LTTR is a critical standardized test method (ASTM C1303 / CAN/ULC S770) specifically designed to predict the aged R-value of foam plastic insulation that uses gaseous blowing agents. The test involves:
- Aging thin specimens of the foam under controlled laboratory conditions (at specific temperatures) to accelerate the diffusion of blowing agents.
- Periodically measuring the thermal resistance over a minimum of 180 days.
- Using a mathematical model to extrapolate the data and calculate a 15-year time-weighted average R-value.
This LTTR value is the mandatory rating used for energy code compliance and performance specification in North America and increasingly elsewhere. It provides a realistic, conservative estimate of in-service performance, unlike the initial, higher “as manufactured” R-value.
