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Community and Q&A

Vapor barrier or vapor retarder paint on open-cell spray foam?

Joe Suhrada | Posted in Green Building Techniques on

I have seen some tout using the vapor barrier paint directly on open-cell foam before covering with Sheetrock, but as of late a general air of negativity towards that approach. I intended to use MemBrain under my drywall, but the open cell installer claims the paint works just as good for half the cost. What are thoughts on this? The stack up is a 2×6 wall with 2×2 interior Mooney strapping for a 7″ thick OC foam fill wall over Zip board, taped of course. Then followed by a fired out ventilated assembly with Hardi siding.

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  1. GBA Editor
    Martin Holladay | | #1

    You forgot to tell us your climate zone or location.

    Vapor-diffusion problems in walls insulated with open-cell spray foam are extremely rare, especially if the walls are finished on the interior with painted drywall. You don't need vapor-retarder paint for this wall. Nor do you need MemBrain.

    There have been reports of vapor-diffusion problems in unvented conditioned attics that have open-cell spray foam sprayed on the underside of the roof sheathing, but only in homes where the spray foam is not protected by drywall. Several factors make these assemblies riskier than walls: the vapor buoyancy issue (which causes indoor moisture to collect near the top of the house), the fact that most roof assemblies can't dry to the exterior, and the lack of painted drywall.

    The same problems don't occur in walls.

    Your spray foam installer is wrong about vapor-retarder paint. Because of the porous nature of cured open-cell spray foam, vapor-retarder paint is worthless when applied to cured foam. It does nothing to reduce the permeance of the foam. This was definitively proved by researchers at Building Science Corporation; they tested the permeance of cured open-cell spray foam before and after the application of vapor-retarder paint, and the paint didn't work. If you want to use vapor-retarder paint (which you don't have to do), the paint needs to be applied to a smooth surface like drywall.

    1. PioneerBuilders | | #33

      Hi Martin, can you direct me to the Building Science Corporation report that you reference here?

  2. Joe Suhrada | | #2

    Thank you! I am in Saratoga County Zone 5 near where it meets Zone 6. If open cell were used on the underside of roof deck, to form a conditioned attic, Joe L. Says the best practice is to put a forced air register and return in the attic. Yet others say to seal the attic from below and include a vapor barrier paint. So it gets to be a confusing issue there. But that is not my issue, at this stage- my issue is the walls with open cell and you have answered that question nicely. However I have done a cost analysis on the attic insulation which I have compared two stack ups. One is a two foot drop heel truss system with R75 cellulose, a ceiling with OSB to hold it, taped seams. Then a drop ceiling system to place lights and ducts within the envelope, vented soffits and ridge vent. The other would be to form a conditioned attic with ducts and lights in the dry wall ceiling and the inclusion of a forced air register/return to assist drying of R50 open cell sprayed on the underside of the roof deck and a sealed attic and no soffit vents. And the R50 is cheaper in bother labor and materials. So it is tempting to go that route. But there seems to be this risk factor of moisture accumulation on the bottom of roof deck. I will also point out that I detest humidity and like to use my air conditioning in summer months, will NOT have any unsealed flame appliances, will use an electric stove, and split duct heating and cooling. How would you mitigate the risk of the open cell on underside of roof? With a register/return in conditioned attic, or a sealed ceiling with vapor barrier?

  3. GBA Editor
    Martin Holladay | | #3

    Q. "How would you mitigate the risk of the open cell on underside of roof? With a register/return in conditioned attic, or a sealed ceiling with vapor barrier?"

    A. The answer depends on your climate zone. In cold climates like yours, the most important detail is an interior-side vapor retarder. In warm, humid climates, the most important detail is a central air conditioning system that is connected to at least one supply register in the attic, and at least one return grille in the attic.

    For more details on these issues, see Open-Cell Spray Foam and Damp Roof Sheathing. You might also want to read Joe Lstiburek's article, "Mea Culpa Roofs."

