapted to suit the site.
- Battens and counter battens set out to create the cavities the product depends on, before the material goes up.
- Laps, tapes and terminations carried out as specified, since these are the parts most often abbreviated on site.
- Coordination with electrical and mechanical trades so that services do not end up occupying the cavity.
- Inspection while the work is still open, because nothing about the installation can be checked afterwards.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The manufacturer's full installation instructions for the exact product and the application proposed.
- The technical documentation setting out how the product's contribution has been assessed and on what basis.
- Details of the tapes, fixings and accessories the manufacturer requires with this product.
- A professional's written view on how the relevant authority locally treats this kind of assessment.
- A record of what was installed, where, and with which accessories, kept with the property documents.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Why is a multifoil proposed here, and is limited depth genuinely the reason it is being considered?
- Do the cavities this product requires actually exist in the construction as it will be built?
- How has the contribution of this product been assessed, and does the relevant authority accept that basis locally?
- What happens to the arrangement if a batten, a cable or a later alteration fills one of the cavities?
- Where do the control layers sit in this build-up, and who has designed and taken responsibility for that?
- How are laps, tapes and terminations being detailed at eaves, ridges, openings and the perimeter?
- Who is inspecting the installation while it is open, since none of it can be checked once it is lined?
- What has to be checked with the relevant authority before this product is ordered?
Blind spots
Commonly overlooked points
- The air spaces are part of the product, and they are the first thing lost when a batten or a cable gets in the way.
- These products are unusually sensitive to installation detail, so a careless lap changes what was specified.
- The basis on which the contribution is assessed varies by product, which is why the documentation has to be read.
- Thin does not mean simple, and the arrangement around the material is usually more demanding than a board would be.
- Once the lining is fixed, nothing about the installation can be verified, so inspection has to happen while it is open.
What this page does not do
- No thermal figure, comparison or performance outcome is stated or implied anywhere in this entry.
- No fire claim of any kind is made here; fire behaviour is a matter for the manufacturer's documentation, the specific assembly, a qualified professional and the relevant authority.
- Whether the contribution of a product of this kind is accepted for a given purpose depends on local requirements that must be confirmed with the relevant authority.
- Build Design Hub does not test, approve, certify or recommend any multifoil product, accessory or installation.
Related entries
Related materials
Materials from the same family, an adjacent role in the assembly, or a shared application.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
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