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Open-Cell Spray Foam in Unvented Attic

Max E | Posted in General Questions on

I need to insulate my unvented attic and the price for 1″ of r6.4(total) closed cell foam is MORE than 5.5″ of r20 open cell foam.

Can I use open cell foam to insulate an unvented attic?

thanks

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Replies

  1. Expert Member
    Zephyr7 | | #1

    It depends on what you're doing, but using open cell is often inadvisable.

    Those numbers sound a bit off too, have you gotten multiple bids?

    Bill

  2. Max E | | #2

    I have gotten multiple bids. All within a few hundred of each other. I'm in climate zone 3 btw

  3. Expert Member
    Zephyr7 | | #3

    That's surprising, but there is some price variation around the country, so that might be why your numbers seem off to me.

    Open cell spray foam has been used in attics, but there can be issues with it absorbing moisture and keeping things humid and musty. I think it's BSC that wrote an article about this. For this reason, the use of open cell spray foam is usually not recommended for attics. Closed cell doesn't have this issue, but it's obviously more expensive.

    I personally would not take the risk with open cell in this application.

    Bill

    1. Max E | | #6

      I would prefer closed cell but I've thing I've noticed is that Matt risinger uses open cell spray foam to insulate his invented attics

  4. Steve Knapp CZ 3A Georgia | | #4

    I'm in 3A and open-cell foam in the attic is quite common. I have it in my own home. But I also monitor my attic humidity and have an HVAC supply in that space. If the levels were high, I would install a dehumidifier. While it might be better to have closed cell (a least a flash), I could not find anyone will to install it.

    1. Max E | | #5

      Thanks for the info. I might have no choice but to go that route duee to price.

    2. Aun Safe | | #8

      Steve, what is the max relative humidity you'd be comfortable allowing your attic to reach before feeling the need to add a dehumidifier? 60%?

      Also, would relative humidity or dewpoint be the critical factor? An attic at 80 F and 60% RH has a dewpoint of 65. An attic at 90 F and 60% RH has much more moisture in the air with a dewpoint of 75. Should a good homeowner be focused on keeping the dewpoint below a certain level, or is RH% the correct factor to be concerned with, even though temperature swings may cause significant changes in the amount of moisture in the air at a constant RH?

      1. Steve Knapp CZ 3A Georgia | | #12

        Hi Aun,

        I have the SensorPush app set to ping me if humidity rises above 50%. During the cooling season, it does this most afternoons but then generally stays below 60%. The attic temperature is usually 5 to 10 degrees warmer than the third floor living space. (We like it warm year-round, so this can mean 80-85 degrees in the attic when it's high 90s outside.) If the weather is particularly wet and humid, the attic can get a bit above 60%. At that point, I'll usually put the Daikin ducted unit, which has a supply in the attic, into dry mode for a few hours and that pushes the humidity back toward 50%.

        I can't answer your technical question but maybe one of the experts will chime in. I'm mostly concerned with the plywood sheathing consistently retaining too much moisture. I feel comfortable that it is not, but maybe I am wrong on this.

        On the plus side, the roof is a simple gable (townhouse) and it receives full sun most days. It's covered in asphalt shingles, so I'm assuming the sheathing is drying to the interior. I kinda counting on that to heat the sheathing and drive out any excess moisture. But again, I would be curious to hear from a few experts on this topic.

        1. Max E | | #15

          Hi, what is the humidity like in the winter and in your opinion where is the biggest source of the moisture. From inside? Or from outside? Thanks

  5. Expert Member
    1. Max E | | #9

      Sorry, I didn't get a notification for this response. Thanks for the links very helpful.

