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My attic is Open Cell Spray Foam – is it OK to air condition the space?

ahha | Posted in Energy Efficiency and Durability on

My attic is Open Cell Spray Foam – I have a 2600 sq ft dehumidifier running. The guy that did the spray foam says I need to run an a/c vent to the attic. One A/C company added a small 4″ auto close duct. They said that would be OK to add without causing to much positive pressure to the attic. Spray foam guy said that was not enough.

He said to disconnect one of the overflow ducts, cap off the connection. Then remove the barometric manual dampener and add an electronic dampener on that piece of the overflow duct (just because the electronic is more reliable) and dump that a/c into the attic.

One A/C company I was going to have to the work said they wouldn’t do it because it could cause too many other problems with positive pressure in the attic and the pressure pushing air back into the house could cause the a/c unit to short run. They also said that open cell spray foam attics were not meant to be air conditioned like you air condition the house.

I really need some feedback because my attic is still running hot, after the open cell spray foam. Hot days in the 92 degree range the attic will run about 97 degrees. The guy that did the spray foam said that I need to run some a/c into the attic to help fix that issue.

Thanks for your help and comments.

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Replies

  1. Expert Member
    AKOS TOTH | | #1

    Ahha,

    You have built an unvented attic which is now part of your conditioned space. That means you need to condition it. This means providing the right amount of supply as well as a return in the attic space. I would check your local code, typically it is around 50cfm per 1000sqft of conditioned space so 130CFM in your case. That is about a 6" supply duct. The return should be near the peak of the roof again sized to handle the 130CFM.

    Since there is both supply and returns in the attic, they won't pressurize anything. Besides making the attic and house more comfortable, adding the right amount of conditioning will also remove humidity which can build up with open cell foam and cause your roof deck to rot over time.

  2. ahha | | #2

    I was reading never to create an air return using attic air.....???

    1. user-5946022 | | #3

      I believe the "attic air" referenced in those types of statements is unconditioned attics.
      By applying open cell spray foam to the underside of the roof deck, between the rafters (please confirm that is what you did) and totally sealing around the perimeter of your attic, your attic air becomes part of the conditioned space. As such you need to be aware of humidity and remove it if needed.
      First you need data. Get yourself a way to measure the humidity at the peak in your attic. As I understand it, it should be about 50%, but Allison Bailes recently wrote an article indicating not to obsess about 50%, and be sure to keep it under 60%. If your attic humidity at the peak stays under that you may not need to do anything.
      Second if your attic humidity at the peak is not under that, you may be able to solve is with a small supply duct.
      Third, if the small supply duct is insufficient, then yes, a larger supply with a return near the peak.

  3. ahha | | #4

    Yes, this is what we did (open cell spray foam to the underside of the roof deck, gable, between the rafters (please confirm that is what you did) and totally sealing around the perimeter of your attic).

    I have a dehumidifier in the attic and keep it at 45%.

    Issue is that my electric bills are higher since I foamed the attic.

    Is it really ok to add an a/c duct. One A/C company I was going to have to the work said they wouldn’t do it because it could cause too many other problems with positive pressure in the attic and the pressure pushing air back into the house could cause the a/c unit to short run. They also said that open cell spray foam attics were not meant to be air conditioned like you air condition the house.

    1. user-5946022 | | #5

      It sounds like the issue you are trying to resolve is the higher utility bills vs. humidity in the attic.
      Is that correct?

      If so, what type of dehumidifier do you have up there? If it is just a room type unit, many of them are very inefficient. However they also have a very low acquisition cost. Built in dehumidifiers, which can run up to $2k, can be very efficient. Be careful though - not all built in units are efficient. Some are just as bad as the room units and cost more...

      Secondly, do you need to dehumidify the attic to 45%?
      - This article by Allison Bailes indicates anything under 70% RH may be be ok
      as in mold could not grow.
      https://www.greenbuildingadvisor.com/article/relative-humidity-doesnt-tell-you-how-humid-the-air-is
      There was also another recent article or comment by Allison (which I now cannot find) which suggested or indicated not being concerned as long as RH is under 60%.
      Perhaps you could save energy allowing the RH to increase.

      Also, you indicate energy usage has increased since spray foaming - if your open cell spray foam attic was previously not being conditioned, you are not conditioning more space. Ideally, if ducts are going through that attic, your total energy use is offset by ducts going through a conditioned space. Also, if the humidity up there was previously high, you are now lowering it, which costs energy.

      You also indicated you try to "keep" the RH at 45%. In my open cell spray foam attic, the RH is lowest in the morning, and increases 5-10% by late afternoon, and then starts decreasing again. I would think it would be expensive to keep it consistent.

      Although there seems to be alot of info here and there on this, I wish GBA would do an article on exactly how to operate a house with an open cell spray foam attic. Mine supposedly has a vapor diffusion port, but no attempt was made to dehumidify or send conditioned air to the attic. The other issue is it may be against code to send conditioned air to the attic. However, the ducted mini in my "conditioned" attic is noticeably leaky, so the attic is getting somewhat conditioned that way. As long as the house humidity stays around 50%, the attic maxes out around 60%, which I hope is ok.

      1. Expert Member
        AKOS TOTH | | #6

        It could also be the attic is still leaky. I've seen spray foam jobs where the installer missed soffit areas in tricky spots and you could clearly see daylight. Air leaks are very hard to see, even something that looks tight can leak significant amounts of air. The best is to do a blower test to check and seal any leaks at the same time.

        In terms of conditioning open cell attics, here is my reading of the literature:
        -if there is no RH issues, there are no issues, do nothing
        -if RH is high add some conditioned supply (return is a code issue unless there is a thermal barrier over foam)
        -if still high, add an exhaust fan near the ridge venting outside

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