Air Leakage Through Spray Polyurethane Foam

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Air Leakage Through Spray Polyurethane Foam

How thick does a layer of spray foam insulation has to be to qualify as an air barrier?

Posted on Sep 25 2013 by Martin Holladay

Many builders use spray polyurethane foam as an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both., raising the question: How thick does the spray foam layer have to be to stop air flow? There's a follow-up question, of course: Is the answer different for open-cell spray foam than for closed-cell spray foam?

As with most building science questions, there is a short answer and a long answer. The short answer is that closed-cell spray foam needs to be at least 1 or 1.5 inch thick to act as an air barrier, while open-cell spray foam needs to be between 3.0 and 5.5 inches thick to act as an air barrier.

We’ll get to the longer answer after some presenting some definitions and data.

What’s the definition of an air barrier?

Most building codes define an air barrier material as a material that has an air leakage rate below 0.02 liters/sec-m² @75 Pa (0.004 cfm/sf @ 1.57 psf) when the material is tested according to either ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. E2178 or ASTM E283.

This definition was adopted by the 2009 International Residential Code as the basis for the code definition of an “air-impermeable insulation.”

Joseph Lstiburek, a principal at Building Science Corporation in Westford, Massachusetts, told me that he helped develop this definition. “The .02 number was based on a suggestion from Gus Handegord,” Lstiburek told me. “He suggested that an air-barrier material should be defined relative to drywall. We tested drywall and the air leakage rate for drywall was a bit under .02. So that was the basis of the standard.” (For more information on the development of this definition, see Is OSB Airtight?)

Differences between open-cell spray foam and closed-cell spray foam

The main difference between open-cell spray foam and closed-cell spray foam is foam density. Open-cell spray foam usually has a density of 0.5 or 0.6 pounds per cubic foot, while closed-cell spray foam has a density of 2.0 to 2.4 pounds per cubic foot.

Of course, spray foam manufacturers have developed foams with a wide range of densities, including densities that fall between the two extremes of open-cell spray foam and closed-cell spray foam. But the open-cell vs. closed-cell spray foam distinction is still useful, since most residential spray foams fall into one of these two categories.

To generalize, closed-cell spray foam is expensive, dense, and vapor-impermeable. It has a relatively high R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. per inch (about R-6 to R-6.5 per inch).

Open-cell spray foam is less expensive, less dense, and more vapor-permeable. It has a lower R-value per inch (about R-3.7 per inch).

Check the Evaluation Service Report

Most insulation manufacturers have their products tested by a third-party testing laboratory. These lab reports are then evaluated by the International Code Council Evaluation Service (ICC-ESThis is the International Code Council Evaluation Service. ICC-ES is a non-profit public benefit corporation that evaluates building products, issuing final reports on code compliance of building products and materials. These reports on then made available at no charge to the building community at large.). This type of evaluation results in a published document called an Evaluation Service Report, or ESR. Most ESRs are published online.

With a little bit of Googling, you should be able to find the ESR for any spray foam product that you are considering. If you can’t find it online, contact the manufacturer and ask for a copy.

The ESR for any spray foam product should include the results of air leakage testing performed in accordance with ASTM E283 and ASTM E2178. These are the results you’re looking for if you want to know the minimum thickness of spray foam that needs to be applied to achieve an air barrier.

The results of this testing are usually reported in a sentence with this format: “The insulation, at a minimum thickness of X inches (Y mm), is considered air-impermeable insulation in accordance with 2012 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. Section R806.5 and 2009 IRC Section R806.4, based on testing in accordance with ASTM E283 and ASTM E2178.”

The data reported in these ESRs confirm that you need a thicker layer of open-cell spray foam than closed-cell spray foam to create an air barrier.

Here are the results of air leakage tests performed on several brands of spray polyurethane foam, as reported in Evaluation Service Reports commissioned by the manufacturers:

This table above shows why you can’t use open-cell spray foam when using the flash-and-batt approach to insulate walls. When installed in walls as part of a flash-and-batt job, spray foam is usually installed at a thickness of either 1 inch or 2 inch. That’s too thin for open-cell spray foam to perform as an air barrier.

Is closed-cell spray foam a “better” air barrier than open-cell spray foam?

The data summarized in the table above should not lead readers to conclude that closed-cell spray foam is a better air barrier than open-cell spray foam.

In fact, either type of spray foam is just as good as drywall at stopping air — as long as the foam is at least as thick as the minimum dimensions shown in the table. Installed in a thick enough layer, either type of spray foam will stop virtually all air movement, even at a pressure difference of 75 pascals.

Why are so many of these ASTM tests performed with 3.5 or 5.5 inches of foam?

In a phone conversation about spray foam manufacturers’ reporting of ASTM E283 and ASTM E2178 test results, building scientist John Straube, a professor of Building EnvelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. Science at the University of Waterloo, pointed out that the builders who consult the ESRs shouldn't jump to conclusions.

