Aged R-Value (a/k/a Long-Term Thermal Resistance or LTTR)
Extruded Polystyrene Rigid Foam Insulation products (XPS) have a closed cellular structure and are manufactured with a blowing agent specifically selected for its ability to facilitate the XPS extrusion manufacturing process and which enhances the thermal performance of the foam.
Over a long period of time (50 to 75 years), the blowing agent s l o w l y diffuses through the thickness of the foam, and air s l o w l y diffuses into the closed cellular structure replacing the blowing agent. The result of this gas movement is the overall thermal resistance (R-value) of the XPS insulation changes over time.
This phenomenon is typically called ‘aging’.
Accurately identifying the aged R-value of XPS is important for a variety of reasons:
- Designers need accurate thermal performance data to determine the heating and cooling loads for homes and commercial buildings.
- Insulation products are compared to one another based on thermal performance
The Canadian standard, CAN/ULC S770-03, defines the long-term thermal resistance (LTTR) of foam insulation products as the 5-year R-value after aging in a laboratory environment. LTTR is a property which may be used to compare performance characteristics of different foam insulation products to one another.
The table below lists the required minimum LTTR values for extruded polystyrene foam insulations when tested according to CAN/ULC S770. Note, the XPS industry recommends using the design aged R-value for XPS insulation thicker than 25 mm (1 inch).