Bulk insulation mainly resists the transfer of conducted and convected heat, relying on pockets of trapped air within its structure. Its thermal resistance is essentially the same regardless of the direction of heat flow through it.
Bulk insulation includes materials such as glass wool, wool, cellulose fibre, polyester and polystyrene. All bulk insulation products come with one material R-value for a given thickness.
Bulk insulation traps air in still layers.
Reflective insulation mainly resists radiant heat flow due to its high reflectivity and low emissivity (ability to re-radiate heat). It relies on the presence of an air layer of at least 25mm next to the shiny surface. The thermal resistance of reflective insulation varies with the direction of heat flow through it.
Reflective insulation is usually shiny aluminium foil laminated onto paper or plastic and is available as sheets (sarking), concertina-type batts and multi-cell batts. Together these products are known as reflective foil laminates, or RFL.
Dust settling on the reflective surface greatly reduces performance. Face reflective surfaces downwards or keep them vertical. The anti-glare surface of single sided foil sarking should always face upwards or outwards.
The total R-values for reflective insulation are supplied as ‘up’ and ‘down’ values. Total values depend on where and how the reflective insulation is installed. Ensure system values provided by the manufacturer relate to your particular installation situation.
Composite bulk and reflective materials are available that combine some features of both types. Examples include reflective foil faced blankets, foil backed batts and foil faced boards.
The properties and uses of some common insulation materials are summarised at the end of this article.