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Community and Q&A

Spray Foam and Batt Insulation

Hatherly | Posted in GBA Pro Help on

I thought I recently read an article on the Building Science website talking about problems installing batt insulation over spray foam – moving the dewpoint to the spray foam layer and causing the batt to get moist where it meets the foam. Now I can’t find it anywhere and am wondering if I understood it correctly.
Can anyone explain this to me better or point me to more information. Thank you so much.

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  1. Hatherly | | #1

    Sorry, I meant to title this Spray Foam and Batt Insulation

  2. GBA Editor
    Martin Holladay | | #2

    Basically, you've got it right. So-called "flash and batt" insulation works, as long as the spray foam is thick enough. Most insulation contractors use closed-cell foam, not open-cell foam, for flash and batt installations.

    The minimum spray foam thickness depends on your climate. The colder the climate, the thicker the spray foam must be. If you tell us where you are building, we can provide recommendations on the minimum foam thickness.

  3. Hatherly | | #3

    In New Mexico - zones 4 and 5

  4. Hatherly | | #4

    Also, this is for flat roof construction.

  5. GBA Editor
    Martin Holladay | | #5

    First, you should know that there is some controversy surrounding this practice. If you have access to a good academic library, you might want to look up an article on the topic in the November 2007 issue of Energy Design Update.

    That article quoted building scientist Joe Lstiburek, who said, "To be safe, use about 1 inch [of closed-cell foam] in climate zone 5, or 1 1/2 inch in climate zone 6, or 2 inches in climate zone 7." Remember, many spray foam operators find it difficult to achieve a uniform spray of 1 inch. If some of the foam is 1 1/2 inch thick, it may be only 1/2 inch thick elsewhere -- which wouldn't be thick enough.

    Moreover, these recommendations usually apply to walls, not flat roofs. You have an additional problem with a flat roof: coming up with a way to ensure that the batt stays in physical contact with the foam, without an intervening air space. That can be tricky, unless you stuff an oversized batt into a tight cavity.

    Furthermore, you have now raised an additional question: should the roof sheathing be vented? Many building scientists would answer that it doesn't have to be -- but you should be aware of the controversies surrounding the issue before you proceed.

    Finally, one manufacturer -- Johns Manville -- has approved the use of flash-and-fiberglass with its spray fiberglass product, Spider, including on sloped ceilings. Because of the controversies surrounding minimum foam thickness, Johns Manville has NOT published a table showing minimum foam thickness requirements for different climates. Instead, Johns Manville instructs contractors interested in flash-and-fiberglass to contact their technical help line (303-978-5280) for climate-specific technical help.

  6. Hatherly | | #6

    Thank you so much for such a detailed answer!

  7. mbloomfield | | #7


    Thanks for the good information. I have two questions:

    1) Why is this not an issue when using just blown cellulose/fiberglass insulation in a wall cavity? Is the idea that the exterior wall is vapor permeable such that any condensation could dry? This seems a dangerous assumption.

    2) We've started to have some clients show interest in using Owens Corning EnergyComplete spray foam ( to seal the wall cavity, then blow in cellulose/fiberglass. What are your thoughts on that system in this context?



  8. GBA Editor
    Martin Holladay | | #8

    1. Many conventional wood-framed wall assemblies routinely experience some condensation against the interior face of the exterior sheathing during the winter. As long as the sheathing is somewhat vapor permeable, however, this isn't necessarily a problem. In spring, the snow melts, the birds start to sing, and sunshine hits the siding, warming and drying the exterior of the wall assembly. As you correctly guessed, closed-cell spray foam installed against the wall sheathing lowers the vapor permeability of the exterior side of the wall assembly. Since it is now hard for the wall assembly to dry to the exterior, it becomes more important to prevent the condensation in the first place.

    2. Although I have no direct experience with the system, I have no reason to believe that the Owens Corning EnergyComplete system won't work well. Although I called flash-and-batt "controversial," I think that such an approach can work quite well, as long as some basic guidelines are followed. The most important issue: be sure the foam is thick enough for your climate.

