How to specify multifoil insulation
What a multifoil clause has to say given the history of its thermal claims, the decisions behind air spaces, vapour control and fire, and how to write it in CAWS and in Uniclass.
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How do you specify multifoil insulation?
To specify multifoil insulation, ask for the core thermal resistance and surface emissivity declared to BS EN 16012 and a U-value calculation for your exact build-up, not an equivalence claim. Show the unventilated low-emissivity air spaces on both sides on the drawings, say where the product acts as a vapour control layer, check its Euroclass against the fire rules, and require third-party certification that covers the construction as drawn.
Multifoil is the most argued-about insulant in the UK. It is thin and quick to fit, which makes it attractive in loft conversions and timber frame where depth is tight, and for years some products were sold with thermal claims, often as equivalence to a thickness of mineral wool, based on comparative field tests that building control did not accept. A multifoil clause therefore has to be exact about what has been measured, to which standard, and for which build-up.
This guide covers Uniclass Pr_25_57_06_57 Multifoil blanket insulation, with the sample clause written for a timber frame wall. Single reflective foils and foil-faced membranes have a separate code, Pr_25_57_06_70 Reflective foil insulation, and the air-space rules below apply to them too.
What does a multifoil insulation blanket clause need to cover?
Some properties are specific to multifoil insulation. Others belong in every product clause. A clause that leaves either out gives the contractor room to substitute.
Properties of multifoil insulation
- Product typeMultifoil blanket, hybrid or breathable type.
- Thermal performanceCore resistance and emissivity, declared to BS EN 16012.
- ThicknessDeclared at a stated compression; varies when fixed.
- Nominal sizeRoll width against stud or rafter centres.
- FacingLow-emissivity outer foils; emissivity must be declared.
- Edge treatmentOverlapped and taped joints.
- TolerancesThickness under compression, as BS EN 16012.
- Reaction to fire classificationNever guessOften low on the scale; check the DoP.
- Acoustic performanceRarely declared; not a reason to choose it.
- Vapour resistanceOften a VCL in itself; breathable types differ.
- Dimensional stabilityMust not sag and close the air spaces.
- DurabilityFoil faces must stay clean to keep low emissivity.
- FixingsStaples and battens that form the air spaces.
- Third-party certificationCertificate or registered detail for the build-up.
- Recycled contentDeclared value for reporting.
- Environmental product declarationFor embodied carbon reporting, where published.
What every product clause needs
- Manufacturer and product referenceNamed product; the claims are product-specific.
- Product standardBS EN 16012 for the declared thermal performance.
- Installation methodFixed with sealed low-e air spaces on both sides.
- Technical submittalsDoP, certificate and U-value for the exact build-up.
- Operation and maintenance informationAir spaces not to be filled or vented later.
- MaterialFoil layers with wadding or foam spacers.
Uniclass Pr_25_57_06_57 Multifoil insulation. Not every property applies to every project; where one does not, say so rather than leaving it blank.
Fields marked Never guess (reaction to fire classification) depend on the project and should never be filled from a default or a previous job. If the information is not available yet, the clause should say so with a TBC and a reason.
Which decisions does the clause depend on?
Most properties follow from the product you choose. These depend on the project, so they need a decision from the designer before the clause is written.
- 01
What the thermal claims rest on
ClaimsMuch of a multifoil's benefit comes from the low-emissivity air spaces either side of it, not from the blanket alone. BS EN 16012 sets how reflective products declare their performance: the core thermal resistance and the emissivity of the outer surfaces. The U-value of the build-up is then calculated to BS EN ISO 6946, giving each air space a resistance that depends on its width and the emissivity facing it. LABC guidance written during the long dispute over these products did not accept comparative field tests as evidence. Ask for declared values and a calculation, not an equivalence.
How to decide
- Ask for the core thermal resistance and surface emissivity declared to BS EN 16012.
- Ask for a U-value calculation for the exact build-up, with each air space modelled as drawn.
- Treat "equivalent to X mm of mineral wool" as marketing, not evidence.
- In-situ measurements to ISO 9869-1 can support a certificate, but they are not the declared value.
