The Open-Cell vs. Closed-Cell Debate

In most closed-cell foams, such as those made by Corbond®, an HFC blowing agent is captured in the foam’s cell structure. This gas has a better thermal performance than the air-filled open-cell foam and gives it a higher overall R-value. However, while HFC — blown closed-cell foam might initially have an R-value as high as R-8 per in., its R-value diminishes as the blowing agent evaporates through cell walls and is replaced by air. Closed-cell foam’s "aged" R-value is roughly R-6 per in. Some manufacturers produce water-blown closed-cell foams. These foams have the same performance properties as HFC-blown foam, but slightly lower R-values, at around R-5.5 per in.

Closed-cell foam’s greater density, 2 lb. per cu. ft. compared with open cell’s 1/2 lb. per cu. ft., also increases its R-value and offers it the rigidity that open-cell foam lacks. Tests at the National Association of Home Builders research center confirmed that closed-cell foam can actually increase the shear strength of conventionally framed walls by 30%. Closed-cell foam also has a low vapor-permeability rating (roughly 0.5 perms at a thickness of 3 in.) and is con­sidered a class-II vapor retarder, meaning it’s semi-impermeable.

OPEN CELL

Density: Vi lb. per cu. ft. R-value: 3.5 per in.

Vapor permeability: Permeable Air barrier: Yes Blowing agent: Water

Open-cell foam, made by companies such as Icynene® and Demilec®, has a greater expansion rate than closed-cell foam. It expands 100 times its initial volume (closed­cell foam expands only 30 times its initial volume), so less of the foam is needed to insulate a house.

Open cell’s one major weakness is its lower R-value, roughly R-3.5 per in. This means that when used in a 2×4 exterior wall, it will create an assembly that’s approxi­mately only R-12, which won’t meet code in most parts of the country.

Installing Lots of Foam Isn’t as Effective as You Think

A lot of energy-conscious architects and builders shoot for the highest R-values they can possibly attain: R-40 walls and an R-60 roof. However, R-values aren’t necessarily an accurate reflection of overall thermal perfor­mance. For example, you would think that an R-40 wall full of spray foam would per­form twice as well as a wall sprayed to R-20 with the same foam, but that’s not the case.

Chris Porter, the building-science and code manager for BioBased Insulation®, explains that "open-cell foam reaches a point of diminishing returns at around
5 in. That threshold is even lower for closed­cell foam, which experiences diminishing returns at around 3 in. or 4 in." Those thick­nesses create assemblies between R-20 and R-24, which by the numbers seem a little weak. Each additional inch of spray foam

More Than One Way to Use Spray Foam: Two Experts Weigh In

Most experts agree that spray polyurethane foam is a revolutionary product. What they don’t always agree on is the way it’s installed and integrated into a building assembly. To shed some light on this debate, energy-efficient building expert Bruce Harley (Westborough, Mass.) and architect Peter Pfeiffer (Austin, Texas) explain how they use spray polyurethane foam to insulate the homes they build.

PETER PFEIFFER No other insula­tion system I am familiar with provides the real R-value that spray foam does, ac­complishes the air-sealing it does, or thwarts vapor flow as well. Closed-cell spray foam greatly re­duces the chance for condensation within the framing of a home. I think it is critical that houses be built to thwart vapor flow correctly. I insulate all homes pretty much the same way. However, in colder climates, I use 2×6 exterior walls and insulate the basement or crawlspace. Peter Pfeiffer of Barley & Pfeiffer Architects is a LEED-accredited architect and building scientist who has spent the past 30 years developing high — performance building-design strategies.

R-17 WALLS

R-19 BASEMENT

Bruce Harley of Conservation Services Group is an energy-efficient construction expert and author of Cut Your Energy Bills Now (The Taunton Press, 2008).

Fiber-cement siding Vapor barrier extends 12 in. up wall and is secured with mastic.

Damp proofing

Consuming fossil fuels to make products intended to conserve fossil fuels makes little sense to a lot of people. All spray foams contain a certain level of petroleum in their A component and in their B component. Manufacturers such as BioBased Insulation, Demilec, and Icynene have created more environmentally benign spray-foam products by reducing the amount of petroleum used in their B component. They replace a portion of the polyol resin, which makes up 20% to 30% of the B component, with a renewable resource such as soybean or castor-bean oil. Apex even has a sucrose-based polyol. Manufacturers say that the transition to bean oil or sucrose doesn’t alter the look or the performance of open — or closed-cell foam in any way.

The amount of soybean, castor bean, or sucrose found in foam varies by manufac­turer, so identifying the “greenest” foam might not be so easy. According to the U. S. Department of Agriculture, only 7% of a spray-foam product needs to be made of a renewable resource to be labeled as a bio­based foam. This, of course, doesn’t factor in the petroleum fueling the crop-cultivation process.

yields little performance. In fact, while the cost of an R-40 wall is indeed double that of an R-20 wall (not factoring in the construc­tion materials used to create deeper cavities for the extra foam), it reduces the heat flow through a wall by only an additional 2%.

For this reason, Porter says that in most parts of the country, 6 in. of foam—be it open or closed cell—is perfectly adequate.

Spray foam is priced based on board feet. Manufacturers don’t price their product. Instead, cost is determined by installers. The spray-foam market is extremely competi­tive, and spray-foam prices can be astonish-