  9. mbloomfield | | #9


    I should have explained it a bit better - the Owens Corning EnergyComplete foam is only a very thin layer of open celled foam that's intended to air seal the cavity, not to insulate. OC recommends (naturally) blown or batt FG for insualtion. The EnergyComplete foam vapor permeance is listed as 40 perm (dry cup) and 110 perm (wet cup) on the data sheet. Would this allow for enough drying to the exterior?

    Thanks again for the helpful information.


  10. GBA Editor
    Martin Holladay | | #10

    A thin layer of open-cell foam is quite vapor permeable. However, I have doubts about the ability of a "very thin layer" of open-cell spray foam to create an effective air barrier. The effectiveness of the air barrier would depend not only on providing adequate thickness, but on the skill of the installer.

    I have no direct experience with Owens Corning EnergyComplete, so it's best for me not to comment on the system's performance. You should probably direct further technical questions to Owens Corning. Of course, any GBA readers with experience are invited to comment.

  11. mbloomfield | | #11


    I will indeed contact Owens Corning, but their website for the product has what I consider to be unfair characterizations of other insulation systems, and it's obvious that the development of the EnergyComplete foam was aimed at selling more of their fiberglass products. While this is understandable, I was interested in some non-biased opinions of the potential effectiveness of the products, which you provided.

    Thanks again,

  12. Riversong | | #12

    The danger of flash-&-batt or any spray foam application on a flat roof is that potential leakage will become trapped in the sheathing, which will be unable to dry in either direction. Closed-cell foams will prevent vapor diffusion drying and open-cell foams can hold liquid water like a sponge.

    If the roof assembly is unvented (probably), then it's important to allow a drying potential downward, which means an air-sealed but vapor open ceiling.

  13. Riversong | | #13

    Don't assume that any opinions expressed here are unbiased. But you're correct to notice that the Owens Corning marketing demonstrates bias and misdirection (such as comparing spray foam to latex caulk for air sealing). The fiberglass industry, particulary Owens Corning, has a long history of deliberate distortion - particularly in regards to cellulose insulation, which they attacked with ferocity for decades.

    This was, of course, necessary in order to market such a nearly useless and dangerous product and to dominate the insulation industry as long as they have.

  14. michael sheahan | | #14

    Thanks everyone for addressing an issue I've been thinking about lately. I'm a few weeks away from insulating my new house on the coast of Maine, zone 6. I've been reading about flash and batt and wondered about the condensation issue. I'm also planning on covering the interior studs with 1" foil faced rigid insulation before the drywall goes on. The walls are 2x6 and the siding will be reverse board and batten so there will be a 7/8" air space behind the 1x12 barn boards. What I don't know is how the extra insulation and the siding affect the propensity for condensation to form on the interior face of the exterior sheathing. To be on the safe side it sounds like I should spray 1 1/2 inches of foam.

  15. Riversong | | #15

    You definitely don't want closed-cell foam both inside and out, as that will trap moisture. If the interior is air-sealed with taped, foil-faced rigid foam (and all penetrations sealed), then the wall assembly needs to be able to dry to the outside.

    You didn't mention what kind of exterior sheathing you're planning. I would recommend keeping it as vapor permeable as possible, and make sure there is a good WRB properly integrated with flashings between it and the siding.

  16. michael sheahan | | #16

    Forgive my ignorance of acronyms but I assume a WRB is a building wrap like typar. The siding is 1x12 white pine boards over 3" pine battens with a 3/8" gap between boards. This should dry quickly with a 7/8" air space between siding boards and sheathing. I shied away from applying closed cell foam boards to the exterior of the sheathing because it would present fastening issues for the siding. If I had to choose between closed cell foam against the interior of the sheathing and rigid foam on the studs before drywall, which would be best? By applying foam boards to the inside of the studs I was thinking of the benefit of a thermal break. From everything I've read on this site, it sounds like foil faced rigid insulation to the inside and a breathable cavity to the exterior makes the most sense.

