Dispelling a Few SPF Thermal Performance Myths
Efficiency and R value of Spray Polyurethane Foam is often misunderstood. This article dispels myths about spray foam R values and long-term thermal reduction. The topics covered and the explanations gives insight to implications that affect insulation quality such as quality of equipment, contractor's installation procedures, and variations among material manufacturers. Issues involving current laboratory testing procedures to determine spray foam R Value are described in detail. Finally, the "law of diminishing returns" as it relates to thickness of foam roof material is described clearly and concisely at the end of the article.
Below: due to the law of diminishing returns, adding more than 6 inches of sprayed foam insulation is almost completely futile.
Why SPF Performance is Hard to Measure
SPF performance is difficult to measure for several reasons. First: The quality of the product is in some ways unpredictable due differing manufacturers formulations, the skill of the applicator, and the installation conditions including temperature, humidity, wind, and the substrate surface. Secondly, spray foam R-value performance diminishes over time on a sharp curve that flattens out at the six month mark. Initial spray R-values are astronomically high, but these subside within 6 months, at which time the losses almost completely flatten out, staying above 6 in R-value. Third, SPF performance is hard to measure because of ASTM testing methods and also due to the fact that there are no long term thermal resistance LTTR regulations or requirements. In effect, lack of a regulated benchmark to measure against prevents manufacturers and contractors from delivering a consistent standard. These factors all provide most of the reasons why Sprayed Polyurethane Foam (SPF) performance is hard to measure.Spray Foam Product Quality is Unpredictable
Polyurethane Spray Foam is unique, in that the product is manufactured on site by the installer. Depending on the skill of the application team, varying results are possible. There are also numerous formulations and manufacturers. Foam varies in performance with each formulation and manufacturer. And to complicate things even further, install conditions including humidity, temperature, substrate, in addition to factors such as thickness also effect the end result. All in all, there are too many inconsistent variables to accurately make blanket predictions about spray foam performance, which gives the system a degree of unpredictability in regards to performance. With all these variables in mind, it’s possible there are formulations of foam that will not lose R-value sharply over time. This article simply refers to most foam formulations, which do lose R-value sharply over time.Spray Foam R-Values Diminish Sharply Until 6 Month Mark
After spray foam is installed, some of the bubbles which are holding blowing agent, slowly absorb air from the outside. These cells lose a slight amount of R-value, however slight. Outside air reaches each successive layer of cells exponentially slower and slower, until the middle layer is reached after a substantial amount of time (or never). The end result is decreasing R-values from the install date forward, although the losses mostly stop at about the 6 month mark. In fact, an equilibrium between air and blowing agent is nearly reached at the one month mark, and from there forward changes are relatively slow. ASTM testing is done at the 6 month mark because very few changes happen after this date. The difference between foam which is 6 months old and foam which is five years old is about 5% to 8% loss in R-value. (See figure above). Notice that the chart becomes nearly flat after 6 months; this demonstrates how R-value losses, due to breaking air bubbles becomes almost nil after 6 months.ASTM Testing Methods Don’t Completely Match Real World Conditions
ATSM testing requires the ‘skin’ to be removed from spray foam while testing, on all sides (top, bottom, etc). This removes a layer of cells that would otherwise be insulating in a real world environment, substantially lowering the R-value of the foam. The foam tested must also be 6 months old; this will greatly underestimate the R-value of new foam, however, this will be accurate for a foam roof at 6 months or more. With the skin removed on all sides, the test results are an accurate prediction for foam which has reached an age of 15 to 20 years.Long Term Thermal Resistance (LTTR) Measures are Unregulated
The debate centers are whether the miniscule changes between R-values at 5, 10, 15, 20 and more years are worth measuring. Generally, R-values are nearly the same between, 5, 10 and 15 years, and between 15, 20, 25, and 30 years (over any 10 year period after year 5, the differences in R-value are very small). Are the differences between these R-values important? Some argue that, however small these changes are, that yes, they are important. On the other side of the argument, there is no standardized way of testing or benchmarking foam LTTR values. Even if both sides agreed that LTTR testing was important, there is no current standardized method available currently to measure this.Conclusion: Real World Results are What Matter Most
Insulation laboratory test results are only as accurate and worthwhile insofar as they accurately predict the performance of a material. With foam this is difficult for several reasons. First, foam is unique as an insulation product because it is literally manufactured on site. The quality of foam varies greatly by manufacturer, applicator, and installation conditions. Foam R-values are not static early on (making them very difficult to measure); however, R-values do stabilize around the 6 month mark; and this is when ASTM testing is done.However, in some ways ASTM testing is unfair or inaccurate. Layers of sealed foam cells, or the ‘skin’ is removed on all sides of the foam before testing, substantially lowering resulting R-values. The test is a good predictor of SPF foam performance at 15-20 years, but it underestimates the R-value of spray foam leading up to this time. Finally, a lack of LTTR testing standards for SPF foam means that to some extent, the industry remains unregulated. Without clear regulations, end results are more like to vary. In the end, those real world results are what matters most.