In short, open-cell foam, tested in accordance with ASTM E 283, provides an air barrier with vapor breathability. Water-blown solutions have less environmental impact than the current HFCs used for most closed-cell spray-foam insulation. And open-cell has about twice the noise reduction coefficient in normal frequency ranges as closed-cell foam. Because the blowing agent in open-cell insulation dissipates as it sets, instead of slowly over time, there is no degeneration of the R-value—a minor point given aged closed-cell R-values still trump open-cell R-values by a magnitude of nearly 100%.

Unlike open-cell foam, closed-cell foam uses chemical blowing agents that come in liquid form and become gasses as they are applied. These gasses expand, 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 used 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 has more obvious advantages over open-cell, and a slightly higher price tag (20% to 30% for the same thickness). It provides both a vapor and air barrier and offers an aged R-value of a whopping 6.5 per inch. Because of its density and glue-like consistency, it remains very strong, providing both compressive and tensile strength to structure comparable to added sheathing, increasing the racking strength of walls by as much as 300%, according to the NAHB Research Center. Because water does not penetrate or degrade the product, FEMA recommends closed-cell foam as a suitable insulation material for flood regions.

The principle disadvantage of closed-cell foam comes with overkill. If you do not require the extra vapor barrier, structural strength, and R-value per inch, then you may be wasting money. As for the added wall strength, while real and substantial, it’s not acknowledged by building codes currently, so you can’t reduce the structural bracing as a tradeoff.

Environmental Tradeoffs

With the phase outs of chlorine-based chemicals, third-generation blowing agents such as Forane’s HFC-365mfc and Honeywell’s HFC-245fa have become widely used in closed-cell foam, lowering concerns about ozone depletion. Although it no longer contains ozone-depleting chemicals, closed-cell foam still poses a high global warming potential (GWP). The average impact of today’s closed-cell foam blowing agents approaches 1,000 GWP versus 1 GWP for water-blown open-cell foam.

A recent article in Environmental Building News argues against using HFC-blown closed-cell insulation due to its high GWP. In computer modeling referenced by EBN, the results showed that closed-cell spray-foam insulation using HFCs can have lifetime GWP payback lengths of as much as 30 to 60 years with insulation depths from 1 to 4 inches, versus less than one year in all cases for cellulose.

Manufacturer BASF says that the article doesn’t take into account the air tightness achieved in a home with closed-cell spray-foam insulation, arguing that the comparative performance advantages of this type of foam in reducing home energy consumption over the lifetime of the product outweigh imbalances at the manufacturing and installation stage. The manufacturer says a third party–verified Eco Efficiency Analysis of its closed-cell polyurethane spray foam (which factored in the production, use, and disposal phases) shows that cellulose insulation has more than twice the GWP than its closed-cell product over its lifecycle; the GWP for fiberglass was four times greater over its lifecycle.