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Return and Supply Vents in a Spray Foam-Insualted Attic

suect | Posted in General Questions on

I have a contractor recommending a supply and return to condition a foam attic in Texas to decrease humidity.  Any thoughts on this?

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  1. Expert Member
    NICK KEENAN | | #1

    By "foam attic" do you mean that the underside of the roof is foamed, so the attic is inside the insulated part of the house? If that's the case then the suggestion is a good idea.

  2. walta100 | | #2

    I understand you are in Texas and everyone has their HVAC equipment in the attic but in my opinion that fact does not make it a good idea for the person paying the 25-50 % larger electric bills. Yes it is a great idea for the builder because easy to do and has a low cost to install.

    It sounds like this is an existing home with HVAC in the attic and spray foam against the roof correct?

    The way I see it there are 3 types of attics vented conditioned and moldy. Since you already have the spray foam venting is off the table. You could conditioned the attic with a dehumidifier that will wear out every other year and heat up your attic or install supply and returns vents in the attic making your HVAC equipment heat and cool the house plus the attic.

    It sounds to me like your contractor is offering you the best of the poor options left at this point.


    1. suect | | #9

      I just placed a dehumidifier up in the attic when the humidity was 65%. I last checked, it jumped to 70%. The air is cooler coming from the dehumidifier vs the attic air at 82F. Any thoughts? Today was the first 90 degree day after a 5 day run of rain and overcast. On the previous cooler days it was in the low 50’s for humidity.

    2. suect | | #11

      What was advised is an ERV, dehumidifier and attic ducting (supply and return) Would all be recommended together as well?

  3. BirchwoodBill | | #3

    I turned our attic into conditioned space by placing plywood between rafters and then installing 2 inches of closed cell foam on top of the plywood roughly R12. That lowered the attic temperature by about 35f in the summer, it went from 130 down to 95f. For winter, when it was -22f, the attic got down to 25f. We still have 16 inches of fiberglass on the ceiling that covers our insulated duct.

    My AcuRite sensors show the humidity levels a bit high at 65%, so having a small supply and return in the attic would probably lower the humidity in the attic by bringing in fresh air. Matt Rissinger is also venting his attic.

    1. walta100 | | #8

      William in my opinion you are playing with fire having your unvented and semi conditioned. It seems likely sooner or later the right conditions will happen the attic temp will fall below the dew point of the air in it and it will rain in the attic.


  4. Expert Member
    ARMANDO COBO | | #4

    I've designed many foam conditioned attics in TX, and many other states, without a problem, and it's a great solution if done correctly, like any other system in a house. All our homes have the HVAC system SIZED, DESIGNED AND INSTALLED PROPERLY, with the humidity controlled at 50% or less, controlled by an IAQ thermostat, and most without a dehumidifier. As conditioned attics are built, you need to install supply air per code.
    What is unfortunate is that here in TX most conditioned attics and HVAC systems are done wrong, so you see all kinds of "fixes". Without knowledge of how the conditioned attic was built, and how the HVAC system was design and installed, it's just a guess by anyone to suggest what needs to be done.

    1. suect | | #5

      I have had many opinions and have trialed and failed. I live in San Antonio hoping to find someone with an answer.

      We have an AprilAire fresh air intake, a 90% Trane furnace, 5 ton evaporator coil and 3.5 ton condenser outside.

      Just turned fan to medium low from high. AprilAire settings 55% high limit for humidity 30% low, 95 high temp 30 low. For awhile no set backs and humidity went high. Even at 55% I need to run dehumidifiers.
      Any suggestions? Or a paid visit?

      1. user-2310254 | | #12


        I looked at the Trane site, and it seems that direct venting is an option on the 92% efficient models. Do you know if the direct vent was installed on your furnace?

        1. suect | | #13

          Hi Steve,
          One was installed venting through the roof but it is an outer and inner sleeve for the intake and exhaust. I have not seen this before.

          1. user-2310254 | | #14

            Hi Sue,

            I don't see this configuration in the Trane instructions. A GBA member with HVAC experience might be able to comment. But I would be temped to have a local technician inspect the installation just to be on the safe side.

    2. Expert Member
      ARMANDO COBO | | #6

      Anyone trying to fix your issues need to see how your conditioned attics was built, how tight your building envelope is, all plug loads, etc. Typically in TX, Builders install 5.5” R20 ocSPF under the roof decking, which is possibly a code violation (if not under the performance code), and possibly, a building science disaster in the making. Sad but true. Did you read about any house failures in TX back in February?
      To add, Your HVAC system needs to be designed with Manuals J, S, D & T correctly performed, which 98% of the time, it’s not done. Most HVAC contractors either don’t have a clue, they are lazy or dishonest. They also know that most Builders build leaky and drafty houses, so they “compensate”.
      IMO, you need to contact an experienced HERS rater or Building Inspector in San Antonio. Someone with high-performance house experience. These guys are trained to do building envelope assessment, energy ratings and based on their findings, suggest recommendations how to fix your issues and possibly a good contractor. You can find them at or at

  5. Expert Member
    PETER G ENGLE PE | | #7

    +1 for Armando's comments above. I'd just like to add that, if you have a conditioned attic with exposed foam insulation, the insulation should be covered with a "thermal barrier" to separate it from the conditioned air space. The standard protection is 1/2" drywall or 3/4" plywood, but most foam manufacturers also have qualified fire-retardant paint systems that are easier to apply, if a bit pricey. As with most materials, it is important to follow the application instructions, especially for paint film thickness.

    1. suect | | #10

      Thanks. The foam does have an orange spray on it. I was advised this is the fire retardant layer. It has become a little hard, is this common?

    2. Jon_R | | #15

      My understanding is that in typical non-storage conditioned attics, you don't need a "thermal barrier". You only need an "ignition barrier". Which most closed cell sprays foams are, even with nothing covering them.

      1. Expert Member
        PETER G ENGLE PE | | #16

        The few muni inspectors I've discussed this with who understand it say that by conditioning the attic with supply/returns from the main HVAC system, it effectively becomes living space and therefore, the requirement for thermal barrier. The concern is smoke being ingested by the return and circulated through the house. If the attic only had access for maintenance and was separately conditioned, it would only require an ignition barrier. Makes sense to me, but I'm still not sure that's exactly what the code says. Sounds like the OP has some sort of fire retardant paint, so that's a good thing.

        1. Jon_R | | #19

          Yes, this is often why "supply only" conditioning is used. Which I suspect makes no difference (at the same CFM), other than meeting code.

          Adding plywood or drywall is expensive, so it's often worth investigating this issue.

  6. suect | | #17

    It sounds like supply and return may not be the best option due to code.

    I would like to try the dehumidifier but at start up the air is cool from the dehumidifier could this cause any concerns?

    Are there any other options to lower the humidity in the attic?

    1. Expert Member
      NICK KEENAN | | #18

      Dehumidifiers are highly efficient space heaters. Once running they should be pumping heat into the space.

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