Rigid Foam

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Rigid foam can be used in walls, roofs, and foundations, for retrofits or new construction. Most varieties of foam have a higher R-value per inch than fiberglass, cotton, or cellulose.

There are three main types of rigid foam insulation: expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate. Key differences are R-value per inch, water resistance, compressive strength, permeability to water vapor, facings, and of course, cost. All foam insulation products are petroleum-derived.

Rigid foam sheets are sold in several thicknesses; most lumberyards carry insulation ranging from 1/2 in. to 2 in. thick. Thicker sheets (up to 6 in. thick) are usually available by special order.

If you are looking for a substitute for rigid foam insulation that isn't made of plastic or petroleum, you might want to consider using cork insulation panels or semi-rigid mineral wool panels.

Rigid foam can solve thermal bridging problems
Because a stud connects the inside of a house to the outside, it can act as a bridge for heat to escape (studs have lower R-values than insulation). Thermal bridging through the studs significantly degrades the thermal performance of the wall. In all climates, exterior foam sheathing improves a wall's performance.

Installing rigid foam insulation over wall or roof framing reduces this thermal bridging, raises the R-value of the wall or roof assembly, and can eliminate or reduce air leaks. Building scientists Joseph Lstiburek and Peter Baker have reported that adding 1 in. of R-5 insulation to a 2x6 wall insulated with fiberglass batts increases the effective R-value of the wall from 14.4 to 19.4—a 35% gain with only a 15% increase in wall thickness. Adding 2 in. of foam raises the R-value from 14.4 to 23.8, an improvement of 65%.

A layer of insulating foam on the outside of exterior walls also helps the framing stay dry by raising the dew point of the surface where water vapor is likely to condense. Remember, though: Thick foam sheathing is safer than thin foam sheathing. To learn more about determining a safe thickness for exterior foam, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

Rigid foam also is very useful for insulating the interior or exterior of foundation walls. In this case, the best practice is to apply rigid foam directly to the concrete, keeping air away from the cool and damp surface and lowering the risk of condensation. To meet fire codes, interior foundation foam will probably have to be covered with a layer of gypsum drywall.

Installing foam on a wall is straightforward
Foam wall sheathing is usually attached to studs or OSB sheathing with cap nails. Foam up to 3/4 in. thick can be attached with a pneumatic cap stapler like the Bostitch SB150SLBC.

If the wall will include vertical strapping to create a rainscreen, the foam sheathing can be tacked in place with just a few fasteners, as the strapping will be screwed into the studs through the foam. For more information on installation techniques, see How to Install Rigid Foam Sheathing.

Foam can be attached to a concrete wall with specialty fasteners like Styro Tapit fasteners or adhesives like PL300 Foamboard Adhesive or Handi-Stick Polystyrene Construction Adhesive.

BLOWING AGENTS ARE AN ENVIRONMENTAL ISSUE

Polyiso has the most benign blowing agents, while those used for XPS are the worst
In the foam manufacturing process, a blowing agent creates tiny bubbles in the foam. These bubbles slow the flow of heat the same way tiny air pockets slow the flow of heat in a fiberglass batt.

Extruded polystyrene (XPS) and polyisocyanurate (polyiso) rigid foam insulation used to be made with ozone-depleting CFC blowing agents, but when ozone depletion was identified as a major environmental problem, the CFCs were replaced with HCFCs through international agreement.

Although HCFCs are more environmentally benign than CFCs, they still cause some damage to the earth’s protective ozone layer. HCFCs were eliminated from polyiso insulation in 2003 — polyiso is currently produced with hydrocarbon blowing agents — but they are not scheduled to be totally eliminated from XPS until 2020. Most XPS manufactured in the U.S. is still blown with HCFC-142b; European manufacturers have converted to non-ozone-depleting blowing agents. Only a few specialized XPS insulation materials in the U.S. are currently produced without HCFCs.

The blowing agents currently used to manufacture XPS in the U.S. are hydrofluorocarbons

(HFCs) with a high global warming potential. Because the global warming potential of these damaging blowing agents is 1,430 times more potent than carbon dioxide, many green builders avoid the use of XPS. For more information, see Calculating the Global Warming Impact of Insulation.

Expanded Polystyrene

Expanded polystyrene (EPS), also known as beadboard, is usually white. It is commonly used for coffee cups and coolers. EPS is the most commonly used insulation in structural insulated panels (SIPs) and insulating concrete forms (ICFs).

EPS can be manufactured in different densities. Although higher densities of EPS have a greater compressive strength than lower densities, EPS is never as strong as XPS; EPS is more susceptible to crumbling at the edges and to other job-site damage. That’s why EPS is rarely used for wall sheathing.

The R-value of EPS depends on its density, with higher-density foams having higher R-values. The R-value range is from about 3.6 to 4.2 per inch.

EPS is the least expensive and most vapor-permeable of the three types of rigid foam; 1 in. of EPS has a permeance of 2.0 to 5.8 perms, making it a semi-permeable material. EPS can absorb more water than either of the other two types, from 2% to 4% by volume; that’s why XPS is usually preferred for below-grade applications. However, high-density EPS can be used below grade.

Extruded polystyrene

Extruded polystyrene (XPS) is stronger, smoother, denser, more water-resistant, and more expensive than EPS. It’s also a better thermal insulator, rated at R-5 per in. XPS is less vapor permeable than EPS. One inch of XPS has a permeance of 1.1, while 2 inches have a permeance of 0.55, making XPS a semi-impermeable material.

Because of its high compressive strength and water resistance, XPS is often used below grade to insulate slabs and foundation walls.

The two most common brands of XPS are Dow Styrofoam, which is blue, and Owens Corning Foamular, which is pink.

For green builders, XPS has two major strikes against it: it contains the flame retardant HBCD, and its blowing agents have a very high global warming potential. For these two reasons, most green builders try their best to avoid the use of XPS.

Polyisocyanurate

Polyiso has higher insulating values (R-6 to R-6.5 per in.) than XPS or EPS at warm temperatures; however, when temperatures drop, the performance of polyiso worsens. For more information on the the cold-weather performance of polyisocyanurate, see In Cold Climates, R-5 Foam Beats R-6.

Because its manufacture doesn’t require the use blowing agents that deplete the ozone layer or contribute to global warming, polyiso board is considered the most benign type of rigid foam from an environmental perspective. However, because it can absorb water, polyiso is not recommended for use under slabs or on the exterior of foundation walls.

Polyiso often comes with a foil facing. One inch of foil-faced polyiso has a permeance of 0.03 perm, which is quite low. When used as exterior sheathing, foil-faced polyiso creates a vapor barrier on the outside of the building; that means that an interior plastic vapor barrier should never be used on a polyiso-sheathed wall.

Foil-faced polyisocyanurate is more resistant to ignition than unprotected XPS or EPS. For this reason, some (but not all) building inspectors allow foil-faced polyisocyanurate to be left exposed on crawlspace walls or in attics without requiring a layer of drywall as a thermal barrier.