  17. Expert Member
    Michael Maines | | #17

    WRB is "water resistive barrier," and it is usually a building wrap such as Typar. The idea is to lap and tape all seams so the house is watertight before the siding is installed.

    What are you using for sheathing under the WRB? If it's permeable, installing foam on the inside of the studs would be your best bet because you reduce thermal bridging and the assembly can dry to the exterior.

    I design and build on the coast of southern Maine and have been using Advantech's Zip wall sheathing, which is not very permeable, but works well with closed cell foam sprayed on its inside surface. Two critical items for the flash-and-fill or flash-and-batt system though: the foam has to be thick enough to prevent condensation 99% of the time, and you can't have poly sheeting inside as a vapor barrier--you need the airtight drywall approach instead, to minimize vapor migrating into your wall cavity but allow it to dry to the inside.

  18. michael sheahan | | #18

    I boarded the house the old fashioned way with 1x12 pine on the diagonal. (I'm building it alone and find boards easier to apply single-handed than sheets of sheathing.) Yes, dry to the outside is my only real option. Thanks for the input. The construction of the house, in Sedgwick, is chronicled at:

  19. Expert Member
    Michael Maines | | #19

    Michael, great looking project! I love the Blue Hill peninsula. Framing hip roofs is one of the more difficult tasks in carpentry. Don't feel bad that it required a sawzall.

    Although wrapping the outside of the structure with foam would arguably be the very best way to insulate, I think your plan of foil-faced on the inside of the studs is a good alternative. Just make sure you do a good job with your air barrier so the wall cavity insulation can perform properly. Properly installed housewrap can make a good air barrier.

    One other idea you might consider is the cross-hatched wall, where you run 2x2 strapping across the inside face of the studs and fill the whole thing with dense-pack cellulose. You give up a little R-value, but the wall can dry to both sides and thermal bridging is reduced. Robert Riversong is very knowledgable about this approach and could give you more information. I haven't done it myself but would like to try it someday.

  20. Riversong | | #20

    I had almost mentioned cross-hatched walls with cellulose as an alternative, but you seemed intent on foam. Even with carefully-taped seams on the foam board, you still have to deal with difficult-to-install and difficult-to-seal electric boxes that must extend 1/2" beyond the surface of the foam board.

    Using polypans and the air-tight drywall system over a cross-hatched wall can be an easier way to get a good air seal, reduce thermal bridging and increase wall R-value.

    P.S. Michael, you really should have started another thread rather than hijack someone else's.

  21. michael sheahan | | #21

    Sorry, the hijacking was unintended. Thanks for all your input.

  22. Brent_Eubanks | | #22

    Since the thread's already been hijacked once, I hope no one minds if I do so again, to direct a question at Robert :).

    Robert, you said
    If the roof assembly is unvented (probably), then it's important to allow a drying potential downward, which means an air-sealed but vapor open ceiling

    That confuses me. I thought that all air barriers were also vapor barriers, though the converse is not necessarily true. Can you explain?


  23. GBA Editor
    Martin Holladay | | #23

    Examples of air barriers that are not vapor barriers include:
    1. Tyvek, Typar, and other plastic housewraps.
    2. Unpainted drywall.

    There are other examples as well. These materials allow moisture to diffuse through them, but they stop air.

  24. Hatherly | | #24

    I don't mind the hijacking at all (I started the thread).It's all good information.

  25. Brent Eubanks | | #25

    Thanks Martin. Have I got it backwards then? Are all vapor barriers also air barriers, but not vise versa? Or is it more complicated than that (some things are one or the other, and some are both)?

