Advantages and disadvantages of spray polyurethane foam

Spray-Foam Insulation
Discover the advantages and disadvantages of spray polyurethane foam-- a high-performance material discovered in the walls and roofing systems of many of today's energy-efficient homes.

Spray-polyurethane-foam insulation is more pricey than other kinds of insulation-- fiberglass batts or blown cellulose, for example-- and it calls for professional setup. So, why do some contractors and house owners swear by it? In a word, performance.

Spray polyurethane foam can be found in 2 variations: open-cell and closed-cell. Both types fill cracks and crevices in walls and ceilings much more successfully than batt insulation, quickly flowing around pipes and wires to produce a reliable air barrier as it cures.

Spray foam is made at the job website from 2 chemicals, the "A" and "B" sides. The two mix at the nozzle of the spray gun, foaming as the elements are united. Installers wear protective clothing and a respirator as they work with it.

Open-cell foam is vapor permeable
Open-cell foam is the lighter and more economical of the two. With an R-value of in between R-3.5 and R-3.6 per in., open-cell foam weighs about 1/2 pound. per cubic ft. Although costs vary, intend on $1 to $1.20 per sq. ft. when filling a 2 × 4 stud cavity.

Installers normally over-fill a stud or rafter bay with the foam and trim back the excess once the foam has treated. This leaves the cavity completely filled.

Open-cell foam makes a good air barrier, but it's vapor permeable. That means water vapor can move through the foam even when the bulk motion of air is obstructed. This ends up being an essential factor to consider when the foam is sprayed in between rafters on the underside of the roofing sheathing to develop a conditioned attic in a cold climate. In the winter season, moisture in the attic air can make its method through the foam and gather on the back side of the sheathing-- a possible mold and decay problem.

In this case, a separate vapor retarder or vapor-retarding paint must be installed over the foam.

Closed-cell foam is a vapor barrier
Closed-cell foam has a much higher R-value than open-cell foam-- about R-6.5 per inch-- and it's a vapor barrier in addition to an air barrier. (One maker, Demilec, states its Heatlok High Lift spray foam has an even higher R-value-- R-7.5 per in.) Closed-cell foam is much denser, about 2 lb. per cu. ft., and it's untouched by water. It forms a tough, dense insulating layer and enhances wall and ceiling cavities structurally.

Closed-cell foam also is significantly more expensive than open-cell foam, costing between $1.75 to $3 per sq. ft. in a 2 × 4 cavity. That's money well spent in some applications. In a wall or roofing where there's limited space for insulation, for example, closed-cell foam uses outstanding thermal performance in a thinner layer than a lot of other kinds of insulation.

Cured closed-cell foam is much more difficult to cut than open-cell foam, so installers do not overfill stud and rafter bays. It's way too much work cutting it back after the foam has cured.

Installers likewise have to be careful not to spray too much foam in a single pass, or "lift." The reason is that the foam develops heat as it cures-- an exothermic reaction. If the foam is too thick, it can catch on fire. Although this is unusual, it has actually happened, and improperly used foam has been blamed for several house fires.

Ensure you know what you're getting
With its really high R-values, closed-cell foam would appear to be a perfect insulation for exterior walls. Simply think: In theory, filling a 2 × 6 stud cavity yields an R-value of 37.75, practically double what a standard-density fiberglass batt would provide.

However, as Green Building Advisor editor Martin Holladay describes in this article, the truth is various. First, the installer will probably not bring the foam all the way out to the edge of the stud. He's more most likely to leave a 1/2-in. buffer so he will not need to cut anything back later. Now the insulation layer is 5 in., not 5-1/2 in.

Then there is the "framing element"-- that part of the wall that isn't insulation: studs, headers, and top and bottom plates. When the R-value of wood (about R-1.2 per in.) is figured into the formula, the whole-wall R-value is more like R-15.4. That's only R-1.9 more than you 'd get with open-cell foam, however at a much higher expense.

Environmental factors to consider
Regardless of some undeniable performance advantages, some home builders and designers will not use spray foam. Supporters of foam-free construction don't like the petrochemical origins of spray foam, or the possibility, nevertheless remote, that incorrectly blended foam will develop a chemical risk or lingering smells in the house.

In the case of closed-cell foam, there is the matter of the blowing agent—the chemical additive that gives foam its froth and its high R-value. Open-cell foam utilizes water or carbon dioxide as the blowing agent, but the industry-standard blowing agent for closed-cell foam has actually been a hydrofluorocarbon (HFC) with a worldwide warming capacity (GWP) some 1300 times higher than CO2.

With the dangers of global warming and climate change becoming better understood, this alone is enough to make some designers run the other method. However, a next-generation blowing agent, Solstice, developed by Honeywell, is a chemical, a hydrofluoroolefin. It has a GWP of 1 or less, making spray-foam solutions that use it no more hazardous in that respect than open-cell foam.

The spray-foam market is slowly switching over to this more recent blowing agent, and the Spray Polyurethane Foam Alliance anticipates the transition to be complete at some point in the next year or more.