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Decreasing Humidity and Temperature in Spray-Foamed Attic

suect | Posted in General Questions on

We have a 2500 sq ft ranch that has spray foam insulation.  We have had humidity issues and have tried to locate the source.  I purchased a device for my phone that works like an infrared camera.  I found the entire north wall in the corner lights up as a hot spot.

The ceiling is angled on that wall only, coffered type.  Going into the attic there is a space along that entire area between finished ceiling and spray foam rafters.  I have read it is not wise to fill in cavities with spray foam, what would be the best approach to decrease the humidity/temperature along this entire wall?

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  1. GBA Editor
    Kiley Jacques | | #1

    Hi Sue,

    There are a number of variables that should inform your approach to tackling high humidity in your attic. Here are a few resources that might be beneficial: High Humidity in a Spray Foam Attic, High Humidity in the Attic, and Measuring (and Understanding) Humidity.

    1. suect | | #2

      Thank you, unfortunately all links state “page not found.”

      We are keeping our humidity in the living space below 50%. The attic humidity increases above this during the day with temperature increases. I will also check the ducts in the attic thinking cool a/c and warming roof may increase humidity?

      This is a new finding and when I am in that portion of the attic I am able to feel the difference.

  2. walta100 | | #3

    The way I see it vented attic work well and conditioned attic work well.

    You seem to have chosen an encapsulated attic that seems destined to fail.

    I think at this point you have no choice but to connect your attic to the conditioned space so the attic is close to the same temp and humidity as the rest of the house.


    1. suect | | #10

      Thank you

  3. JC72 | | #4

    Daytime heating will drive moisture out of the roof sheathing into the attic space. This is a normal occurrence and not a problem when the attic space is vented to the outside.

    If your entire attic has spray foam in the rafters then this bare spot should also be addressed with spray foam.

    With an insulated unvented attic it can be a problem which would require either a dehumidifier or if your local build code allows a small HVAC return and a supply vent.

    You also want to consider the sources of humidity. For example make sure your bathroom vents are not leaking moisture into the attic space. Can lights in the bathrooms can leak air into the attic space as well.

    1. Jon_R | | #7

      > This is a normal occurrence

      I don't disagree, but one should also check for the non-normal case where rainwater gets to places it shouldn't and is then driven out by rising temperature.

    2. suect | | #8

      I’m not sure if we could foam the area. It would be inside a cavity that maybe about 2 feet deep.

      We just installed the UltraAire whole house dehumidifier. The company suggested a supply to the attic but not a return. Venting with a bath fan was suggested to exhaust moist air. Any thoughts?

      1. Expert Member
        NICK KEENAN | | #14

        When you're running air conditioning, if outside air leaks in it can cause condensation when it mixes with cool inside air. If you slightly pressurize an area -- by having a supply but no return -- the air will go out rather than in through the leaks. So this works but it's kind of a hack, it's better to seal the leaks. During the heating season you don't want air flowing out because the inside air is warmer and more humid and can cause condensation within the wall.

    3. suect | | #11

      Thank you! I will address them.

  4. Jon_R | | #5

    You can leave the attic disconnected from interior space while lowering humidity if you install a dehumidifier in the attic. Don't worry about the attic remaining warm - this lowers relative humidity.

    1. suect | | #12

      Thank you. So far I have noticed the humidity increases as it warms and decreases as it cools. Would this be concerning?

      1. Expert Member
        NICK KEENAN | | #13

        Are you measuring this with a device? How does the attic temperature and humidity compare to the outside?

        If both the temperature and humidity are increasing there is a dramatic increase in the amount of water in the air. To put it in perspective, if you're at 70F and 50% RH the amount of water in the air increases by:
        * 20% going to 70F and 60% RH
        * 70% going to 85F and 50% RH
        * 100% going to 85F and 60% RH

        So if both humidity and temperature are increasing somehow a lot more moisture is getting into your air. There are two possible sources. One is that humid air from outside is getting in and bringing humidity. The other is that liquid water is somehow getting into your attic and evaporating. In both cases the solution is to block the leak, but the methods differ.

        I would be more worried about undetected liquid water.

        1. Expert Member
          NICK KEENAN | | #15

          To put those numbers in perspective, imagine an attic that is 1,000 square feet in area, 12' at the peak and zero at the eave. That's 6,000 cubic feet. At 70F and 50%RH the air in that attic contains about 4 pounds of water. At 85F and 60% RH that air contains about 8 pounds of water. The difference is about half a gallon. That's a huge amount to be coming in as vapor. If you really are seeing an increase in both temperature and humidity I'd be looking for places that water is getting in.

  5. Expert Member
    Peter Engle | | #6

    Nobody has mentioned it, but if you can feel that one wall is noticeably hotter than the rest of the attic, it is entirely possible that the wall is either not insulated properly, or that there are exterior air leaks somewhere in the wall, or both. Your IR image doesn't help much, but used properly, you might be able to find local defects in the insulation with an IR phone attachment. You should fix air leaks first, improve insulation second, and if the humidity stays high, run a dehumidifier. It shouldn't take much dehumidification to condition a properly spray-foamed attic.

    1. suect | | #9

      Thank you! I would like to seal the area if possible. The cavity Is about 24” plus deep by 2-3” wide by 15 ft long. What would be the best approach? I hope to find a specialist but most prefer a complete project.

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