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Community and Q&A

Attic air barrier/vapor retarder

Julie Brown | Posted in Green Building Techniques on

I’d like to get some feedback on this detail (attached). Background: <700 SF seasonal home in Climate Zone 6. 2×6 at 24″ walls are insulated with dense pack cellulose. Rafters are 2x12s at 16″oc (snow country) and ceiling is 2×8 T&G douglas fir laid across 2×10 (actual) bottom chord ~44″ apart which are exposed to the living area below. It is 1-1/2 story (just lower level and loft) and I am venting the roof. Heat is radiant slab from tankless boiler augmented with a wood stove when we are there (the only penetration in the ceiling besides the hatch, both sealed well). There is no air conditioning and never will be (just mountain air). To get a min R-49 and a vapor retarder at the rafter bays we are using closed cell spray foam and I have installed SmartBaffles in each bay giving 2″ of venting (love SmartBaffle by the way, super easy, installed myself). The spray foam goes all the way to the top wall plate. I have used Henry Blueskin VP as my air barrier on the exterior of the walls (with Homeslicker over that for a drainage plane). The air barrier membrane ties in to the spray foam at the bottom of the rafter bays. Above the attic floor however, I am switching to Roxul which will be applied continuously over the floor and over the top of the spray foam (which stops at the solid blocking in each bay). Since the attic plank flooring is not air tight I wanted a AIR BARRIER and also felt it would not hurt for it to double as a vapor retarder. So I am using 4 mil poly which is lapped and sealed and yes, will be meticulously sealed over the top of the blocking in the rafter bays so it can be as continuous as possible with the spray foam below. Here is where I am looking for feedback: My insulation contractor suggests I spray 1″ of foam over the planking instead of the poly. He thinks it will cause moisture problems but I did not get a lot of reasoning as to why. (We are installing the poly, not him.) I am resisting spray foam in the attic for a number of reasons: 1.) I am only using spray foam where nothing else will do the job as well (sloped rafter bays). While spray foam has its benefits I do not think it is the be-all to end all (I will defer that discussion to others already held in GBA). 2.) Spray foam is irreversible and we may do other things in the future with our attic space. At least with poly and batt I can remove it without damaging the planking. 3.) Spray foam is expensive (Also, I have never used this contractor before and have inspected enough bad spray foam jobs to know it can be screwed up. I don’t want to increase that potential). 4.) Spray foam is flammable and I do not want all of that over my family’s head…our chimney passes through the attic and while it is insulated & separated from the insulation I would rather not increase the combustible materials around it. I would have to spend more money covering it with a thermal barrier. 5.) What would the moisture problem be? The 1.5″ thick wood planking has hygric buffering capacity. The poly has a perm rating of 0.08, while 1″ of foam would be more than 1.0 perms (We would need at least 1.5″ or 2″). The surface temp of the poly should be warmer than the air hitting it as it has R-38 on top of it and I can always add more insulation. I don’t see what the spray foam would do differently (or better)? Thoughts? Comments?

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  1. Expert Member
    Dana Dorsett | | #1

    Polyethylene caulked and taped at the seams & edges is more than adequate as an air barrier over your 2x6 t & g planking, no need to go with a flash-foam there. But there's definitely a better & cheaper way to go.

    The 8" of closed cell foam between 16" o.c. rafters is a waste of foam. The higher than average framing fraction and short path through the framing makes it FAR lower performance than R49 over deeper rafters.

    In zone 6 you can go unvented as long as at least 50% of the R value is low permeance foam adhered to the underside of the roof deck. A 2 x 12 is nominally 11.25". If you installed 4" of closed cell foam on the underside of the roof deck the foam-R would be R24-25-ish and you would have 7.25" of space for fiber.

    A standard density R25 batt designed for 2x8 framing performs at about R24 when compressed to 7.25",(see: ) so with that stackup you be at the prescriptive R49 value, with the prescriptive minimum ratio using literally HALF the foam, but also with 27-30% less thermal bridging due to the longer path through the ~R1/inch (assuming douglas-fir) wood.

    With that stack up you do the whole roof, including the mini-attic un-vented for less foam, less money, less fuss, and it delivers higher performance. Rather than 4mil polyethylene on the attic floor, put something semi-permeable on the underside of the batts & rafters, such as 2-mil nylon (Certainteed MemBrain). You could use 2-mil nylon under the gypsum board part of the ceiling too, but it's not necessary if you have R25 closed cell foam with R24 fiber below.

    If you installed rock wool, cellulose, or high density batts in the 7.25" space it would push the ratio to less than 50% foam, but not so far that it's a real hazard. With cellulose the moisture buffering capacity of the material would handle it, but if denser fiberglass or rock wool the 2-mil nylon would be cheap insurance against mold getting started in the insulation. (At 12-15 cents per square foot, installed the sheet nylon it's a heluva lot cheaper than another inch of closed cell foam!)