  4. Joe Suhrada | | #4

    Martin, I have read that article and the resulting comments under it at least a half dozen times. I am still confused. I guess I would lean towards placing OSB on the bottom of the lower truss chords and cover with Sheetrock and reduce sealed penetration to as few as possible if your above comment is correct. Or perhaps use three inches of closed cell on the underside or roof sheathing before open cell is applied? That might seem to be a (costly) solution.???

  5. Joe Suhrada | | #5

    IN Mea Culpa, Joe calls for both a vapor barrier AND ducts to condition the attic space. What is the point of the vapor barrier if we ar sending household air up there anyway?

  6. GBA Editor
    Martin Holladay | | #6

    There are few issues in residential building science that are more contentious right now that the question of why so many conditioned attics with open-cell spray foam on the underside of the roof sheathing are experiencing sheathing rot.

    Joe Lstiburek has advanced more than one theory about what's going on over the last few years.

    If any evidence is needed that this is still a hot topic, it's worth noting that building scientist Bill Rose just this morning posted a comment on GBA challenging Lstiburek's theory of "hygric buoyancy." You can read his comment here:
    Open-Cell Spray Foam and Damp Roof Sheathing.

    If you want to install open-cell spray foam against the underside of your roof sheathing, you should be aware of this current controversy, and read as many of the (competing) opinions on the causes of these problems as you can.

    I don't think there is anything wrong with a belt-and-suspenders approach (interior vapor retarder + active conditioning and dehumidification).

  7. Expert Member
    Dana Dorsett | | #7

    The BSC folks estimate that vapor barrier latex only performs at about 5 perms when applied directly to foam, whereas it's rated at about 0.5 perms when applied to wallboard, a full order of magnitude difference.

    But using a "smart" vapor retarder such as OSB or purpose-made flexible sheet goods such as Intello Plus or MemBrain on the conditioned space side of the foam should go a long way toward limiting the RH pumping that goes on in some of these roofs.

    I'm not buying the "hygric buoyancy" theory either. Humidity stratification by that mechanism would occur in any tight attic, not limited to those insulated with open cell foam.

    1. Bruce Fergusson, CIH, PE | | #34

      "Humidity stratification by that mechanism would occur in any tight attic, not limited to those insulated with open cell foam."
      And it does. We have seen more than one attic w/ inadequate venting and we see signs of chronic stratification, evidenced by mold growth.

  8. Joe Suhrada | | #8

    Dana, my inclination is to add the OSB since it offers the vapor barrier, and stiffens the truss chords on the bottom. It is also nice if I want to fir out for the dry wall to run wires up in there instead of the full drop ceiling, or attach something like wood wainscoting later on. But essentially my question to you, Dana- is if I add the vapor control layer then do I also use the "belt and suspenders" approach of running a register and return up there, or add "leaky ducts" and in doing so -DOES THAT NEGATE THE BENEFIT OF THE VAPOR RETARDING LAYER? It would seem like by doing the register and return we are introducing more conditioned air into the space and if in fact- the conditioned attic is in and of itself part of the overall envelope now, the vapor retarder/barrier layer seems rather moot (???) I am trying to stay away from the closed cell foam, obviously.

  9. GBA Editor
    Martin Holladay | | #9

    The reason why experts recommend that you install a forced-air supply register and a return-air grille in your attic is to lower the relative humidity of the air in your attic. This is always a good thing. Since your attic is within your home's thermal barrier, there is no energy penalty to this approach.

    In winter, supplying heat tends to make the attic air more dry.

    In summer, the supply air from your central air conditioning system is also dry.

    Drying the attic air is good.

    The HVAC register and grille will not undermine the performance of your vapor retarder in any way. Both measures can work together to lower your risk.

  10. Joe Suhrada | | #10

    Thank you! Excellent info. Obviously there will be no humidifier or other device on the furnace if this route is taken.

  11. Expert Member
    Dana Dorsett | | #11

    If you're pushing conditioned air into the other side of the vapor retarder it is rendering it's vapor retardency moot. The vapor control needs to go between the conditioned space air and the foam to have any effect.

  12. Joe Suhrada | | #12

    So Dana and Martin then diverge on having the register and return ducts?

  13. Expert Member
    Dana Dorsett | | #13

    Only if you expect the vapor retardency of the OSB to have any effect would there be a divergence.

