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Closed-Cell Foam Beats Open-Cell Foam

Higher R-values and lower permeance make closed-cell foam a winner

Neal Ganser of Bozeman, Montana, is a spray-foam consultant and the former president of Corebond Corporation.

By Neal Ganser

Replacing long-obsolete insulation materials with closed-cell or open-cell spray polyurethane foam is a big step in the right direction for energy efficiency. Of the two types of foam, it is closed-cell foam that truly answers all of the building envelope’s requirements for long-term sustainability. Compared to open-cell spray foam, closed-cell foam has a higher R-value and is less vapor-permeable.

Higher R-Value per Inch

Closed-cell foam encapsulates a high-R-value blowing agent which acts somewhat like the argon in a gas-filled insulated window. The most commonly used and safest blowing agent is made by Honeywell Corporation; it’s a non-ozone-depleting, environmentally benign chemical. This blowing agent produces a closed-cell foam with an aged R-value of about 6.2 per inch.

The blowing agent used for open-cell foam is carbon dioxide, which is off-gassed as the foam cures and is eventually replaced with air. This off-gassing also contains some sensitizing agents and various catalysts. The process takes about 30 days (or more, depending on thickness), and can have an impact on people working in the building. The R-value of open-cell foam is about 3.6 per inch.

While open-cell foam is cheaper to buy per inch, it takes nearly twice as many inches to achieve required R-values, and so it ends up close to the same cost per installed R-value. Accommodating the required thickness of open-cell foam may require oversized structural members, since even R-19 walls require 2×6 studs. A customer should buy insulation by its aged R-value — based on testing by a third-party laboratory — not by inches.

Lower Permeance

To avoid moisture problems in any wall or roof system, the indoor and outdoor climates must be kept apart. This requires that four criteria be met: thermal isolation, air isolation, moisture isolation, and reversibility.¹ The reversibility requirement accounts for daily and seasonal reversals in the direction of vapor drive through wall and ceiling assemblies.

Closed-cell spray foam meets all of these criteria, while open-cell foam does not. The vapor permeance of 2 inches of closed-cell foam is less than 1 perm, making it a natural moisture resister. The permeance of 3 1/2 inches of open-cell foam, on the other hand, is more than 16 perms — roughly ten times the permeance of closed-cell.

Because of its extremely high moisture permeance, open-cell foam fails at moisture isolation, forcing the builder to decide which side gets a vapor retarder. The outcome of this decision precludes reversibility — that is, the ability of the wall to resist moisture or to dry out in either direction.

Although air leakage is a more important mechanism of moisture transport than vapor diffusion, it is silly to assume that the laws of moisture diffusion are suspended for open-cell foams. Because open-cell foam allows moisture to diffuse through it, it can allow condensation to form behind exterior sheathing in winter or behind a vapor retarder or wallpaper in summer. Diffusion-related moisture problems in walls insulated with open-cell foam are not a lot different from similar problems with fiberglass-insulated walls — except that the moisture dries more slowly, because foam doesn’t allow air movement.

While it is possible to use open-cell foam successfully in some climate zones where vapor drives are predominantly in a single direction — in other words, climate zones where you can accurately predict which side of a wall to apply the vapor retarder — these zones tend to be hot, humid coastal areas subject to hurricanes and flooding. Closed-cell foam is the only insulation approved by FEMA for flood zones.

As a bonus, closed-cell foam imparts strength to whatever construction it is applied in. It can be used in any climate zone in the United States without a lot of extra stuff (like vapor retarders and thick walls) to get it to work and not cause moisture damage.

Look at the refrigerator in your kitchen. Refrigerators haven’t used fiber insulation for over forty years, and they don’t use open-cell foam either. Closed-cell foam is the insulation the others can only ‘wanna be.’

¹ Exerpted from “Walls that Work” seminar, © 2003 Corbond Corporation

To read another perspective on the issue, see “Open-Cell Foam Beats Closed-Cell Foam.”