Write it asThermal performance: Core thermal resistance and emissivity declared to BS EN 16012. Wall U-value calculated to BS EN ISO 6946 for the build-up as drawn. Submit. - 02
The air spaces are part of the product
Air spacesA low-emissivity surface only works if it faces an air space. Fixed tight against plasterboard, insulation or timber, a foil face adds almost nothing. The air spaces have to be unventilated, of the width the calculation assumed, and closed at their ends, which in practice means battens on both sides and taped joints. A blanket that sags against the lining, a gap that connects to a ventilated cavity, or a service void later packed with insulation each remove part of the resistance the U-value counted on.
How to decide
- Show the battens and the width of each air space on the wall and roof sections.
- Close the ends of the air spaces at heads, sills, floors and junctions.
- Say in the O&M information that the air spaces must not be filled or ventilated.
Write it asAir spaces: 25 mm unventilated air space each side of multifoil, formed by treated softwood battens, ends closed. - 03
Reaction to fire and the fire rules
Never guessMultifoils are made from plastic films, foams and waddings with aluminium layers, and many are declared at the low end of the Euroclass scale. Regulation 7(2) requires materials that become part of an external wall, or specified attachment, of a relevant building to be A2-s1,d0 or A1, and anything located within a space forming part of the wall counts, so multifoil in the timber frame wall of a relevant building is ruled out. Approved Document B asks for the same in residential buildings with a storey 11 m or more.
How to decide
- Relevant buildings and residential buildings with a storey 11 m or more: not multifoil in external walls.
- Other buildings: read the Euroclass on the DoP and check it against the fire strategy, especially where the product is exposed in a void.
- Check cavity barriers and fire-stopping where the air spaces run past floors and party walls.
- Keep the product clear of flues and heat sources by the manufacturer's distances.
Write it asReaction to fire: Euroclass as declared to BS EN 13501-1. Not to be used in external walls of relevant buildings. Submit declaration of performance. - 04
Vapour control: warm side or cold side
MoistureMost multifoils have foil outer layers that are close to vapour-tight, so wherever one goes it behaves as a vapour control layer. On the warm side of the insulation that can be useful, provided the joints are taped. On the cold side a vapour-tight layer can trap moisture in the structure. Breathable multifoils with perforated or permeable outer layers are made for that position, and some are certified to act as the breather membrane too. The declared vapour resistance of the actual product goes into the BS 5250 condensation risk assessment.
How to decide
- Warm side: a taped multifoil can serve as the VCL if its certificate says so.
- Cold side: only a breathable multifoil with a declared vapour resistance.
- Run the condensation risk assessment with the declared value, not a generic one.
Write it asVapour control: Multifoil taped as the vapour control layer on the warm side, where the certificate permits. Submit condensation risk assessment to BS 5250. - 05
Certificates, registered details and their scope
AcceptanceBecause a multifoil's performance in use depends on the build-up around it, a certificate or registered detail is only as useful as its scope. A certificate for a pitched roof over rafters says little about the same product inside a timber frame wall. Warranty providers and building control bodies look for third-party evidence, such as a BBA certificate or an LABC registered detail, that covers the construction as drawn. If the drawn build-up sits outside that scope, the U-value has to be justified another way, often with more conventional insulation.
How to decide
- Read the scope: element, orientation, air space widths and companion insulation.
- Match the drawn section to a certified or registered build-up, or ask the manufacturer to calculate yours.
- Send the evidence to building control before ordering.
Write it asThird-party certification: Current certificate or registered detail covering this wall build-up. Submit before ordering.
The values under "Write it as" are examples. The right value depends on the location, the use and the fire strategy.
Which standards should the clause cite?
Cite the product standard the declaration of performance refers to, the code of practice for installation, and the test method behind each value you set. A value without its test method cannot be checked against a submittal.
- BS EN 16012
- Thermal insulation for buildings. Reflective insulation products. Determination of the declared thermal performance
How reflective and multifoil products declare core thermal resistance, surface emissivity and thickness under a stated load.
In the clause "Core thermal resistance and emissivity declared to BS EN 16012."
- BS EN ISO 6946
- Building components and building elements. Thermal resistance and thermal transmittance. Calculation methods
The U-value method, including the thermal resistance of unventilated air spaces bounded by low-emissivity surfaces.
