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Moisture Problems With Open-Cell Spray Foam in Attic

NikoFL | Posted in General Questions on

I am about to insulated the attic of my 1982 house in florida with open cell foam. I’m reading everywhere that open cell foam can lead to moisture problems in an insulated attic. Some say ‘conditioning’ the attic will help. But in order to minimize air exchange between living space and attic space, I don’t want to put supply or return registers of my central AC in the attic.

This leads me to my question. Is it feasible to just put a dehumidifier (e.g. Aprilaire self draining) into the attic? It seems like a simple answer to a complex problem, so would like to get this confirmed here.

Thank you

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  1. user-2310254 | | #1

    Installing open cell against the roof sheathing is more problematic in colder (greater than CZ4) climates. It is important to monitor conditions (can be done with a low-cost remote sensor) in the attic to ensure that humidity is kept in check. You also can install a supply in the attic to help condition the space.

    You might need mechanical dehumidification, but you might not. Keep an eye on conditions and make this investment if necessary.

  2. larkomundo | | #2

    I am in CZ 4 and have done lots of homes with encapsulated attics. Steve is giving you good advice. These attics need to be conditioned or at the very least, humidity controlled. In new homes, I include the attic as just another room in the Manual J load. They are provided with the necessary airflow, same as any room in the house. Then I design a damper and an auto changeover thermostat to shut off airflow if the attic gets too hot or too cold. That thermostat does NOT control the equipment. It simply operates the damper. I never install a return in an attic and just allow unintentional leaks in the ceiling to provide pressure balance. In no way do I believe I have all the answers. My motto has become, "if I admit I don't know everything, at least I know one more thing than the other guy". And the more I see sick folks, the less I like spray foam. But many builders love the stuff. So I suggest the best I know.

  3. Expert Member
    PETER G ENGLE PE | | #3

    Steve is on the right track. A low-cost humidity monitor will let you track moisture over time. If the attic spends more than a few hours at a humidity of 90% or above, you'll need to do something to dehumidify it. A simple dehumidifier can work, but depending on the extent of the moisture sources, can cost a fair amount to run. You can also install vapor diffusion ports along the ridge to allow the humidity to diffuse through the foam and out the ridge.

    1. AnnieL | | #5

      Hi Pete, figured I'd reach out based on your answer above, which pertains to my situation. I am in need of insulation ASAP in humid Tampa, Florida. I moved here from Colorado (no humidity experience)... We recently sucked the nasty old insulation out of attic, replaced 20+ year old AC/ducts, and installed a vapor barrier in crawl space (to improve air quality/allergies) -- all of that has helped tremendously. Planned on spray foaming right away, but paused the job because I'm learning that foam can cause major humidity issues. I don't really want to have a separate dehu installed in the attic. Some people say I should ideally have 3 dehus... attic, crawl space, and living area... it's a bit overwhelming to understand on the fly. I've not heard of a vapor diffusion port. The roof is new and has a ridge vent... so does a roofing contractor pull the ridge vent off and replace it with vapor diffusion ridge? Is this pricey? Should I do it before foaming, or wait and see... do it afterwards if needed? (since the foam will seal the ridge vent -- seems like it would be more expensive to do it retroactively, after foaming?) This is a very entry-level house; so I'm trying to be reasonable with expenses... we will live in it part time (aging parents are here). Rest of year, it will be a rental. Is spray foam the best insulation choice for my situation? Or should I just do a quality air seal, vapor barrier foil, and blown insulation --- in order to avoid humidity risks?? Many MANY thanks for your helpful advice!!

      1. NikoFL | | #6

        Hi Annie, I posted the original question and I am in the Tampa Bay area as well and still haven't figured out what to do with my house. I have moved away from spray foam though and will likely do an unvented attic with air permeable insulation on top of the attic floor and a vapor diffusion port. I don' t need the space of the attic for storage or anything - I just want to mitigate risk of my duct system sweating - therefore unvented. 2020 florida building code allows this (R806.5).

        BTW not sure if GBA allows this or has a privacy friendly functionality to share info but would love to start talking to people locally about project like this. If you are interested let me know.


        1. Expert Member
          Akos | | #7


          If you have your air handler in the attic, you want the permeable insulation in the rafters, not on the floor. This way the air handler is inside now conditioned space. The diffusion vent at the ridge is still needed.

          If your rafters are not deep enough for a lot of insulation, you can hold batts in place with straps (last picture in the slideshow):

          Make sure to air seal the soffit area to create a proper unvented roof.

          Your unvented attic will need some conditioned airflow, usually the air leakage in the ducts is enough but in case it is not, a supply vent will also be needed.

          1. NikoFL | | #8

            Thank you for your comment. The air handler will not be in the attic only duct work. I was going back and forth on the location of the insulation and my reasoning for choosing top of the attic floor is the fact that my primary goal is indoor air quality - and therefore I don't want any air exchange between the house and the attic (it will have open fibrous insulation).

