Weighing the Merits of Spray Foam Insulation
A homeowner gets conflicting advice on how to insulate a 90-year-old Cape Cod home
Scott Jacobs’ 1,100-sq. ft. Cape is a perfect candidate for an energy upgrade. The 90-year-old house is gutted, and Jacobs wants to insulate it well even if his budget is not unlimited.
The house, located in Climate Zone 6, now has a 1/2-in. thick layer of rigid foam on the exterior walls. Jacobs’ plan is to insulate the house from the inside with spray polyurethane foam.
“This is my problem,” Jacobs writes in a Q&A post at GreenBuildingAdvisor. “Three companies have provided estimates now. Two say open-cell foam, 7 in. to 8 in., on the roof and 3.75 in. on the walls. One company just quoted 7 in. open-cell foam on the roof and 2 in. closed-cell foam on walls. Estimates vary between $4,500 and $5,500.”
First, Jacobs wonders, how do the 2 in. of closed-cell foam compare with 3 3/4 in. of open-cell foam? And second, for someone with not much money to spend, would installing rigid foam insulation in the rafter bays himself be a reasonable option?
That’s the topic for this month’s Q&A Spotlight.
Open-cell vs. closed-cell foam
Jean-Paul McGraw sums up some of the basic differences between these two types of insulation, including R-values and cost.
“The advantages of closed-cell foam compared to open-cell foam include its strength, higher R-value, and its greater resistance to the leakage of air or water vapor,” McGraw writes.
“The disadvantage of the closed-cell foam is that it is denser, requires more material, and therefore, is more expensive. Even though it has a better R-value, typically the cost per R is still higher than open-cell foam. The choice of foam can also be based on the requirements for the other performance or application specific characteristics such as strength, vapor control, available space, etc.”
Open-cell spray foam has an R-value of about 3.5 to 3.7 per inch and typically uses water as a blowing agent, while closed-cell foam has an R-value of about 6 per inch, he adds.
Costs, at least in McGraw’s area, average about 80 cents a square foot for open-cell foam and about $1.20 per square foot for closed-cell foam. With those R-values in mind, Jacobs will have trouble meeting energy code recommendations for his region if he goes with open-cell foam.
“A house in [Climate Zone] 6 requires R-49 insulation in the attic,” Armando Cobo says. “If your intention is to have a conditioned attic, 7.5 in. [of open-cell] foam is R-28, then you also need 3 in. minimum rigid insulation on top of the roof decking. If you want to install [closed-cell] foam, you would require 7.5 in., and it can only be installed in 2-in. applications.”
“The companies that are proposing 7 inches of open-cell spray foam for your roof are only offering R-26,” adds GBAGreenBuildingAdvisor.com senior editor Martin Holladay. “That isn't much. In your climate zone, as Armando points out, you really want at least R-49. Don't let a spray-foam contractor talk you into accepting insulation that is less than the minimum code requirements.”
Energy codes aren’t everything
Energy codes may call for R-49 in the roof, but that doesn’t mean it’s absolutely necessary, some commenters argue.
Among them is A.J. Builder in upstate New York, who writes that “code R-values do not factor in how well spray foam works compared to fiberglass of the same R value. Most times spray foam is not installed to code R values and yet it performs much better than fiberglass.”
Others go even further. Meeting the R-49 requirements is a “waste of money,” Eric Price writes.
“Eric is right,” adds John Pfeiffer. “IMHO the law of diminishing returns really takes a bite out of using more than 2 to 3 in. of closed-cell (depending on your location, I'm in southern N.Y., zone 4a). Don't trust the codes for telling you what is the most cost-effective way to insulate a house. They are written by people wearing suits and working in offices, not contractors.”
An R-40 roof may leak half the amount of heat as an R-20 roof, he says, but savings may amount to only pennies per square foot at the expense of adding twice the amount of insulation at double the cost.
“Also, consider what happens if your roof has a leak,” he adds. “Open-cell is a sponge and will need to be taken out and replaced; closed-cell won't show the leak and will soak the plywood until something gives.”
Our expert’s opinion
GBA technical director Peter Yost added this:
Scott, it sure sounds as though you would like to optimize energy performance, taking advantage of the framing cavities being open, and do it as economically as possible, including DIY. Given all that, here are my recommendations:
- Get a sense of where to spend your money – the best two ways to do this, in my opinion, are to get a whole-house performance assessment done by a BPI-certified technician, and to use the LBNL Home Energy Saver Pro Energy Assessment Tool. I have used this on more than one project, and using the detailed assessment path, I have come within $100 of total household annual utility bills, and their recommendations are useful.
- Seriously consider the DIY rigid-foam approach to cavity fill. Yes, this is labor-intensive, but it is relatively easy to do, and you often can employ scraps of rigid foam or salvaged rigid foam insulation. The key is to not be too fussy about how your cuts fit, because you can’t cut it tight or well enough to be your 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. anyway, so save the air sealing for later, when you spray foam all your joints and perimeters.
- Others have given solid guidance on choosing open-cell or closed-cell foam. On this, let me add that if you do choose a spray foam approach to cavity fill, be sure to use a certified installer.
- Evaluate your investment options. Martin and I just both wrote useful blogs on the topic of payback analysis – give those a look.
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