Low Energy House » Multifoils - Multifoil Insulation
Proprietary multifoils incorporate double and multiple layers of metalised film, fibrous materials and cellular plastics. Their low emissivity surfaces resist the passage of radiant heat and the materials in the core resist the convection and conduction heat
Multiple layer products fall into two categories:
- Double layer foil products have a low emissivity (low-e) facing bonded either side of thin foamed polyethylene or bubble sheet core
- Multifoil insulations consist of multiple low emissivity layers separated by a variety of very thin layers of wadding or foam
To achieve the levels of thermal insulation required by new dwellings, double and multifoil layers are generally used alongside bulk insulation products.
Multifoils – Multiple Combinations of Insulating Materials
Multifoils consist of several sheets of metalised film separated by an insulating core. A range of multifoil insulation products are manufactured using different combinations of thin plastic foam, plastic bubble sheet and non woven fibrous wadding with plain and reinforced aluminium foil.
To be effective, the outer layers must face an unventilated cavity in the construction. The whole assembly may also contribute towards the airtightness of the building if it is installed with tightly sealed joints and quilt edges, using proprietary tape provided for the purpose.
Multifoils and the Unventilated Air Space
The condition of the airspace is important because it is recognised that for low emissivity facings to deliver their best performance they must face an unventilated airspace. For optimum performance, the unventilated airspace should have low emissivity surfaces on both sides of the void.
The amount of thermal resistance provided is also dependent on the depth of the airspace and the direction of the heat flow. If a low emissivity layer is not adjacent to an air space then there will be no reduction of radiant heat transfer and subsequently no thermal resistance benefit. If the airspace is ventilated then the thermal resistance benefit is substantially reduced.
Multifoils - Location of Reflective Coatings
Locations where reflective layers are fitted next to unventilated spaces
- Partial-fill in masonry cavity walls
- Room in the roof and other traditional roofs insulated at rafter level
Testing Multifoils
The thermal performance offered by some reflective foil products is a matter of some controversy. Where very low values of emissivity are claimed, emissivity values should be confirmed by third party accreditation organisation such as the British Board of Agrément (BBA).
The benefits of using multifoils in rafter situations are that they: save space, are easy to cut and fit, produce minimum waste and are lightweight, durable and quick to install