  1. gregbehm | | #1

    Environmentally benign blowing agent?
    Neal, could you elaborate on your statement that the "most commonly used and safest blowing agent is made by Honeywell Corporation; it’s a non-ozone-depleting, environmentally benign chemical"? As I understand it, closed cell foams use HFC blowing agents such as HFC-245fa, chemicals with high global warming potential (GWP). That's not what I'd described as environmentally benign.


  2. Paul Eldrenkamp | | #2

    spray foams: are we being too cavalier in their use?
    I have four things to add to this discussion.

    One: Spray foams contain fossil fuels and have, by one accounting I have seen, 25 to 70 times the embodied energy of cellulose. Not a deal breaker, but something to think about.

    Two: A small-scale foam contractor we have worked with hired a helper about a year ago and did not provide enough training or protective equipment. One day, as the helper was driving home after work, he started losing his eyesight. It was (mercifully) "only" a 2-3 day side effect of the foam application work he’d been doing. But imagine how scared he was until he found out he would indeed get his full eyesight back. Bottom line: The occupational hazards of spray foams are real.

    Three: A colleague of mine who splits time between working for my company and working for a state agency that tracks public health issues sent me this summary of a recent webinar:

    A group of professionals from EPA, OSHA, NIOSH, CPSC (Consumer Product Safety Commission) formed a SPFI (spray foam insulation) workgroup to address exposure concerns they have around the use of SPFI in homes. The group is particularly concerned because of the dramatic increase in the use of this class of material and the lack of communication of risks to workers and the general public who may use DIY products. There is still a great deal that isn’t known about these products, but plenty that is known. There are also apparently many misleading marketing claims about the lack of toxicity and safety of using them.

    SPFI is a 2-part product, one of which contains a diisocyanate. This compound is known to be the leading cause of occupational asthma. There have been deaths attributed to diisocyanate exposures. It is also a sensitizer, meaning exposure to it can make you hypersensitive to it. This is very problematic because polyurethanes are everywhere, once you are sensitized, you could get triggered by a something as common as a floor coating.

    NIOSH did an investigation September 2009 at 4 different worksites using SPFI. They found many of the same concerns that lead them to issuing an alert in 2006 for the industry that makes truck bed liners using a product with diisocyanate.

    Recommended practices for workers applying SPFI:
    • Cover all skin with disposable suit and gloves
    • Wear full face respirator with supplied air
    • Isolate work areas from other occupied areas
    • Exhaust air to outdoors with an appropriate filter
    • HVAC system should be shut down before using SPFI

    Helpers also need to wear full skin protection and full face respirator. Skin contact can cause sensitization.

    NIOSH saw a high percentage of workers not wearing respirators during their field studies. There is particular concern that many workers are not well educated, fairly transient and not well trained.

    Cure time for SPFI varies between 1 hour and 1 week, depending on formulation of SPFI, the ambient temperature, humidity and other factors. Off-gassing during this time could be harmful to residents, especially children.

    EPA is launching a diisocyanate website with information that they have. They are very clear that they don’t know everything there is to know about this product. They feel they know enough to say that workers should be better protected than they are now. They are doing research on cure times and concentrations of toxic monomers that are released during curing.

    OSHA will do free consultation for small and medium businesses. OSHA is hiring more people to do enforcement around this issue due to increase use of SPFI from stimulus funding. There was some discussion about no worker protection guidelines connected to weatherization work resulting from stimulus money.

    Four: It's really easy to get people upset about possible health hazards in their homes, especially if we glibly reassure them there are no risks to certain products whether we know that to be true or not. In building, as in any other endeavor, it's easy to convince ourselves something is true if we really want it to be true. I don't know how safe spray foams are. I do know they're used a lot, and I also know that the quality and safety control within the industry is (at least in my market area) VERY uneven. I also know that there's rampant confusion regarding what building codes really require for thermal and ignition protection; that what sales reps say often contradicts what their own literature says with regard to ASTM testing among other things; and that, of all the newly prevalent products we use to make homes perform better, spray foams worry me the most in terms of potential liabilities.

