Multi-layer insulation blankets have recently been added to NBS section P10. NBS Technical Author, Charles Stirling, strips back the layers to reveal his findings on their uses and efficiency.

Introduction

A wide range of multi-layer reflective foil insulation systems has been launched onto the product specification market over the past few years. Of particular interest to the construction industry have been claims that relatively thin layers of composite insulation can provide enhanced thermal performance. These systems rely, principally, on the low emissivity, reflective nature of metal foils to reflect long wave radiation back into the building.

Ideally suited, you would think, to applications where shallow roof spaces or narrow wall cavities have to incorporate both ventilation and thermal insulation. In addition, some products have a high vapour resistance and these can be utilized as vapour control layers.

Reflective foil technology was originally introduced in warmer climes, where there is a need to reduce overheating from radiant sunshine.

Product range

Multi-layer reflective foil insulation systems, typically, fall into three discrete categories:

Reflective foils: These include reinforced metal (usually aluminium) foils or plastics membranes laminated or coated with a reflective treatment. Uses include:

  • Sarking underlay
  • An additional layer placed over conventional horizontal roof insulation
  • Impermeable membrane below solid concrete ground floor slabs
  • Suspended ground floor insulation draped over joists
  • Wall insulation suspended in vertical cavities
  • Insulated wrapping to ducting.

In addition to thermal insulation applications, reflective foils are also used as vapour control layers, and as fire protection around structural members and to protected areas.

Laminated membranes: Composite, flexible materials comprising a series of laminations which typically include aluminium foil; polyethylene sheet; mineral wool fleece; encapsulated polyethylene sheet (‘bubble wrap’) or polypropylene membranes. The thickness of these laminated membranes can range from 3−30 mm. Uses include:

  • Sarking underlay
  • As an additional layer placed over conventional horizontal roof insulation
  • Suspended ground floor insulation draped over joists
  • Wall insulation suspended in vertical cavities
  • Insulated wrapping to ducting.

Laminated boards: Reflective foil laminated to 25−150 mm thick, rigid polyurethane, expanded or extruded polystyrene, or mineral wool boards or quilt. Uses include:

  • Rigid and flexible sarking insulation
  • Ground floor insulation placed between joists or laid below solid concrete slabs
  • Wall insulation placed between masonry leaves or structural framing (timber or steel)
  • Wrapping to ducting.

Rigid boards have the advantage of being self supporting, i.e. they can be fixed to structural components to form rigid or semi-rigid infill panelling.

Thermal performance

Some debate has ensued, within the UK construction sector, over claims by multi- layer foil manufacturers that the thermal performance of relatively thin laminated membranes can equate to that of much thicker layers of conventional (mineral wool) insulation.

Conventional calculations of insulation value are based on the thermal conductivity or resistance of the insulant; i.e. conductive heat losses, based on the long established ‘hot box’ testing procedures. In contrast, reflective insulants are marketed on their ability to reflect long-wave radiative energy thus achieving low levels of heat transfer. A detailed review of the effectiveness and appropriateness of these test methods and approaches has recently been published by the Department for Communities and Local Government (Multi-foil Insulation; Report no. BD2768, June 2009).

The efficiency of the reflective foil is dependent upon a low surface emissivity (a measure of surface radiation), i.e. the higher the polish the better the performance. However, as with conventional insulation products where thermal conductivity can be impaired by moisture and compression, emissivity can be degraded by dust, contamination and tarnishing of the surface finish. Therefore, the long term performance within a construction context may be difficult to assess and predict.

Constructional detailing

Multi-layer insulants are recommended for use in a range of construction related applications; their installation typically relying on the need for an adjoining cavity facing the surface reflective foil. In some situations a cavity is required on both faces. In addition to providing a degree of thermal insulation, those products which have a high vapour resistance may also function as a vapour control layer; requiring joints, laps and penetrations in the multi-layer insulant to be effectively lapped and sealed. Where these high vapour resistance products are utilized at rafter level, sufficient provision for ventilation, in accordance with BS 5250, is required.

Where laminated membranes are draped within cavities, care must be taken on site to ensure that an effective gap or void is retained between adjoining construction elements. Construction operatives are more accustomed to fully filling cavities and voids with insulation and particular care must be taken to ensure that membranes are sufficiently taut and that they do not make contact with adjoining layers or subsequently fitted linings.

Laminated board products provide their own rigidity and are typically less likely to suffer from draping or sagging.

Joints, laps and penetrations in both membrane and board systems must be effectively taped and sealed as most systems rely on complete air tightness for maximum performance. Particular care must be taken to ensure continuity of seal at wall/floor and wall/roof interfaces and around openings.

Conclusions

The reflective technology integral to multi-layer insulation can assist in reducing thermal losses from buildings. Given this alternative approach to thermal performance, specifiers now need to consider surface emissivity and thermal resistance when specifying thermal insulation products. Many manufacturers have third party accreditation for their products that should be scrutinized; all manufacturers produce detailed guidance that should be carefully followed.