  6. Expert Member
    Armando Cobo | | #10

    Most of my work is in TX's CZ3, where I never, ever, design an unvented attic with ocSPF, unless a) Its has rigid foam on top of the roof decking, used in Flat, tile or metal roofs; and b) installing 2" ccSPF against the roof decking, followed by ocSPF, used in shingled roofs. All thicknesses of the insulations depend on the brands used.
    Here is a must read blog from last Friday: https://www.greenbuildingadvisor.com/article/misleading-energy-reports-used-to-sell-spray-foam

    1. Max E | | #13

      Ha. An installer was trying to sell me on the 5.5 inch open cell foam and he said it would be r20 and the plywood deck and shingles would get me to r30. He must have thought he was talking to the typical homeowner

  7. Jon R | | #11

    Joe Lstiburek: "Again, I repeat, it is ok to use open cell low density spray foam in conditioned attics….but the attics need to be conditioned. ..."

    https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-016-ping-pong-water-and-chemical-engineer

    1. Max E | | #14

      I think I'll have to have a way of conditioning the attic but I don't I will go open cell foam. Why risk it? Worst case I can buy reclaimed xps and spend several days installing it using the cut n cobble method.

      1. Expert Member
        Zephyr7 | | #17

        If you aren't building a vented roof assembly, cut and cobble is likely to be even more risky than open cell spray foam would be. Open cell spray foam could potentially retain moisture, but at least it won't have issues with air leaks. Cut and cobble is likely to develope air leaks over time, which could cause major condensation issues on the underside of the sheathing.

        Bill

        1. Max E | | #20

          I'm curious why you say that. I'm thinking cut n cobble with spray foam at the perimeter would be just as air tight as full spray foam no?

          1. Steve Knapp CZ 3A Georgia | | #23

            There's evidence that, over time, the foam sealant separates from the surrounding framing as it expands and contracts. At least that is my understanding of the issue.

          2. Jon R | | #25

            There may be some material cost and performance advantages to using cut-n-cobble rigid foam followed by foaming over the whole thing (not just the edges) with two part spray foam (open or closed cell). At some thickness, it should form a durable air seal.

  8. Steve Knapp CZ 3A Georgia | | #16

    Max,

    On your moisture source question above, it's probably from occupants and exterior air infiltration. My townhouse was built in 2006. While I've tightened it a bit, it's still fairly leaky.

    If you can find an installer who is amenable, I would go with 1 or 2 inches of closed cell follow by air permeable insulation. That would be safer based on what I've read. You also could consider installing a vapor diffusion port. (See Joe Lstiburek site for more on that.)

    1. Max E | | #21

      Thanks for the info!

  9. Nola Sweats | | #18

    I'm in zone 2A with open-cell foam. I couldn't find a closed cell foam with ozone-friendly blowing agents in my area, so I went with water-blown open-cell. I did experience high attic humidity in the springtime, when the a/c and heat weren't running as much -- this is exactly what the models predict. I added a small a/c vent into the constricted end of the attic with a return and an LG dehumidifier in the middle of the open area of the attic. I've posted on this site about my very positive experience with that simple setup, and I've just updated the charts showing the historical heat/humidity levels. Look for the Q&A update today on "ping pong" moisture graph.

    1. Max E | | #19

      Thanks for posting. So if we can use open cell and condition the attic couldn't I also use blown cellulose/ fiber glass at the ceiling and just condition the attic space with a dehu?

      1. Nola Sweats | | #24

        You don't need an a/c vent in the attic if you have a dehumidifier, if that's what you're asking. I have both because I tried the a/c vent first and added the dehu later. The a/c vent probably keeps the attic a little cooler in the summer, but it's not necessary for moisture management.

        I'm not sure why you'd want insulation on the ceiling if you've already foamed the roofline. It would make the attic warmer in summer, cooler in winter, which wouldn't be good if your ducts (and seasonal storage items) are in the attic.

        1. Max E | | #26

          All the bedrooms are downstairs and I feel that foaming the roof and insulating the ceiling would result in virtually zero heat loss at the ceiling.

  10. Expert Member
    Zephyr7 | | #22

    No, not without a vent channel. Open cell spray foam allows moisture migration, but it's a good air barrier in the thicknesses typically used for these applications. Fiber insulation is NOT an air barrier, so you have a LOT more moisture migration than you would with a vapor open material that is also an air barrier. This is similar to why we like the "air tight drywall method".

    Bill

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