  3. homedesign | | #3

    Yep...the Blue Haze
    I agree about the installation hazard.
    I did a stupid, stupid thing and got "too close" to the action when they were spraying my house....
    The workers called it the Blue haze.
    My eyesight was still foggy the next day.
    It had finaly cleared up by the next afternoon......
    The installers admitted to me that they did not always take the proper precautions.....and the consequences could last for days.....or longer...who knows.

  4. Joe Chalat | | #4

    closed cell and epdm membrane roof
    I've designed several additions to houses where I ended up with an epdm membrane roof adhered to the roof sheathing. The 1/4" per foot taper is achieved by ripping a diagonal along the wood rafters. My problem is If I use closed cell in this application, I'm trapping the sheathing between a layer of epdm on top, and closed cell below. This does not allow any drying in either direction. Have you seen this application?

  5. Sam | | #5

    Open cell vs Closed cell sealing foams
    I've been having a helpful (for me) discussion over on GreenBuildingTalk with folks about the "Great Stuff" closed cell polyeurethane foam sealant. I'm not a fan, for a few reasons outlined in that discussion. For the purposes of sealing around the windows and doors of my house, I've found that I much prefer working with DAPtex Plus, a latex based foam sealant (which I gather is open cell, though DAP's DAPtex Plus product page doesn't say so). My only reservation about ditching Great Stuff at this point is the issue of moisture resistance, which the Great Stuff website lauds as being a differentiating factor between its polyeurethane product and latex competitors. I think I understand the moisture permeance problem you are describing here, but I wonder if it should be a big worry when for me when I'm working on sealing gaps around a window vs foaming an entire wall for insulation. If I prefer working with a latex foam when sealing windows and doors, is the moisture permeance problem of latex foams a big enough performance problem that I should give them up and instead hassle with polyeurethane foam?

  6. Riversong | | #6

    Turning the Facts Upside Down?

    I'm no advocate of either open or closed cell spray foams, so I have no dog in this fight.

    But your claim that closed-cell spray urethane foam meets the requirement that you call "reversibility" - the ability of a thermal envelope to be able to dry in either direction - is simply false. Like any vapor barrier, 2 pcf foam prevents drying. If there is a relatively impermeable material on the other side of the assembly (such as self-adhering membranes on walls or roofs) then the sandwiched wood sheathing cannot dry if it ever gets wetted by an imperfection in the weather barrier any time in the life of the building.

    Additionally, because it is hydrophobic, any water leakage will be fully absorbed by wood framing and sheathing materials, increasing their saturation and the probability of mold or rot. Making an assembly impermeable to both liquid and vaporous moisture is a valid strategy only in a perfect world in which nothing ever leaks.

    In the real world, some vapor permeance is essential to the long-term durability and livability of a house. And in mixed climates (most of the US), where the seasonal moisture drives reverse, it is important for a thermal envelope to be able to dry in both directions.

    Closed-cell spray foam fails this requirement.

  7. DK | | #7

    Open Cell foam
    Open cell foam will Never off Gas. The common blowing agent in closed cell foam, HFC245fa can't be good in an indoor environment. typically 17% of the off gassing takes place in the forst 6 mo. Calculated R-Value diminishes from 7.4 / in to about 5.? over time. (LTTR-Long Term thermal Resistance).
    Open cell foam is a consistant R-Value, for a life time, and no HFC145fa.Typically water blown, flexible, and a vapor permience that will not TRAP moisture like closed cell foams.
    Incolder climates a vapor retarder coating should be applied to the open cell foam, as a code requirement. (Class ll or lll vapor retarder. will not trap moisture)
    My opinion,....EPA will Ban HFC245fa within 3 years as a blowing agent.
    Enjoy you R-Value,...closed cell fans. It won't last long.

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