Straube explained that when spray foam manufacturers report that (for example) 1.5 inch of spray foam, or 5.5 inches of spray foam, is an air barrier, that doesn't necessarily mean that the manufacturer is reporting the thinnest possible layer of material that would meet the definition of an air barrier.

“The chosen thickness for testing has nothing to do with airtightness,” Straube said. “Manufacturers choose the dimensions of the spray foam based on the thickness of foam that can be reliably applied. Airtightness is almost always an afterthought. They don't worry about the reported thickness too much, because it passes.”

I asked Straube whether he agreed that, broadly speaking, it takes a thicker layer of open-cell spray foam than closed-cell spray foam to create an air barrier.

“Yes, I agree,” Straube said. “You need more open-cell than closed-cell. Closed-cell foam really does have closed cells. Well over 90% of its pores are not connected, so air can’t flow through it. Air can only flow through the pores that are open. XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. and polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. are similar products — similar to closed-cell spray foam — and all these foams have good airtightness numbers at a thickness of 3/4 inch and 1/2 inch. They are all tight.

“However, open-cell foam is different. The pores are larger, and many of the pores are interconnected. It slows the air down. So to create an air barrier, the required thickness is more.”

BSC’s Thermal Metric testing

Three years ago, the Building Science Corporation began publishing the results of its Thermal Metric Research Project, a multi-year effort to measure how air leakage affects heat flow through walls.

The test results included some surprises, including the apparent finding that walls insulated with closed-cell spray foam were leakier than walls insulated with open-cell spray foam, especially at cold temperatures. How could these results be explained?

One of graphs published in a article on the Thermal Metric Project (“Air Leakage Degrades the Thermal Performance of Walls”) has been cited as evidence that open-cell spray foam may do a better job of reducing air leakage than closed-cell spray foam. That graph (reproduced below) shows the increase in heat flow attributable to air leakage in five different types of walls.

The increase in heat flow is greater in the wall insulated with closed-cell spray foam (Wall 6) than in the wall insulated with open-cell spray foam (Wall 5) — apparently indicating that the closed-cell spray foam wall is leakier. When tested under a 10 Pascal pressure difference, the heat flow through the open-cell spray foam wall was 16% higher, and the heat flow through the closed-cell spray foam wall was 23% higher, than when the same walls were tested without any air flow.

In an e-mail discussion of these results, Dana Dorsett, a longtime GBA subscriber who contributes regularly to online discussions, wrote, “To be sure, both types of foam outperformed all of the fiber insulation tested, but a 23% performance hit is decidedly worse than a 16% performance hit. In fact, the performance hit for the damp-sprayed cellulose wall was essentially identical to that of the closed-cell foam wall (24% instead of 23%, which is probably within the measurement error of the equipment).”

“It’s a bee’s fart”

I telephoned Straube and Chris Schumacher, a principal at Building Science Laboratories in Waterloo, Ontario, to discuss these results, as well as to ask questions about air leakage through open-cell and closed-cell spray foam. The three of us chatted in a conference call.

Schumacher said, “It's hard to say whether there is a meaningful difference between the open-cell spray foam wall and the closed-cell spray foam wall. Is the difference more than the noise? Maybe — but only just.”

At that point, John Straube chimed in. “It’s a bee's fart. If one more person opens or closes the door that day, that difference is gone. It's not that significant.”

Leakage through spray foam is so low it's hard to measure

In the spray-foam-insulated walls that were tested as part of the Thermal Metric project, the air wasn't leaking through the foam; it was leaking through cracks in the wall assemblies — for example, cracks between doubled framing members. Schumacher and Straube agreed that either 1 inch of closed-cell spray foam or between 3.5 and 5.5 inches of open-cell spray foam is an effective air barrier.

"The air leakage rate through a foam material is so low that a variation of 30% [from one brand to another] doesn’t have any meaning," Straube said. "It isn't easy to distinguish between these materials. You can’t hold test labs accountable for differences in reported air leakage if one tests at 0.012 and another at 0.017. You can't believe that that difference is meaningful. It is more likely due to extraneous leakage in the apparatus. The leakage is so low that it is tricky to measure.”

The leaks occur elsewhere

I asked Straube, “What if a builder in a hot climate wants to insulate a wall with the flash-and-batt method? How much spray foam should the builder install to get a good air barrier?”

Straube responded, “I advise builders to target 2 inches of spray foam, since up to 2 inches can be installed in one pass. Once you have to do a pass, you may as well install a reasonable amount of foam. If you ask for 2 inches, you will have an optimum use of the labor required to install it. So aim for 1 1/2 to 2 inches. That said, you don’t need 2 inches for an air barrier — 1 inch would work. You don’t get leaks through the spray foam material in any case. The leaks are elsewhere.”

