Polyisocyanurate, or PIR, is a plastic used for foam insulation board in building construction. It has superior thermal and flame retardant properties compared to other insulation materials, with a minimum R-value of 5.6-8 per inch. PIR is similar to polyurethane but has a higher concentration of MDI and passes the FM 4450 fire test without thermal barriers. However, it undergoes thermal drift, reducing its insulating properties by about 20% within the first two years of use.
Polyisocyanurate is a form of plastic also known as polyiso or PIR, which is chemically related to polyurethane plastic (PUR). It is used for many of the same applications as polyurethane, but primarily applied as a form of foam insulation board in building construction. It can also be obtained for use as a liquid or sprayed foam that is blown into walls or crawl spaces to fill air cavities as a thermal insulator. As one of the newest insulation materials on the market as of 2011, it has become the preferred insulation material in 60% of all commercial construction in the United States and Canada due to several characteristics that the material possesses which make it superior to other types of isolation.
Insulating materials are often placed in interior wall locations where electrical wiring also runs, and this requires them to have flame retardant and thermal properties. Polyisocyanurate has a melting point of over 392° Fahrenheit (200° Celsius). When used as a foam board, it is a material made up of tiny closed cells that contain hydrochlorofluorocarbon (HCFC) gas, which is non-flammable, inert, and a very good insulator.
These structural and heat resistant properties give polyisocyanurate a minimum R-value for insulation of between 5.6 and 8 per inch, with a standard sheet of 2 inches thick. R-value is an industry-standard method of measuring resistance to heat flow, and the R-value of polyisocyanurate is generally equivalent to that of polyurethane foam, but far superior to other forms of insulation. Polystyrene insulation has an R-value of approximately 4.3 with a sheet equivalent thickness of 3.1 inches, fiberglass beating an R-value of 3.3 with a thickness of 4 inches, and plywood an R-value of 1.25 with a thickness of 10.9 inches.
Polyurethane plastic and insulating foam have been manufactured since the 1930s, when they were first used by the military and aerospace industries. It became commercially popular in the 1970s, and polyisocyanurate hit the market in the United States and Europe in the late 1970s. Both types of foam insulation board use thermal barrier coatings of plastic sheet or laminate or other materials such as gypsum or perlite to increase their ability to retard heat loss and prevent the spread of fires. The key difference between the two materials is that polyisocyanurate passes the Factory Mutual Calorimeter (FM 4450) fire test without such thermal barriers, whereas polyurethane does not.
Both types of foam are also thermosetting plastics, which means that once they are made or sprayed into place, they take on a rigid shape that cannot be reformed and restored by melting. Another similarity between PIR and PUR is that they are products from the reaction of methylene diphenyl diisocyanate (MDI) with polyol compounds. However, polyisocyanurate has a higher concentration of MDI in the final product than polyurethane.
A potential negative feature of using polyisocyanurate is that, as it ages, it undergoes a process known as thermal drift. This means that, within the first two years of use, part of the HCFC gas contained in the cellular structure escapes into space and is replaced by the normal atmosphere, reducing the insulating properties of the foam by about 20%. HCFCs are also potential ozone-depleting gases, although they have a much weaker effect on the ozone layer than the chlorofluorocarbon family of gases that have been banned from use worldwide.
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