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Community and Q&A

Attic moisture issues

237ScottWilson | Posted in General Questions on

I am having issues with moisture on the underside of the roof in my attic. My house is 90% covered with solar panels. I live at the New Jersey shore and the humidity level is generally high. I am thinking that because the sun does not warm up the roof the moisture just does not go any where. Should I seal the soffit with foam board and attach to rafters making it sealed? Maybe use plastic sheet to help to make it more air tight?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name? (I'm Martin.)

    Properly built attics are naturally dry. If an unconditioned attic is having moisture problems, it's a good bet that the cause of the problem is a leaky ceiling that allows indoor conditioned air to enter the attic. There is also a good chance that the house has a damp basement or crawl space.

    The solution is to seal the air leaks. Here are links to two relevant articles:

    "Air Sealing an Attic"

    "Blower-Door-Directed Air Sealing"

  2. 237ScottWilson | | #2

    Hi I am Scott Wilson
    I have a dehumidifier in my basement and one in the main part of the house they are both set to 45% and I can still see water droplets on the nails in the attic .

    1. GBA Editor
      Martin Holladay | | #3

      It's still probable that your ceiling (the floor of your attic) is leaky. Frankly, the fact that you have installed a dehumidifier in your basement is a good indication that you've got a damp basement.

    2. Expert Member
      Dana Dorsett | | #5

      Setting a basement dehumidifier to 45% is unnecessarily low. Anything below ~60% is enough to mitigate the "stinky basement" smell and mold.

      As long as the roof isn't leaking the biggest wintertime source of attic moisture is air infiltration from the conditioned space through the attic floor.Air sealing the house, at the attic floor & basement band joist can do a lot for reducing 24/365 outdoor air infiltration, which is the major driver of summertime indoor humidity levels. With the upper floor ceiling/attic floor well sealed the attic ventilation should purge the moisture levels to something reasonable and keep it there until/ unless there are roof leaks. Pay attention to sealing both ends of any flue, electrical, or plumbing stack chases or balloon framing that extends from the basement to the attic.

  3. 237ScottWilson | | #4

    My basement is finished and has a bedroom and bathroom in it the dehumidifier is more a preventative measure. I have noticed since switching from eletric base baord to mini splits for heat and AC there is definitely more moisture in the house in the winter months. The dehumidifiers have no problems holding the humidity level. I have painted the bathroom ceilings with non permeable paint and put 200cfm fans into the bathrooms.

  4. 237ScottWilson | | #6

    The 45% setting was an attempt to reduce the reduce the moisture in the attic and it does not seem to have an effect.

    1. GBA Editor
      Martin Holladay | | #8

      If operating a basement dehumidifier has the effect of reducing your attic moisture problem, that fact confirms that you have air leaks through your ceiling.

      1. Expert Member
        Dana Dorsett | | #10

        But he's saying having dehumidifiers set to 45% in both the basement and the conditioned space upstairs is having NO effect.

        And the reason why is that 45% RH air in the CONDITIONED space (as in "not the basement") is still too high

        The problem still likely be air leaks through the ceiling if it's actually staying at 45% RH @ 70F in the conditioned space in winter. The dew point of 45% RH/70F air is 48F, which is WAY above the average outdoor temperature in winter along the Jersey shore, which runs in the mid-30s F:

        Holding the line at 30-35% in the conditioned space in winter would be appropriate for mitigating against it, but the dew point of 30%RH/70F air is 37F, which is still above the mid-winter average outdoor temperature, but probably not the mid-winter average temp in the attic.

        It's just silly/wasteful to keep the basement at 45% RH, winter or summer 60% is enough down there.

        1. GBA Editor
          Martin Holladay | | #11

          Somehow, I missed the word "not" in Scott's sentence.

  5. Expert Member

    This could get complicated. There is at least one other poster named Scott Wilson on GBA.

  6. Peter Yost | | #9

    Hi Scott -

    I agree with Martin that your problem is likely a air-leaky ceiling; have you had a blower door test done on your home? If not, that would help in understanding the situation.


  7. 237ScottWilson | | #12

    If it is a leaky ceiling then spray foam the attic floor (Sheetrock)with 1 inch of closed cell? I want to increase the R value in my attic as cheaply as possible I have 2X8 joist and I use the attic for storage.

    1. Expert Member
      Dana Dorsett | | #13

      It doesn't take an inch of clo$ed cell foam to air seal sheetrock. Detailing all the electrical & plumbing penetrations with the appropriate sealants, as well as non-combustible metal air-dams on flue chases with fireproof sealants on the seams is a lot cheaper and more environmently friendly than an inch of closed cell foam.

      If there is sufficient headroom to lay down a course of 2x6 joists perpendicular to the 2x8s you can then fill it up to the top with cellulose and hit R47 at center cavity, but it would meet IRC code on a U-factor basis due to the dramatically lower thermal bridging. If there's room for another course of 2x8s it would meet/beat code on an R-value basis. That approach was quick and effective for a co-worker of mine a couple of years ago when working on his 1920s bungalow. He even re-used the old plank flooring boards for the storage deck on top of the perpendicular joists.

  8. 237ScottWilson | | #14

    Even if I air seal the attic floor wont moisture raise throw the sheet rock? If my roof is 90% covered with solar panels is there a benefit to leaving the attic vented? Would I be better off sealing the vents, foam board to rafters and dehumidifier in attic?

    1. Expert Member
      Dana Dorsett | | #15

      >"Even if I air seal the attic floor wont moisture raise throw the sheet rock?"

      Sure, some moisture will still get through via vapor diffusion, but it's a miniscule amount.

      Even a square inch of air leak moves more moisture via air transport \ than the entire ceiling's worth of vapor diffusion through sheet rock painted with standard latex ceiling paint.

      With solar panels shading the roof deck the roof deck won't warm up as much to help dry it out. That makes the attic venting even MORE important than it would be without the panels. But if the ceiling is really leaky almost no amount of venting can compensate for the air-transported moisture.

      1. 237ScottWilson | | #16

        Thank you.

  9. BazookaSkill | | #17

    Solving moisture problems can be complicated. The problems are usually created by an assortment of causes and often require a combination of efforts to eliminate. It's a matter of achieving a balance between adequate ventilation and adequate moisture input, to create a healthy living environment. Eliminating attic bypasses is the main strategy to avoid moisture problems in attics. Adding more attic ventilation is not the right solution. Additional roof vents allow more air to escape from the attic and that pulls more warm moist air from the house into the attic. The real solution is to seal off your attic bypasses and install adequate levels of insulation. This should eliminate frost from accumulating in your attic.

    1. GBA Editor
      Martin Holladay | | #18

      While I agree with most of your advice, one of your statements is odd.

      You wrote, "It's a matter of achieving a balance between adequate ventilation and adequate moisture input, to create a healthy living environment."

      I disagree. Moisture input into an attic is always undesirable. You want to minimize moisture input. The ideal amount of conditioned air leaks into an attic is none.

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