Read More About Pentane-Blown Polyiso and LTTR
In 2002 the U.S. Environmental Protection Agency published its ruling on acceptable substitutes for ozone-depleting substances in the foam blowing sector. Pentane has since become the blowing agent of choice in the manufacture of polyisocyanurate (PIR) foam. Different blends of pentane isomers are used by different manufacturers, however. Since pentane isomers each have different physical properties it is important to note that the characteristics of different PIR brands may vary.
Yet the industry has gained sufficient knowledge and experience to form general conclusions that debunk some of the misconceptions that still exist - - particularly surrounding R-factors and Long Term Thermal Resistance (LTTR). For example, many engineers and end-users of polyiso do not realize that the published LTTR values for a particular polyiso insulation may not be applicable to low-temperature insulation applications. This is because LTTR is measured at "ambient" temperatures (within conditions that could reasonably reflect roofing application, for instance). At lower temperatures, such as those experienced by chill water or liquid natural gas insulation, the diffusion rate of gases in and out of the cells is materially reduced, and at cryogenic temperatures virtually non-existent. Hence the decay in R-factor that LTTR is meant to reflect is likely not representative.
Other factors that affect the actual long-term insulating value of polyiso insulation include the application of facings, vapor retarders, and the application (such as in pipe insulation where the inside of the insulation is not exposed to air. These factors are exceedingly difficult to quantify yet can have appreciable affect.
It may be helpful to recap that LTTR became an issue when it was discovered that certain foam insulations such as polyurethane, polyisocyanurate, and extruded ploystryrene lost some insulating value over time. The amount of the R-factor reduction varied with the chemistry, thickness, and application of the foam, but a ballpark reduction over the life of the foam could be typified as 15% - - sometimes more sometimes less. Since the majority of the degradation in R-value appeared to occur in the first six months, the industry began using a measured R-factor after aging the foam for six months. This reduction in insulating value occurred as the insulating gases within the closed cells diffused out and were replaced by air, which has higher (poorer) thermal conductivity. Research has demonstrated that during the first eight months or so following manufacture (assuming ambient temperatures) this process is dominated by CO2 diffusion out, combined with air infiltration. It is during this initial phase that the vast majority of R-value degradation occurs. During the 8 to 15 month period the diffusion rate of CO2 levels off and the diffusion of pentane out of the cells can be detected. Beyond 15 months the reduction in R-value is exceedingly small.
Another common misconception about pentane-blown polyiso is that, since pentane has higher thermal conductivity that HCFC-141b (a previously used blowing agent) the resulting foam necessarily has lower R-value. The fact is that it may - - or it may not. Atlas Roofing Corporation, for instance, has stated that their pentane-blown polyiso boardstock has the same R-value as their previous generation of polyiso. There are several reasons why pentane's higher conductivity does not necessarily result in lower R. For one thing, as described above, the insulating value of polyiso foam is dominated by CO2 in the cells, with much less impact from pentane. Also, there are structural properties of a pentane-blown foam that offset any reduction caused by the pentane gas within the cells: 1) the cells in pentane-blown foam are smaller than 141b-blown foam, making energy transport across the foam more difficult, 2) the closed cell content is generally higher, and 3) the lower solubility of pentane and stronger cell walls may have influence.
More on the subject in later newsletters!