The R-value of a material describes its thermal resistance — how much the material inhibits the transfer of heat. The higher the R-value, the more effective the level of insulation.

Insulation - terminology

 Material R-values are supplied with bulk insulation and refer to the insulating value of the product alone.

System R-values should be supplied with all types of insulation for intended applications. This is the actual peformance level that can be expected when the insulation is installed.

R-values can differ depending on the direction of heat flow through the product. The difference is generally marginal for bulk insulation but can be pronounced for reflective insulation:

• Up R values describe resistance to heat flow upwards (sometimes known as 'winter' R values).
• Down R values describe resistance to heat flow downwards (sometimes known as 'summer' R values).

• How are they measured?

  R-values can be measured depending on the direction of heat flow (upward or downward). They’re known as ‘up R-values’ (resistance to upward heat flow, also called winter R-values) and ‘down R-values’ (resistance to downward heat flow, also known as summer R-values).

  Foil-insulated systems tend to have higher down R-values. In hot, humid climates where houses are naturally ventilated, high down values and lower up values are appropriate for roofs and ceilings.