  2. Julie Brown | | #2

    Thanks for the input, not arguing with the approach you suggested at all, but I have a vented roof, it is a done deal and so is the spray foam in the rafter bays (already installed). So with that in mind, do you think the 4 mil poly is an issue on top of the 2x8 attic floor, or would Certainteed Membrain be better there? I still have time to make the switch (if I can get the material in time...that has been a practical issue in the area I am building).

  3. Expert Member
    Dana Dorsett | | #3

    With a fully vented attic a polyethylene air barrier on the conditioned space side of the assembly between the insulation and ceiling planking is fine.

  4. GBA Editor
    Martin Holladay | | #4

    I disagree strongly with Dana on this point. The 4 mil poly is thin and delicate. As insulation workers install the mineral wool above the poly, it will get damaged -- if not now, then at some point in the building's history. Each fastener penetration through the poly is a tiny air leak, especially if the fastener distorts the poly. Lapped seams are likely to be imperfect.

    Above the board ceiling, the 4 mil poly is a perfectly adequate vapor retarder. But it is a lousy air barrier -- and in this location, a durable air barrier is essential.

    Assuming the boards have not been installed yet, I urge you to start with a layer of taped drywall before you install the boards. If you don't want to use drywall, you can use a tough Eurpean air barrier membrane like Intello Plus.

    If the boards have already been installed, then the suggestion to use 1 inch of closed-cell spray foam as an air barrier is an excellent suggestion.

  5. D Dorsett | | #5

    If using foam as the air barrier, 3" of open cell foam would be more effective, 1000x greener, and higher R than 1" of closed cell foam at about the same price.You may still want to put down the polyethylene first as a slip-surface between the foam & planking, since the planking will undergo seasonal dimension changes with temperature and humidity.

  6. GBA Editor
    Martin Holladay | | #6

    Good point. I agree with you that 3 inches of open-cell foam is more environmentally friendly than 1 inch of closed-cell foam.

  7. Julie Brown | | #7

    Is it the durability issue alone or other reasons that you feel the 4 mil would make a poor air barrier in this instance? Ordinarily I would agree with you, but I am confronted with cost issues about using more spray foam or an intelligent or self-adhered membrane over the surface of the attic floor. Additionally, I want this to be completely reversible when we modify the building later (the building is <700 sf and the attic is under 400). i shudder at thought of scraping away disposing foam in a few years. considered laying gwb on top poly but it seems duplicative, costly can't get full size sheets up there.

    Knowing that workmanship would be critical to the performance I spent a lot of time installing the poly myself and meticulously sealing the edges around every rafter bay, studs at the gable walls, and seams with strips of left-over self-adhered membrane. It was NOT fun and I could not have hired someone to do this, I admit. The plank floor is blind-nailed so there are no exposed fasteners. But you bring up a good point and I will probably lay down the batt myself as well to make sure the poly does not tear underfoot. This attic by the way will not be used for storage and we have a "catwalk" down the middle if we need access (built on top of a strip of rigid foam laid over the poly).

    I acknowledge that spray foam has its place and can make things easier, but it has its drawbacks too. However, if I had to rely on the workmanship of hiring someone I would probably go another route, more similar to what you suggest. I appreciate the input.

  8. GBA Editor
    Martin Holladay | | #8

    Establishing a durable air barrier at your ceiling is extremely important. Due to the stack effect, the pressure of the air near your ceiling (with respect to outdoor pressure) is higher than anywhere else inside your house. Here is where you really want a durable air barrier.

    I'm a big believer in durable air barriers liked taped drywall, taped plywood, Ice & Water Shield, and similar materials. 4 mil poly? Not so much.

    It's your house, and you know how tight the poly is, and whether it's likely to get damaged when the insulation is installed. Don't forget the cable guy, the electrician, and other tradespeople who may enter your attic over the next 30 years.

    If you're happy with the 4 mil poly, that's all that matters.

  9. Jon R | | #9

    Sounds like a good application for a fluid applied air barrier. But I don't know how the cost compares to the other options.

    Would be interesting to see test data on fully adhered membranes vs non-adhered.

  10. Julie Brown | | #10

    Jon, I don't think fluid applied would work as this attic floor is 2x8 planking, so there are joints and some knot holes. The attic floor/ceiling joists are exposed below the attic floor, there is no finished ceiling. But I would like to see data on self-adhered vs. non-adhered. If I weren't weighing down the poly with insulation and taping it, I would not use it.

    Martin, I could not agree more on the importance of a durable air barrier. Knowing I plan to add on in a few years, I think this detail will work for a while and be easily removable (and was cheap). At that point I will probably use a different material. I will definitely change the detail to box out where the attic floor meets the rafters and at least fill that with spray foam as having to strip-in self-adhered in and around the rafters and blocking was a major pain that I wouldn't repeat.

    Thanks again for the input.

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