Open- and closed-cell spray foam both insulate by trapping air or gas in a plastic matrix. But that’s where the similarities end.
Open-cell foam costs slightly less for the same thickness, but offers lower R-value. The blowing agent used to install open-cell insulation is water, which reacts with air to become carbon dioxide. Because CO2 expands quickly, the bubbles tend to burst before the plastic sets — hence “open cell” — producing a spongy, lightweight foam.
The industry describes the foam as “half-pound” material, which means the foam weighs 0.5 pounds per cubic foot. This density yields an R-value of approximately 3.6 per inch, equivalent to most traditional insulations.
Open-cell foam tested in accordance with ASTM E 283 provides an air barrier with vapor breathability. Because the open cell structure allows some vapor to pass through, open-cell foam is a good choice in hot, humid climates, and under roof sheathing, such as in conditioned attics, where water vapor caught between insulation and sheathing could promote wood rot.
Open-cell has about twice the noise reduction coefficient in normal frequency ranges as closed-cell foam. Because the water blowing agent in open-cell insulation dissipates as it sets instead of slowly over time, there is no degeneration of the R-value.
Unlike open-cell foam, closed-cell foam uses liquid chemical blowing agents. These gasses expand as they are applied, but not as quickly as CO2, allowing the polyurethane plastic to set before the bubbles burst. This yields dense foam weighing nearly 2 pounds per cubic foot and without the capillary characteristics of open-cell, it remains impermeable. The blowing agents perform like the inert gasses between the panes of high-performance windows, adding to the insulating qualities of the foam. Unlike open-cell foam, closed-cell foam rarely requires any trimming, with little or no jobsite waste.
Closed-cell’s obvious advantages compensate for its slightly higher price tag (20% to 30% for the same thickness). It provides both a vapor and an air barrier, and offers an aged R-value of a whopping 6.5 per inch. Due to its dense, glue-like consistency, it remains very strong, increasing the racking strength of walls by as much as 300%, according to the National Association of Home Builders Research Center. (These structural properties are not acknowledged by current building codes, so you can’t reduce structural bracing.) Because water doesn’t penetrate or degrade the product, the Federal Emergency Management Agency recommends closed-cell for flood regions.
Due to its higher cost, if you do not require the extra vapor barrier, structural strength, and R-value per inch that closed-cell provides, you may be wasting money.
Although they no longer threaten the ozone layer, today’s closed-cell foam blowing agents approach a global warming potential (GWP) of 1,000 versus 1 GWP for water-blown open-cell foam. A recent Environmental Building News article referenced computer modeling results showing that HFC-blown closed-cell spray foam can have GWP payback periods of 30 to 60 years at thicknesses from 1 to 4 inches; cellulose payback is less than one year.
Manufacturer BASF counters that the performance advantages of this type of foam in reducing home energy consumption outweigh imbalances at the manufacturing and installation stage. BASF says a third party–verified Eco Efficiency Analysis of its closed-cell foam (factoring in the production, use, and disposal phases) shows that cellulose has more than twice the GWP of foam over its life cycle; the GWP for fiberglass was four times greater over its life cycle.
Several chemical companies also are expected to introduce fourth-generation ozone-safe blowing agents in the next few years, with a radically improved GWP of 15. The EBN article acknowledges that this would change the equation.
—Fernando Pagés Ruiz develops ecologically minded affordable housing in the Midwest and mountain states.