    If you put the OSB between the foam and the attic space AND put some HVAC air into the attic you'd get some benefit from both.

    With the OSB at the attic floor with HVAC air injected to flow through the attic it's a bit like penning a herd of sheep with a really great fence, but a fence with very large open gates. The fence may still be there, but sheep still come & go as they please, unaffected by the presence of the fence.

  14. GBA Editor
    Martin Holladay | | #14

    I think Dana and I agree: "The vapor control needs to go between the conditioned space air and the foam to have any effect."

  15. Joe Suhrada | | #15

    Where the second floor meets the attic: So then I will put the OSB under the taped and sealed Sheetrock, taped seams, no unsealed electrical boxes, no canned lights. I will paint the Sheetrock with latex primer and paint. I will add conditioned air directly from a ducted mini split, as well as a return. I will make sure the register is carefully sealed where it goes up top so extra unconditioned air doesn't sneak into the attic. Ultimately the OSB doesn't add a lot to the cost, obviously and as I mentioned above I appreciate the benefit it offers in the way of stiffening across the bottom chord. Perhaps it adds a benefit, but for $800 or so it seems worth it.

  16. Expert Member
    Dana Dorsett | | #16

    "I will paint the Sheetrock with latex primer and paint. I will add conditioned air directly from a ducted mini split, as well as a return."

    I don't get the picture- draw me one!

    Putting the OSB in for structural reasons is perfectly legit, in which case you already have a vapor retarder between the attic space and the room below, which is fine as long as you don't have air exchange between the attic space and the room below.

    A better placement of the vapor retarder is tight to the foam though, which would allow you to ventilate that attic space with conditioned air, which guarantees that it stays pretty close to the moisture levels of the rest of the house.

  17. Expert Member
    Malcolm Taylor | | #17

    If engineered trusses need additional stiffening it is included in the bracing schedule. There may be good reasons for you to want to sheath the ceiling, but I wouldn't do it to beef up the trusses.

  18. Joe Suhrada | | #18

    These are not cathedral trusses. These are 4/12 standard trusses with a attic space, so putting the OSB tight up to the foam is not possible. The idea behind the foam on the underside of the roof deck is to bring the entire building inside the envelope. I know it could be an issue with moisture accumulation, that is why we are having this thread. So Dana, you are saying that the OSB is moot, if I run registers and return up there? Martin is calling fr the belt and suspenders approach. Am I wasting time and money with the OSB or what? Martin your point in well taken, I know the bracing does the job, but the OSB adds stiffness as well, even if it is not required for engineering purposes. It is certainly not FOR stiffness, the only reason I am doing it is the vapor control if it indeed helps in that area. My cost analysis is that it is cheaper to spray R50 open cell foam up there with traditional trusses, with closed soffits rather than use an energy saver truss and do R75 cellulose with open soffits. The open cell is immeasureably better in air sealing and over all envelope control. I am not interested in closed cell and am trying to develop a stack up I can justify and have confidence in.

  19. GBA Editor
    Martin Holladay | | #19

    Only you can decide how you want to detail your attic. I wasn't advocating the belt-and-suspenders approach; I merely said that I didn't think there was anything wrong with that approach.

    There have been reports of damp roof sheathing behind open-cell spray foam in attics. Several possible remedies have been proposed. If you like the sound of the remedies, you are ready to design your insulation system. If the entire issue makes you nervous, then open-cell spray foam may not be for you.

  20. Joe Suhrada | | #20

    I received a proposal for using Icynene Classic Plus which is a class III vapor barrier at 5" according to the spec page. Can anyone offer input on this product and how it might perform in the attic underside of roof deck? This is the Icynene website

  21. D Dorsett | | #21

    Almost any 0.7lb polyurethane will be a class-III vapor retarder at 5", but so what? A coat of standard latex paint on wallboard is also a class-III vapor retarder, and more vapor tight than most 0.7lb foams would be at 5" (or 5.5", in a 2x6 cavity.) Class III vapor retarders are between 1 & 10 perms. To be protective of roof decks from interior moisture drives you'd want to be in the 1-perm range or a bit less, not the 8-10 perm range. The spec sheet shows 20.7 perms @ 2", which would be 7.5 perms @ 5.5"

    But the R4/inch is really quite good for an open cell foam- most 0.7lb foams are in the high- R3s.