What should spray foam installers worry about?

I asked Schumacher about what advice he would give to a spray foam contractor who was installing spray foam as an air barrier, other than the need to address obvious stuff like the crack between the bottom plate and the subfloor, and cracks between double studs. Schumacher bristled at my use of the word “obvious.”

“You talk about ‘all the obvious stuff,’ but let me tell you, when I work with builders, their assumption is that the wood-to-wood joints are insignificant sources of air leakage. We had to demonstrate that these leaks are a problem to a majority of builders. Maybe these leaks are obvious to your readership, but they aren't obvious to most builders.”

Straube explained, “If you are sealing leaks with spray foam, the major worry is whether the materials are dusty or wet. You have to worry about dirt and moisture. We've found that spray foam almost never seals well to steel, because steel often has oils on it from the manufacturing process. The oil doesn’t allow foam to seal to the steel. With concrete, the quality of the seal depends on whether there is dust, or residue from a form-release agent. At a commercial building, if the contractor is attempting to seal the spray foam to the concrete, I’ll say, ‘Is the concrete clean enough?’ They'll say yes, because maybe they swept it. But they didn’t vacuum it or wipe it with something wet — so there is still dust.”

What about the choice between open-cell spray foam and closed-cell spray foam?

When it comes to air leakage, Schumacher said, “what really matters is the experience of the installer and the quality of the installation — not the choice between open-cell and closed-cell.”

Martin Holladay’s previous blog: “Quality Issues With Brick Buildings.”

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Image Credits:

  1. Rick Duncan, Spray Polyurethane Foam Alliance

Sep 25, 2013 11:20 AM ET

The flexibilibity matters
by Dana Dorsett

In the Thermal Metric Project testing done by the Building Science folks, the leakage of the wall sealed with closed cell foam varied by a factor about two over the tested temperature range of 0F to 108F, whereas the leakage of assembly sealed with open cell was essentially the same at both ends of the range. The closed cell wall was tighter than open cell at one of the temperature extremes, but dramatically leakier at the other.

There can be alternative possible explanations of that, but it's likely that the rigidity of the closed cell foam and the differences in the coefficients of expansion of the foam and the structure wood may have been expanding and contracting the leakage at those "obvious" leakage points such as the bottom plate or between doubled top plates, whereas the flexibility of the open cell gave it enough mechanical compliance that the size of those leaks didn't change much with temperature. It would be possible to test that hypothesis, but it's not clear that it matters.

Bottom line is that for air-sealing an assembly it's both cheaper and greener, and at least as effective to use open cell foam rather than closed cell foam. An inch of the typical 2lb closed cell foam has as much polymer as 3-4" of 0.5-0.7lb open cell, and most closed cell foam installed in north America uses HFC blowing agents with a far heavier environmental impact than the blowing agent for open cell foam (water.)

Use of closed cell is only necessary in stackups where it's lower water vapor permeance is an important part of the moisture management aspect of the design (which can usually be designed out.)

Sep 25, 2013 11:35 AM ET

Response to Dana Dorsett
by Martin Holladay

I agree with your most important point: that open-cell spray foam is less environmentally damaging than closed-cell spray foam, and should be chosen by green builders who need to use spray foam (at least for those applications where open-cell spray foam is appropriate).

Because open-cell spray foam is associated with damp roof sheathing when the foam is sprayed against the underside of roof sheathing, I think that builders should be cautious before using open-cell spray foam in this location. If anyone wants to build an unvented insulated roof assembly, it's always better to install rigid foam above the roof sheathing than any type of spray foam on the underside of the roof sheathing.

To return to the questions raised by the Thermal Metric Project results, I spent quite a bit of time talking with Straube and Schumacher on this issue, proposing different ways that open-cell spray foam might be considered less leaky than closed-cell spray foam at cold temperatures. Straube and Schumacher strongly resisted the implications that meaningful differences in leakage were occurring. As Straube noted, "it's a bee's fart."

The discussion was complicated by Schumacher's insistence that the first round of testing -- the testing that was used to create the graph reproduced here -- had results that displeased the manufacturer of the open-cell spray foam. That manufacturer insisted that the open-cell spray foam wasn't installed properly, and Schumacher eventually agreed that there may have been problems with the wall assembly. (Problems like that never happen on a job site, right? But that's another issue entirely...)

As a result of that discussion, the open-cell spray foam wall was disassembled and/or removed, and a new assembly was created for a second round of testing of the open-cell spray foam wall. Schumacher no longer stands behind the results of the first round of open-cell spray foam testing. Curiously, though, Schumacher had no explanation for why a wall with poorly installed open-cell spray foam might have performed better, not worse, than a wall with closed-cell spray foam.