  26. GBA Editor
    Martin Holladay | | #26

    Not all vapor retarders are air barriers.
    Kraft facing, as usually installed, is a vapor retarder but not an air barrier.
    Vapor-retarder paint is definitely not an air barrier.
    Even polyethylene is not an air barrier unless the seams are sealed with Tremco acoustical sealant or tape. If you depressurize a house with a poly vapor retarder with unsealed seams, you'll see how the poly does not act as an air barrier.
    In all of the cases I cited, the materials still function as effective vapor retarders -- even though they leak air.

  27. Anonymous | | #27

    Martin, On another direction. Has a nationally recongnized testing agency done any testing on the "Flash & Batt/blown" wall or ceiling systems. I can't seem to find any studies or reports that are un-biased.


    David C. James

  28. Robert Riversong | | #28


    What kind of testing are you looking for? The big selling point of the flash-and-fill or flash-and-batt systems is a minimal (and low cost) spray foam installation as an air barrier with fibrous insulation as the primary thermal barrier.

    However, given the priority that air-tightness has been given in energy-efficient and durable construction, simply spraying foam in the stud (or rafter) cavities will not constitute an air barrier, since much air leakage occurs between foundation and sill and between floor decks and wall plates. An effective air barrier must be continuous around the entire 3D building envelope.

    In fact, contrary to what Martin stated above (#23), not all polymeric housewraps - even with taped seams - qualify as an air barrier. They are considered weather-resistant-barriers (WRB), as they resist some of the wind and create a secondary drainage plane for rain that gets through the cladding. At best, they can constitute an air barrier material, but must be incorporated into a full 2D assembly and 3D enclosure.

    According to the Air Barrier Association of America (

    Air barrier systems typically are assembled from materials incorporated in assemblies that are interconnected to create enclosures. The recommended minimum resistances or air permeances for the three components are listed as follows:

    Material 0.02 l/sec-m² @75 Pa (0.004 cfm/sf @ 1.57 psf) – 25 mph equivalent
    Assembly 0.20 l/sec-m² @75 Pa (0.04 cfm/sf @ 1.57 psf) – 25 mph equivalent
    Enclosure 2.00 l/sec-m² @75 Pa (0.4 cfm/sf @ 1.57 psf)– 25 mph equivalent

    An air barrier material must have these qualities:
     low air permeance
     continuous, contiguous
     inflexible
     durable

    And must be able to:
     resist positive & negative pressures
     transfer all wind loads to structure
     resist displacement under pressure
     be flexible in joints between dissimilar materials
     resist seismic, thermal, swelling, shrinkage and creep movements
     seal at all penetrations
     separate spaces with differing temperature or humidity regimes
     be continuous along all 6 sides of enclosure

    Some materials that are not air barriers by this standard include:
     uncoated concrete block
     plain and asphalt impregnated fiberboard
     expanded polystyrene (EPS)
     perforated house-wraps
     asphalt impregnated felt (15 or 30 lb.)
     tongue and groove planks

  29. DAVID | | #29

    Hi Robert,

    Looking for an approved, ASTM test or a supplier of spray foam/ fiberglass/ cellulose who has gone through a testing proceedure that provides evidence that the combination of spay foam and fiberous materials do work together and have shown to do so over the lifespan of the home.

    Many companies have come on board with these methods but don't have testing to back it up with.

    A 1/2" of 2lb spray foam would appear to be less than a minimum to insure a air barrier unless supported by a blower door test. How does the building inspector assure the material is an air barrier? And is there an double inspection for both the foam and the batt/blown materials?

    2lb foam has great qualities but I have seen it break away from the stud bays and leave 1/8"gaps, especially after the house has settled and after the wood studs have dried out.

  30. Robert Riversong | | #30

    Yup, there are as many reasons to avoid the use of petrochemical foams in a house as there are promotional claims by manufacturers and installers.

    No insulation system functions in isolation and so a standardized test of insulation, whether homogenous or mixed, does not tell you how it will work within the complex hygro-thermal system that comprises a house, nor how it will work in a given climatic zone.

