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Why Flash and Batt Makes Sense

Closed-cell spray foam and fiberglass batts can be used together to maximize performance and minimize cost

Closed-cell spray-polyurethane foam may be the best-performing insulation available today. It seals against air infiltration, it boasts more R-value per inch than other forms of insulation, and it blocks the passage of water vapor. It’s also the most expensive option, and it requires dedicated equipment for large-scale installations.

[To read more of Michael Maines’s article from the February/March 2011 issue of Fine Hombuilding, click the link below.]

Why Flash and Batt Makes Sense.pdf

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14 Comments

  1. lesprice | | #1

    I just joined GBA, and am surprised that there have been no comments on this. I am building a custom home and my builder plans to use Flash&Batt for the exterior walls. I'm OK with that. My question is why not use it on the roof deck to achieve a conditioned attic?

    1. Expert Member
      Malcolm Taylor | | #2

      lesprice,

      You can. It's one of the methods Martin recommends here: https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling

      1. GBA Editor
        Martin Holladay | | #4

        Les and Malcolm,
        Here is a link to another article in which I discuss the use of the flash-and-batt method for unvented roof assemblies (cathedral ceilings): "Flash-and-Batt Insulation." (See the section that begins with the heading, "Code requirements for flash-and-batt roofs.")

        That said, I agree with Michael Maines (in his comment below): the environmental impact of spray foam is enough to limit my enthusiasm for this method.

        1. lesprice | | #5

          Martin....thanks for commenting. I am in climate zone 4, and most or all of the foaming contractors in my area (KnoxvilleTN) only seem to use open cell foam in residential attics. So I remain confused. To my simple, newbie mind, it's a conflict between using open cell (with the issue of moisture diffusing up to the roof sheathing, but being able to spot a leak if it happens) or closed cell flash and batt (which solves the moisture problem, but hides any potential leaks and may cost more). Is it possible that in my climate, open cell with some attic dehumidification is OK?

          1. GBA Editor
            Martin Holladay | | #6

            Les,
            I have discussed these issues in several articles, including these two:

            "Open-Cell Spray Foam and Damp Roof Sheathing"

            "High Humidity in Unvented Conditioned Attics"

            I hear a lot of stories about cathedral ceiling failures -- and these stories have made me conservative when it comes to cathedral ceiling details. Here is the last sentence of the second article listed above: "Considering the risks and hurdles described in this article, I do not recommend the installation of open-cell spray foam on the underside of roof sheathing in any climate. In this location, closed-cell spray foam is much less risky."

    2. Expert Member
      Michael Maines | | #3

      Lesprice, I wrote that article ten years ago and in the meantime I've learned a lot about the negative aspects of foam insulation, and now try to use other types of insulation whenever possible. When I have to use foam in a framing cavity, if the R-values will meet the building code I still prefer a flash and batt or flash and fill assembly. In most cases I can find a simpler approach that is not potentially harmful to the installers or home occupants, and that has a smaller environmental impact than foam.

  2. lesprice | | #7

    My thanks, especially to Martin, for his work in dealing with this complex issue. It seems to me that my situation is not uncommon - I have to build on a slab, so the ducts must be in the attic. And I want that attic volume to be useful, therefore I must condition the attic. If conditioning an attic saves energy, one would think that the Department of Energy would have something to say on this subject, but if they have put anything out since all the experience that Martin speaks of has come to light, I haven't found it

    1. GBA Editor
      Martin Holladay | | #8

      Les,
      Q. "If conditioning an attic saves energy, one would think that the Department of Energy would have something to say on this subject."

      A. They do, and they have funded many research projects and papers on the topic. See, for example, these resources, which I found in 60 seconds of Googling:

      "A Literature Review of Sealed and Insulated Attics—Thermal, Moisture and Energy Performance"

      "Vented to Unvented Attic"

      "Improving the Efficiency of Your Duct System"

      "Compact Buried Ducts in a Hot-Humid Climate House"

      "Better Duct Systems for Home Heating and Cooling"

      1. lesprice | | #10

        Martin...thanks again. I should have been more specific that the "subject" is selection of insulation materials for unvented attics. None of the links you cite, or any of the first 6 pages of results when I did a Google search on "closed cell vs open cell foam in unvented attic" had anything from DOE giving objective advice to a consumer trying to make decisions .

        I will say that the "vented to unvented attic" article had some useful information, e.g providing HVAC supply and return to the attic. But on materials selection it just says "All International Residential Code (IRC) requirements for air-permeable and air-impermeable insulation at the roof sheathing shall be fully met." duh!

        I don't want to give up on the conditioned attic. Your recommendation seems to be to go with closed cell. Does that mean R49 (code requirement?) divided by 7 to get 7 inches? Sorry if I am totally off the mark.

        1. GBA Editor
          Martin Holladay | | #11

          Les,
          Your math is correct -- 7 inches of closed-cell spray foam adds up to about R-49 (or about R-45 for some brands). But most builders would use the flash-and-batt or flash-and-fill approach, to reduce the environmental impact of the spray foam.

          If you decide to use the flash-and-batt approach, the minimum thickness of the spray foam layer depends on climate. For more information, see "Flash-and-Batt Insulation."

          1. lesprice | | #13

            Martin...using the numbers in your referenced article, in my climate zone (4A) I need R of 15 from the foam, or about 2.5 inches. If a fiberglass batt gives 3.7R per inch, then I need another 9 inches to get to 49. Does code really require 49 for an unvented attic roof deck? I looked my notes from my discussion with a local foaming contractor, who said he would use 7 inches of open cell foam for R of 28.

            By the way, I did find something from a DOE laboratory:
            https://basc.pnnl.gov/resource-guides/unvented-attic-insulation
            https://basc.pnnl.gov/resource-guides/vented-vs-unvented-attic
            They show a figure with foam that looks like flash and batt, but, amazingly, they never mention whether it is closed cell or open cell.

          2. GBA Editor
            Martin Holladay | | #14

            Les,
            Q. "Does code really require R-49 for an unvented attic roof deck?"

            A. The prescriptive table of the IRC (see image below) requires a minimum of R-49 for ceilings. There are ways around that requirement, however, if you want to skimp; see this article for more information: "Three Code-Approved Tricks for Reducing Insulation Thickness."

            Q. "A local foaming contractor said he would use 7 inches of open cell foam for R of 28."

            A. Want to know why he said that? Read this article: "It’s OK to Skimp On Insulation, Icynene Says."

    2. Expert Member
      Malcolm Taylor | | #9

      Les,

      Consider building an unconditioned vented attic and providing a service space for the equipment and ducts by either using scissor or plenum trusses.

      1. lesprice | | #12

        Thanks, but I really do want to make use of all that volume in the attic.

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