Sep 25, 2013 3:09 PM ET

Responding to future comments
by Martin Holladay

I'll be on vacation from Sept. 26 to Oct. 6, so I won't be able to answer any questions or comments posted here (or for that matter, on any other GBA page) until I'm back in my office on Oct. 7.

That said, I hope the dialogue continues, and that there is a healthy collection of comments for me to read when I return.

Sep 25, 2013 5:16 PM ET

I agree the differences are small...
by Dana Dorsett

The differences in air tightness of the ocSPF and ccSPF assemblies were small, and the difference between the ccSPF wall and the cellulose wall was even smaller (and that's with damp spraying, not dense packing!) From a practical point of view it's irrelevant, but from an academic inquiry point of view the anomalies are of interest.

The curious factor that the ccSPF leakage changed with temperature, whereas the ocSPF leakage did not isn't a reason to choose one over the other. At these leakage levels with the different temperature dependencies of leakage with different cavity insulation are purely academic- the major leakage in the assemblies was not through the insulation, but through other parts of the framing, which masks the actual leakage through the foam itself. But understanding the root of that phenomenon better may still provide insight.

Clearly vapor retardency is a deciding factor in some assemblies, as noted. Air tightness alone is not a guarantee that there will be no moisture problems- it's necessary, but not sufficient.

Since spray foam is always applied against some other material, it's really the combined air-tightness of the substrate + foam that's relevant. Cheap half-inch OSB may not meet the definition of an air barrier per the ASTM E2178 test, and an inch of open cell may not either. But an inch of open cell sprayed onto OSB almost certainly would. And the air leakage of a wall air-sealed with a flash-inch of open cell on an OSB wall would, as in the Thermal Metric Project assemblies would be primarily at the framing joints outside of the insulation cavity. It's actually kind of silly to be testing slabs of spray foam without the relevant substrate, since it's the air tightness of the assembly that matters the most, not the absolute air retardency within some arbitrary layer.

Sep 25, 2013 5:29 PM ET

Edited Sep 25, 2013 5:31 PM ET.

Sealing seams vs. sealing seives
by Charlie Sullivan

[Edit: this is a long-winded way of saying some of the same things that Dana says in the post just above, which I didn't see before I posted]

In the test, a free-standing sheet of foam is tested to see whether it woks as an air barrier. That would be the relevant test if you were spraying the foam onto an air-permeable material to make an air barrier. Notice I said air-permeable, not just vapor permeable. For example, if you had a wall that was braced with diagonals and did not need structural sheathing, and you stapled window screen material up instead of sheathing and sprayed the foam on that. Nobody would do that, but the air barrier test is proving that 1.5" of ccSPF, or 3.5" would work all by itself as an air barrier.

In real life there is some kind of sheathing that stops airflow over most of the surface area, and the foam only needs to stop the air flowing through the cracks. Because the crack area is very small, we can tolerate much higher leakage per unit area through the foam, compared to the spec for using it as an air barrier over the whole surface area.

To get an idea of how that works, consider 1280 sq. foot 2-story house, 32' x 20' footprint, 16' high walls. The volume is 10,240 cubic feet; the surface area of the envelope is 2304 sq feet, or 72 4x8 sheets--sheathing or ceiling wallboard.

At 75 Pa, with a barrier material that just met the spec over the whole area (0.004 CFM/ft2), it would leak 9 CFM, or 0.05 ACH_75. That's pretty negligible for sure, but you'd care if it got much worse than that. For example, maybe 1.5" of open-cell sprayed on window screen would give 3-4 times that much, or 0.15 to 0.2 ACH_75. That probably OK, if it is the total for the wall, including the cracks that have been sealed now and all the surface area. But in a real application it should work better:

If we only need to seal the seams, we have much less area to worry about. Below I estimate that the effective area to be sealed might generously be 300 sq. ft. In that case, the leakage with 1.5" of ocSPF seems like it would be less than 4 CFM, for less than 0.025 ACH_75. I think that's plenty good enough.

My conclusion is that the thickness needed for an air barrier has little to do with the thickness needed to provide air sealing, and that 1.5" of ocSPF is fine from that perspective.

How I got 300 sq. feet: There are 1728 linear feet of edges of the 4x8 sheets needed for the envelope. The linear feet of seams is less, because some seams are where two panels are next to each other and my math so far double counts that. But then there are more seams because of windows and doors and other penetrations, so let's call it 2400 linear feet of seams to seal. The actual cracks might be 1/4", but if the air coming in a 1/4" crack spreads out as it works its way through open cell spray foam, a rough and conservative effective crack width to use might be the thickness of the foam. With an effective crack width of 1.5" and 2400 linear feet of cracks, that 300 square feet.

Aug 7, 2017 2:41 PM ET

Would be interesting to see
by Jon R

Would be interesting to see discussion of the validity of the third paragraph here:

Roughly: movement->cracks->air leaks, well after the blower door test.

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