    You can read building science screeds on various systems in various climate zones for some perspective on this, but ultimately there''s no way to judge a particular system without understanding the complex and interdependent thermal and hygric mechanisms and forces that interact within the entire building envelope and interior conditioning system (as well as with the occupants).

  31. RH | | #31

    It is a difficult process because there are a lot of different materials and you have to figure out how your system is going to work. OSB and plywood transport and store water differently. Closed cell spray foam has a low perm rating. Closed cell effectively tops out at about an R25 even when installed at thicknesses that should have a higher rating. Two layers of rigid insulation creates a more tortuous path for air and vapor leaks. Fiberglass insulation is porous and susceptible to convection and wind washing.

    So how do you deal with all the variables. Thermal bridging throw the studs acts like a radiator transporting heat through the wall. Spray foam in the inside does not eliminate thermal bridging. Foam board on the outside will prevent the thermal bridging. Put enough foam board for your climate to eliminate the potential for a dew point in the wall. Moisture movement is through 2 methods bulk/liquid (a leak) and from vapor. If a correct drainage plane is developed bulk water will not be a problem as the solution has been built into the structure. The next step is to deal with vapor. In this respect I like cellulose as it blocks air flow better thus stopping the source of vapor. Then the use of a plastic vapor barrier is not needed and SHOULDN'T be used. By chance any moisture that gets in needs a way out. Drying to the inside is needed, a plastic barrier would trap the moisture.

  32. Riversong | | #32

    Moisture movement is through 2 methods bulk/liquid (a leak) and from vapor.

    Actually, moisture movement in and through building envelopes can be by:
    - bulk liquid, driven by gravity, kinetic momentum or air pressure (wind)
    - liquid, driven by concentration gradient, surface tension or capillary suction
    - water vapor, driven by temperature or vapor pressure gradient, including radiant solar drive
    - water vapor, driven by air movement (convection) through thermal gradient (stack effect) or air pressure differential

    In winter, most moisture movement is from inside to out, though liquid water can diffuse inward at the same time that water vapor is diffusing outward. In summer, almost all moisture movement is from outside to in, particularly when the sun is shining on wetted surfaces or saturated reservoir materials.

    Hygro-thermal energy and mass flux is a very complex set of interrelated forces, resistances, storage capacities and vulnerabilities. If we are committed to building highly energy-efficient homes that are also durable and livable and healthy, then we have an obligation to understand moisture dynamics on an elemental level.

    Almost nobody in the building trades does.

  33. cld | | #33

    How does all of this apply to Zone-2 (south central texas). I'm in a dry/hot zone and I'm primarily dealing with baking temperatures on the roof surface. Doesn't this potential moisture issue only apply in cold zones?

  34. GBA Editor
    Martin Holladay | | #34

    You're right. In your climate, you don't have to worry about cold-weather condensation problems in a flash-and-batt installation.

  35. Susan | | #35

    I have a 110 y.o. home that the ceiling and walls have been gutted to their true 4" depth. (original studs) My plan is to add 1" to the rafters and fill the ceiling cavity with closed cell to reach an R30+. After reading much. and speaking with two insulation companies I am considering filling the walls with 1" of closed cell and then filling the remainder with blown in cellulose. There is nothing but wood siding on this house - no interior sheathing, wrap, etc. I am confused about the vapor issue between closed cell and cellulose blown in (to the inside) - There is no venting - do I need it? Any thoughts on any of these ideas?

  36. GBA Editor
    Martin Holladay | | #36

    You need to tell us your climate or your location for anyone to respond usefully.

    In general, it's a bad idea to spray foam directly against the back side of siding, because it will complicate any future siding repairs.

  37. Fernando | | #37

    I have a question about using foam board insulation along with Roxul to up the R value. It's for my basement and I am planning to put up Ty-Par against the concrete block foundation, then install 2" thick foam board, then frame with 2x4 and install Roxul between the studs and finally vapor barrier and dry wall. I can't seem to find any information on wether this would be ok or would it cause a double air barrier. Any thougths?

