Q. Is it True that the R-value of Closed-Cell Foam Diminishes Over Time?
A. Closed-cell is made up of millions of tiny cells filled with a blowing agent developed by Honeywell called HFC-245fa. Gases other than air in the cells, known as “captive blowing agents,” can increase the thermal resistance of the foam. However, the 245fa agent leaks out of the cells over time and is replaced by air, thereby decreasing the R-value. This phenomenon is known as “thermal drift” and applies to all foams that incorporate captive blowing agents. This thermal drift can be ongoing for years, even decades. While closed-cell foam is required to state an aged R-value, the foam is only aged 90-180 days prior to testing. Many years later the final aged R-value can be much lower than what was intended. Many studies have been conducted to determine what the resulting R-value is after a number of years, but the results can be difficult to ascertain because so many variables come into play. In short, in the absence of an accepted method to determine this aged R-value, the National Roofing Contractors Association believes that users should select an R-value of about 5.5 per inch when calculating thermal performance for closed-cell foam. This is well below the stated R-values provided by most manufacturers. This creates a quagmire when designing a flash-and-batt system and determining its long-term performance. (Open-cell foam simply uses water as the blowing agent and the cells are filled with air to create its insulating value – thus always ensuring a stable and continuous R-value).