  22. Joe Suhrada | | #22

    Thank you for that explanation. It is quite the dilemma. I am thinking that a humistat controlled dehumidifier is an important device in such a house.

  23. Aj Builder, Upstate NY Zone 6a | | #23

    Talk to Doug at

    He lives in Saratoga and is my sprayfoam man. I trust his advice.


  24. Joe Suhrada | | #24

    I think Doug has left there and now they have Al. He is recommending the above Icynene product, AJ.

  25. Robert Hronek | | #25

    I would go with the simplest approach that has better long term performance. Since you are using trusses it rules out putting a barrier on the interior side of the foam. As pointed out paint will not work. I dont think you could get a membrane to work either. To much chance of a bad install.

    Any sheathing rot will take away any savings you may get from going that direction.

    In my opinion the cellulose option is the safer approach.
    One option you havent talked of is rigid insulation above the roof deck.

  26. Expert Member
    Dana Dorsett | | #26

    The Icynene products that WOULD work are their semi-open 2lb foams, MD-R-200 or MD-R-210.

    MD-R-200 is 1.3 perms @ 3" (R15-ish), and about R5.1/inch. For the 7" of foam in the walls that would come in at 0.56 perms, well into class-II vapor retardency, but a hint more vapor open than half-perm paint (which is never that precise anyway.) To hit R49 in the roof takes 9.6", for a vapor permeance of 0.4 perms. That's pretty vapor tight, but it's not a vapor barrier, just a hint tighter than half-perm paint on wallboard.

    MD-R-210 is 2.2.lbs density even more vapor tight but about R4.9/inch, making it a less desirable solution, if the cavity fill is to be just one product. It drops below 1 perm @ 2.4", so at 7" it would be about 0.34 perms.

    Either one is going to be pretty expensive compared to 0.7 lb foam.

    Bang for buck: At the roof it would be cheaper & better to put R25-R28 rigid foam above the roof deck, and 6-7" of 0.7 lb polyurethane under the roof deck. With half or more of the total R above the roof deck the vapor retardency on the interior stops mattering so much in climate zone 5, and is sufficient for dew point control in zone 6 with class III vapor retarders, which the 0.7 lb foam would be, if using 0.7lbs goods.)

    At the walls, 2" of MD-R-210 on the exterior sheathing with the rest being half-pound foam would work, but an all half-pound foam approach with MemBrain on the interior would also work, and would probably be less money.

  27. Joe Suhrada | | #27

    The issue with the foam is that I have these square cut rafter tails and I expect the top chords to be 6" so with the 6" of XPS foam (or better) I would have these monlithic 12" thick plus overhangs (and they are 2feet deep ) and rakes. It isn't attractive whatsoever. This is the detached garage for example.

  28. Joe Suhrada | | #28

    This is garage. Just to illustrate the rafter tails.


  29. Joe Suhrada | | #29

    I am going to post a link to the quote so you guys can see what they are suggesting. AJ, this is particularly with you in mind. I had spoken to Doug there last spring and he seemed to know his stuff. I am not do sure shout this new guy!

  30. GBA Editor
    Martin Holladay | | #30

    Q. "With the 6 inches of XPS foam (or better) I would have these monlithic 12 inch thick plus overhangs (and they are 2 feet deep ) and rakes. It isn't attractive whatsoever."

    A. This illustration from the Building Science Corp. shows how you can create a fascia and soffit that disguise the presence of the rigid foam.


  31. Joe Suhrada | | #31

    I am using a square cut fascia and two door over hangs where the bottom of the top chord of my truss is the soffit.


  32. GBA Editor
    Martin Holladay | | #32

    I get that. But if you are worried about what 6 inches of exterior rigid foam would do to the appearance of your soffit and fascia, you could always redesign your soffit and fascia.

    But it's your house. The BSC suggestion addresses aesthetic concerns. You get to make any aesthetic decision you want.

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