Thermal resistance value indicates the insulating performance of a material. The higher the R or RSI value, the more resistant that material is to heat or cold transfer—meaning, the better the insulation it is.
The R value corresponds to the imperial system, while the RSI value is its equivalent in the metric system.
This is not the case for all insulations made of plastic. In fact, insulation that contains gases other than air during their manufacturing process can see their insulating value drop over time.
In this sense, Underwriters Laboratories Canada (ULC) has developed the CAN / ULC-S770 standard, in which all foam plastic insulation that are composed of a blowing agent other than air, such as extruded polystyrene (XPS), polyisocyanurate (PIR) insulation and spray polyurethane (PUR), must be tested for long-term thermal resistance (LTTR).
According to this same standard, in Canada, long-term thermal resistance value (LTTR) must be used in concept and assembly designs.