  38. GBA Editor
    Martin Holladay | | #38

    As long as you omit the "vapor barrier" (I assume you mean polyethylene) on the interior of your proposed wall assembly, the wall should be fine. (Assuming, of course, that your basement doesn't suffer from existing moisture problems.)

    A framed wall on the interior side of a basement wall should never include polyethylene.

  39. Fernando | | #39

    Thanks Martin. No moisture problems yet (let's hope it stays that way). The vapor barrier would be a 6 mil poly that I was thinking of using. I was basing it on the following article since I am in a cold zone.

    Then I read an article on that omitted the 6mil poly. I'm assuming that omitting the 6 mil allows any moisture in the wall to dry out so to speak. Would I even require the Ty-Par since the foam board would provide protection?

    As an overall system, I'm thinking of the foam board on the walls, the sub floor is going to be Dri-Core to allow air flow beneath, then 2x4 framing on top of the dri-core against the foam, then top that off with the Roxul to obtain an R23. Basically I'm looking for a somewhat equal substitute for sprayfoam since it's a pricey option. Thanks again for the help.

  40. Mike | | #40

    I am currently renovating an old home is south west iowa. My insulation palns are to spray an 1" of closed cell in the exterior wall cavities against the lap sheeting. Then place batt insulation over the spray foam to gain an r-16 or so. I am also spraying an 1" in the rafter bays and soffits to seal the attic where I will install my air handler. My spray foam supplier says this is a safe practice, but after reading a few of your posts i'm not so sure. Will i be creating a water vapor trap between the spray for and the batts? Is there a better way to set up this application to avoid moisture infiltration?

  41. GBA Editor
    Martin Holladay | | #41

    I'm not sure what you mean by "lap sheeting." By any chance, do you mean "lap siding"?

    It sounds like you have an older house without sheathing.

  42. Mike | | #42

    yes it is a 110 year old home with 4" cedar siding on the exterior tar paper then 8" lap siding or sheathing on the studs.

  43. GBA Editor
    Martin Holladay | | #43

    What do you mean by "8" lap siding or sheathing on the studs"? Do you mean 8 in. wide boards that are used as sheathing? Or do you mean siding that was later covered with a second layer of siding?

  44. Mike | | #44

    boards used as sheathing that have alternating grooves cut in the top and bottom so they lock into one another

  45. GBA Editor
    Martin Holladay | | #45

    OK, I got it. You're describing a typical flash and batt job. In your climate, it will work -- although the resulting R-value won't be particularly high. The spray foam will do a good job of air sealing, as long as it is carefully installed.

    If you want to learn more about flash and batt jobs, search the GBA Web site using the search terms "flash and batt."

  46. Anonymous | | #46

    If money wasn't an object, which it is, what would be the best way to insulate your home? I recently used cellulose in the walls and used 5 1/2" of open cell foam on the bottom side of the roof deck. It seemed to have worked very well, but for the money, was wondering if there's a better system. I'm in South GA if it's relevant.

  47. GBA Editor
    Martin Holladay | | #47

    The only problem with the insulation you installed is that it doesn't meet minimum code requirements. In south Georgia (climate zone 2), a ceiling should be insulated to at least R-30. You chose to insulate your ceiling to about R-20. That's not enough.

    If you want to use open-cell spray polyurethane foam, you should have installed at least 8 inches to meet code.

  48. user-1055215 | | #48

    I was wondering if it is possible to undertake the flash/batt approach to wall insulation using rigid foam closed cell insulation instead of spray foam? One would naturally need to tightly seal any seams or gaps between or around boards. My motivation would be avoiding the cost / mess / health & saftey issues associated with spray foam. My home is in the northeast of the U.S. Thanks for any thoughts that any of you may have.

  49. GBA Editor
    Martin Holladay | | #49

    Yes, it's possible -- but it's slow, fussy work.

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