All About Insulation

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There are three types of rigid foam insulation: (1) expanded polystyrene (EPS); (2) extruded polystyrene (XPS) and (3) polyisocyanurate, also called polyiso. Although most rigid insulation is made from various polymers (foam plastics), builders also can purchase rigid insulation made from basalt rock and slag (the same materials used to make some mineral wool insulation products) from Roxul ( www.roxul.com ).

Expanded polystyrene (EPS), also known as beadboard, is the least environmentally damaging product in the rigid foam board family because it is not manufactured using ozone-depleting chemicals. Most people are familiar with EPS, as it is used to make coffee cups and packing beads for shipping.

EPS is made from polystyrene beads mixed with liquid pentane, a hydrocarbon used as a "blowing" or expansion agent, which expands to form millions of tiny bubbles in the finished product. The agent diffuses out of the foam, and the spaces fill with air. Air is a poor conductor of heat, so the millions of tiny bubbles effectively block heat transfer through the foam. (In Insulfoam, steam, rather than pentane, is used to produce air spaces.)

EPS can be molded into large sheets with R-values ranging from about 3.8 to 4.4 per inch, depending on the density of the material. However, air spaces in EPS can accumulate and retain water. Because water is a good conductor of heat, some form of moisture barrier may be required to prevent this problem, especially when EPS is used around foundations. To make the product more durable, waterproof, and thus suitable for in-ground use, EPS is now being manufactured with thin-foil and plastic facings.

Also known as blueboard, extruded polystyrene (XPS) is a closed-cell insulation board made from polystyrene and a hydrochlorofluorocarbon (HCFC) expansion agent. HCFCs are much less damaging to the ozone layer than their chlorofluorocarbon (CFC) predecessors, but this new generation of blowing agents still releases chlorine atoms when exposed to sunlight, destroying ozone molecules in the stratosphere. Whereas a molecule of CFC destroys 100,000 ozone molecules, a molecule of HCFC only destroys about one-fifth as many—20,000 molecules.

XPS is more expensive than EPS, but it has a slightly higher R-value, about R-5 per inch. XPS also tends to be more consistent in density, has a higher compressive strength than EPS and is much more resistant to moisture.

Manufactured using HCFC-141b, polyisocyanurate is the least environmentally friendly foam-board insulation. Although less damaging to the ozone layer than CFCs, HCFC141b is the worst of the new generation blowing agents. Several companies now offer HCFC-free polyiso, in anticipation of HCFC's eventual phase-out.

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