In the clause "U-value calculated to BS EN ISO 6946 for the build-up as drawn."
- BS EN ISO 8990
- Thermal insulation. Determination of steady-state thermal transmission properties. Calibrated and guarded hot box
Laboratory measurement of heat transfer through a whole build-up, including its air spaces.
In the clause Cite it where hot box evidence is asked for: "Build-up tested to BS EN ISO 8990. Submit report."
- ISO 9869-1
- Thermal insulation. Building elements. In-situ measurement of thermal resistance and thermal transmittance. Heat flow meter method
On-site measurement of a wall or roof. Some manufacturers use it as supporting evidence alongside declared values.
In the clause Not a substitute for declared values; cite it only if in-situ testing is a project requirement.
- BR 443
- Conventions for U-value calculations
The BRE conventions UK U-value calculations follow, including how air spaces and reflective surfaces are treated.
In the clause "U-value calculation to BS EN ISO 6946 following the conventions in BR 443."
- BS EN 13501-1
- Fire classification of construction products and building elements. Classification using data from reaction to fire tests
The Euroclass declared for the product as supplied.
In the clause "Reaction to fire: Euroclass as declared to BS EN 13501-1."
- BS 5250
- Management of moisture in buildings. Code of practice
Condensation risk where a vapour-tight multifoil sits within a wall or roof.
In the clause "Condensation risk assessed to BS 5250 using the product's declared vapour resistance."
Confirm the current edition of each standard at every issue. Avoice's State of UK Specifications 2026 found a withdrawn standard cited in 40% of specifications at contract issue.
For the regulations behind these choices, see Part L and the Future Homes Standard, Approved Document B explained, or search the Avoice Regulations library.
How do you write it in CAWS?
In CAWS, multifoil is a product clause in P10. NBS Source lists multifoil for timber frame walls against clause 212 Insulation fixed across inner face of stud walling, which suits the common arrangement of mineral wool between studs and a multifoil across their inner face, with a battened service void in front. The timber frame type clause describes the whole wall and refers to the insulation clauses.
The multifoil clause carries the evidence of performance, the air spaces and how they are formed, and the joints. It relies on clause 190 for the insulation between studs, clause 310 if a separate vapour control layer is used, and clause 320 for the breather membrane on the outside. Lining boards on the battens are written in K10.
- Location
- Timber frame external walls, across inner face of studs
- Insulation
- Multifoil blanket, type I06
- Manufacturer
- Submit proposals
- Product reference
- Submit proposals
- Evidence of performance
- Core thermal resistance and emissivity declared to BS EN 16012
- Certificate scope
- TBC Manufacturer has not confirmed that its certificate covers this wall build-up. Resolve before ordering.
- Air spaces
- 25 mm unventilated air space each side, formed by treated softwood battens, ends closed
- Wall U-value
- Calculated to BS EN ISO 6946 for the build-up as drawn; submit
- Joints
- Overlapped and taped with manufacturer's tape
- Vapour control
- Taped multifoil as the vapour control layer where the certificate permits; otherwise as clause 310
- Reaction to fire
- Euroclass as declared to BS EN 13501-1; not for external walls of relevant buildings
- Insulation between studs
- As clause 190
- Services
- In batten void; penetrations through multifoil taped
- Lining
- Plasterboard on service battens, as section K10
Illustrative, not from a real project. One property is left as a TBC with a reason so it can be tracked and closed before issue.
Clauses the type clause relies on
- P10/190
- Insulation fitted between studs The insulation between studs that the multifoil works with.
- P10/310
- Air and vapour control layer A separate VCL where the multifoil is not certified to act as one.
- P10/320
- Breather membrane The membrane on the cold side of the sheathing.
Clause numbers vary between NBS versions and practice masters. Clause 212 is the number NBS Source gives for multifoil fixed across stud walling; check that the clauses it refers to exist in your own P10.
How do you write it in Uniclass?
Uniclass puts the multifoil inside the wall system it belongs to. Ss_25_10_32_90 Timber wall framing systems describes studs, sheathing, insulation between and across the studs, air spaces and lining, and the U-value the whole wall must meet. The multifoil is a product clause under Pr_25_57_06_57.
