Comparing Open- and Closed-Cell Foams

A version of this article appears in the November/December 2010 issue of Home Energy Magazine.

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Q. Home performance contracting is new to us at our residential heating and cooling company in the Northeast. We are trying to do best by our customers as we learn on the go. One issue seems to be more difficult to solve than others: how best to insulate attics that are home to heating-and-cooling systems.

Encapsulating attics with spray foam (spraying the underside of the roof and the attic walls) appears to be a great option, but what type of foam is not as clear. Arguments for closed-cell foam include a higher R-value, better air barrier, and improved structural support. Open-cell advantages seem to include being able to detect roof leaks, as water will permeate the foam and drip into the attic; lower installed cost; and less fume exposure during installation. What do you recommend in our region and why?

Speaking more specifically to the issue of open cell versus closed cell in the event of a roof leak, does one have an advantage over the other? If two neighboring houses, one with open cell on the roof deck and one with closed cell on the roof deck, both have roofs that begin to leak, is one house better off than the other? Is the homeowner with open-cell foam more likely to detect the leak due to the water running through the foam and marking the ceiling below it? Is this true in theory but not in practice? Is it such a rare occurrence that this small fact should not weigh into the decision-making process?

I understand that this is all irrelevant if the homeowner is diligent about checking and maintaining their roof, but we would prefer to be safe than to be sorry.

Nate Kukla
Philadelphia, Pennsylvania

A. You are correct that closed-cell foam has a higher R-value and that it’s a better barrier to moist air (a better vapor barrier as well as a better thermal barrier). Its greater density does provide some support, and although it’s never a good idea to look at insulation in structural terms, one can walk on closed-cell foam, which one can’t do on the open-cell product. Of course, these benefits come with a greater cost—and the cost per R is still higher with closed- than with open-cell foam. Closed-cell foam would be a good choice where small framing sizes need the greatest R-value per inch (such as roofing applications), as long as the cost is within the homeowner’s budget.

As you noted, closed-cell foam is a better vapor barrier because of its porosity. This is one of the key issues when it comes to choosing between the two forms—porosity is important. If your insulation situation must consider high vapor pressure, then closed-cell foam insulation is the better choice. Note that in many areas open cell is not approved for exterior use. For this reason, open-cell foam is also not used below grade where it could absorb water. 

Open-cell foam is more flexible than closed-cell foam, so if framing members expand and contract with the weather, open-cell foam will flex with the wood. Closed cell does not have the same characteristic; thus if a leak develops, direct evidence of that leak may not be visible. But remember that no system is perfect, and diligence in periodically surveying for leaks is more important than planning for them. Also note that in many regions you are required to install a moisture barrier with open-cell insulation anyway.

Because of the benefits you listed, most (but not all) insulation contractors lean toward closed-cell foam being the better choice. Without knowing more about your specific situation, I would not want to make the choice for you. Hopefully this gives you a little more guidance; also remember to be aware of local code requirements as you make your decision. 

My colleague, Home Energy Technical Editor Steve Greenberg, weighs in on the question of roof leaks:

Roof leaks are always problematic as to where the water becomes evident versus where the leak occurs. Any insulation sprayed on the roof deck will make it somewhat more problematic and the closed-cell product is somewhat worse than the open-cell product in this respect.

Independent of the leak issue, depending on the climate, there might also be problems with water vapor condensing and even freezing in the open-cell foam. The closed-cell foam would be much less likely to have this problem.

Hope this helps!

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