Multi foil insulation materials are made from metalised foils sandwiched with various wadding and foam materials. They claim a thermal performance equal to that of 200mm of mineral fibre (a thermal resistance of at least 5 m²K/W) however this value is based on a manufacturer’s own test regime and only gives a comparative value rather than an absolute value.
None of these products comply with internationally recognised manufacturing standards and the associated quality assurance procedures. For example our PUR and PIR boards comply with harmonised European Standard BS EN 13165 which is the basis for CE Marking, independent assessment under the Keymark Scheme and all of our quality procedures.
There have been doubts cast about the validity of the claims made for the thermal performance of these products. Tests have been carried out, both independently and by at least one multi foil manufacturer, in accordance with the recognised standard, BS EN ISO 8990, The Guarded Hot-Box Method. These tests give a thermal resistance value of between 0.6 m²K/W and
1.9 m²K/W, the actual value depending on whether there are airspaces on one or both sides of the material.
Only one multi foil product is currently approved by the British Board of Agrément (BBA), the most widely recognised certification body in the UK. The thermal resistance of the product, with unventilated airspaces both sides, is 1.69 m²K/W. The lack of certification on other products can lead to problems with Building Control Officers, we have heard of several instances of multi foil insulants having to be removed from site and expensive replacement works undertaken.
As an alternative to laboratory measured values the Building Research Establishment has undertaken in-situ tests, in accordance with ISO 9869. These tests conclude that the claimed thermal resistance of 5 m²K/W is considerably over estimated and that values of between
0.6 and 1.7 m²K/W are more realistic. These values are in agreement with both the laboratory and BBA measured values. Click here to view the BRE report.
The revised Approved Documents (effective 6th April 2006) refer to a number of 2nd Tier documents including BRE document BR443, Conventions for U-value Calculations (available to download at the BRE website). This document specifically states that the thermal performance of multi foil insulants must be measured using either:
1. BS EN 12664 or BS EN 12667 (Guarded Hot Plate) for the core material to which is added the effect of the low emissivity facings.
2. BS EN ISO 8990 (Guarded Hot Box) for the overall resistance of the core plus the low emissivity facings.
When calculating the U-value in accordance with BS EN ISO 6946 and BR443 the effect of the thermal bridge effect of timber battens should be included – this effect is very important as the thermal resistance of the insulation is minimal, perhaps even zero, at this point since:
· The low emissivity reflective foil element of the material is reduced to zero as there is no airspace.
and
· Compression of the foam and wadding materials reduces the resistance of the base material to almost zero.
Instead of a claimed U-value of 0.16 W/m²K or 0.20 W/m²K the above test and calculation methods would actually give a U-value in excess of 0.50 W/m²K!
In an attempt to clarify matters Local Authority Building Control bodies have been issued with guidelines in respect of multi-foil insulation - LABC Technical Guidance Note 06/001b. This means that under the April 2006 Building Regulations the thermal resistance of multi-foil insulation materials must be measured in accordance with BR443, i.e. for calculation purposes a thermal resistance of 1.7 m²K/W must be used. The Guidance Note may be viewed here.
The National House Building Council, NHBC, has gone a step further and totally banned the use of multi foil insulation materials until further notice. Their comments may be viewed here.
In conclusion multi foil insulants simply do not perform as claimed. Under the Approved Documents (2006) they do not achieve the limiting (worst case allowable) U-value for a pitched roof of 0.35 W/m²K or achieve the area weighted U-value of 0.25 W/m²K.
The manufacture of these materials is not covered by recognised British or European Standards and therefore they cannot be CE Marked or be subject to rigorous and independent quality checks that we at Recticel voluntarily subscribe to.