            I think BSC calls it an unconditioned unvented attic (

            I understand there are drawbacks of unconditioned unvented attics with vapor diffusion port as opposed to conditioned unvented attics. Especially with leaky duct work. However, I am redoing all my ductwork with great attention to detail and have it leak tested so I should have minimum leakage.

            Maybe I am missing something but that's how far I got :)

            Love the conversation - keep it coming.


          2. Expert Member
            Akos | | #10

            As long as you seal up your ducts and registers your unconditioned unvented attic with insulation on the attic floor should work. Usually where the flex is run from the air handler to the attic is also big air leak that is not easy to air seal, this should get some extra attention as well.

            I think you would still need to seal up your soffit vents, you want the dewpoint of this attic volume to track the house not outdoors to reduce the chance of condensation on the ducts.

            Stuffing batt insulation into holes doesn't do much if any air sealing, air leaks need to be sealed with something solid such as rigid foam insulation sealed in place with canned foam.

          3. user-5946022 | | #11

            The BSC article you linked does indeed discuss unvented unconditioned attics. There are two issues you may wish to delve into the details on:
            1. Ducts transmitting air going through an unconditioned space: You state the air handler is not in the attic. So you are conditioning the air in a conditioned space (good), then sending that conditioned air, through ducts, through an unconditioned space (not good because that tempers the air to closer to the temperature of the unconditioned space) and then back to a conditioned space. The ways to deal with this are to bring the ducts into a conditioned space by either dropping soffits below the ceiling in select areas (closets, at perimeters, in hall, etc.) or building an enclosure in the attic over the ducts and air sealing & insulating over and around that so that the ducts end up in the conditioned space, even if that is part of the attic. The aggravation of doing this is often costlier than simply including the attic in the conditioned space by bringing the insulation to the underside of the roof deck. This is especially true if you have a low slope roof (5 in 12 or less).
            2. Air sealing: Based on your posts I presume you are aiming for an above average level of air sealing between the conditioned and unconditioned spaces. In order to meet your goal of limiting air exchange between the house and attic due to fibrous insulation, air sealing will be just as important as duct sealing.

            In regards to the fibrous insulation, some fibers are less objectionable than others. Consider blown cellulose - at least it is not itchy if you need to move it aside to access a box or similar. Note that there are also methods of insulating the underside of the roof deck that do not involve spray foam, although they are less common and perhaps more difficult to obtain a proper job. Also, if you have not already done your roof, insulation on top of the roof deck could address many of these issues.

        2. AnnieL | | #9

          Hi Niko, thanks for responding. I posted my own question yesterday, separately from your post -- but nobody replied to my question haha. Bit of a crash course, learning all this on the fly... It's interesting information; but not in an advance planning situation like you. We started having allergies, knew it was the house, and needed to resolve the cause quickly. All would be entirely complete by now; the only reason it is not complete is because of all the mixed/confusing info on spray foam -- I suspended adding insulation until I felt confident in the best approach. I've talked to so many people. I much prefer to do spray foam for easy rehab in attic and for air handler; but I will probably do fiberglass or cellulose in order to avoid air quality issues... my main concern humidity. But I do realize a small % of jobs also have lingering off gassing issues. Seems like spray foam solves a lot of problems... but comes with some risks of its own.

  4. Nola_Sweats | | #4

    I'm in your position, and I agree with the earlier posters. I'm in New Orleans, so a similar climate to yours. I put in open cell a few years ago, fully aware of the moisture risk. For a few months, I monitored temp and humidity with a $50 "SensorPush" pod that sends readings to my phone.

    When I started seeing high humidity levels in the attic, I bought a stand-alone LG dehumidifier from Home Depot with a hose attachment to drain into my a/c drain. It has worked well to keep humidity below 60% for about two years so far. I posted my costs somewhere on this site but can't remember them now; it's not much, maybe $1-$2 per week of electricity? The only maintenance is to put something in the drain line occasionally like you would with an a/c condensate line, to keep it from clogging.

    I did also open a small vent into the attic from a duct that used to go into an interior closet, but that wasn't enough on its own -- the humidity issue is worst in spring and fall, when your a/c isn't running much and so is not dehumidifying. In summer, the a/c runs so much that the dehumidifier almost never kicks on. In winter, the air's usually dry enough that the issue doesn't arise.

  5. walta100 | | #12

    I think you are looking for the free lunch when you make an unvented attic and refuse to condition the air in the attic to more or less the same as the other conditioned spaces.

    The way I see it you are playing Russian roulette with mold and rot. Yes some people do win at this game and get the free lunch but they tend to be some place dry or cold from here it sounds like you live in a warm damp environment aka swam.

    The idea if moving the insulation from the attic floor to the roof line is a marginal improvement at best. The roof line will have a much larger surface area to cover with insulation and this larger area will always lose more heat. Adding another R of insulation on the roof line cost 5 times as much as it would to the floor so they tend to scrimp buy fewer R at the roof line.

    The way I see it the smart move would be to get the HVAC out of the attic.


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