The air spaces belong in the system clause, because they are made by the battens and lining rather than by the product. That keeps them visible: a later change to the lining or the service void is a change to the system that affects the U-value, not a detail left to site.
- Description
- Timber frame external wall, mineral wool between studs, multifoil across inner face
- Studs
- As structural drawings
- Insulation between studs
- As mineral wool product clause
- Multifoil
- Pr_25_57_06_57 Multifoil blanket insulation (I06)
- Air spaces
- 25 mm unventilated, battened, both sides of multifoil
- Sheathing and membrane
- As product clauses
- Wall U-value
- As calculation for the certified build-up
- Manufacturer
- Submit proposals
- Product reference
- Submit proposals
- Product type
- Multifoil blanket for warm-side use
- Core thermal resistance
- Declared to BS EN 16012
- Emissivity of outer faces
- Declared to BS EN 16012
- Thickness
- Declared, with the compression it was measured at
- Vapour resistance
- Declared value
- Certificate scope
- TBC Manufacturer has not confirmed that its certificate covers this wall build-up. Resolve before ordering.
- Reaction to fire
- Euroclass as declared to BS EN 13501-1
Illustrative. Performance of the finished system sits in the system clause; properties of the product sit in the product clause.
In NBS Chorus the multifoil product clause also carries the NBS reference 45-45-70/360. Use whichever referencing your practice masters use, but do not mix CAWS and Uniclass references in one specification.
What goes in the spec, and what goes in the schedule?
The specification defines each type once. The schedule and drawings say where each type goes.
In the specification
What the product is and how it performs and is installed. Written once per type.
In the schedule or drawings
Where each type goes and how much of it. Changes with every plan revision.
Write each multifoil build-up once and let the wall and roof types schedule say where it goes. A roof and a wall are different build-ups, even with the same product.
What are the common mistakes?
Most problems are a missing property, a property filled with the wrong kind of value, or a decision left silently open.
- An equivalence claim used as the thermal value. "Equivalent to 200 mm of mineral wool" is not a declared value. Ask for BS EN 16012 figures and a calculation.
- Foil fixed tight to the lining. With no air space the low-e face does almost nothing. Battens are part of the specification.
- A vapour-tight multifoil on the cold side. It traps moisture in the structure. Use a breathable type there, or move it.
- Air spaces left open at the ends. Moving air carries the heat away. Close them at every junction.
- A certificate for a different element. A roof certificate does not cover a wall. Check the scope against the drawn section.
- Multifoil in the walls of a tall residential building. Cavity contents are part of the external wall. On a relevant building they must be A2-s1,d0 or A1.
How Avoice helps
Avoice's spec writer drafts the multifoil clause from your wall and roof types and keeps the evidence questions in front of the team.
- Drafts from your drawings and schedules
It reads the wall and roof types from your sections and schedules, writes the P10 clause or the Uniclass timber frame system, and links the multifoil product clause to each type that uses it.
- Leaves a TBC where the evidence is open
If the manufacturer has not confirmed its certificate covers your build-up, the line stays TBC with the reason, and it goes into the project TBC register to close before tender.
- Writes in your format and your style
CAWS or Uniclass, adapted from your practice's own master clauses.
- Checks the clause against everything else
QA/QC flags a multifoil clause on a wall type used above 18 m, a section drawing with no battens where the clause counts on air spaces, or a withdrawn standard. Approved Documents B and L are in the Avoice Regulations library for checking the requirement itself.
- Keeps product data in one place
The DoP, certificate and U-value calculation sit in the Product Room linked to the clause. Edit the linked product once and every type that uses it updates.
Related guides: How to specify mineral wool insulation, How to specify vapour control layers, How to specify breather membranes.
Software can draft and check. The designer still chooses the product and is responsible for the specification.
Draft your multifoil insulation blanket clause in Avoice
Upload your drawings and schedules. Avoice drafts the clause in CAWS or Uniclass, cites the standards and flags anything your information does not settle.
This guide is for general education and is not professional advice. Check current editions of standards and Approved Documents, the manufacturer's data, and the requirements of your project before relying on any clause.