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Musings of an Energy Nerd

Why Is My Attic Damp?

The cause depends on whether you have an unconditioned attic or a conditioned attic

If attic air is humid, and roof sheathing is cold, condensation or frost can accumulate on the underside of the sheathing. The source of the attic moisture is usually indoor air that enters the attic through ceiling cracks. Photo: Reuben Saltzman, Structure Tech

If you’ve got a damp attic, you may think that diagnosing the problem is complicated. And you may be right. But in the vast majority of cases, damp attics have one of just two causes.

Let’s look at the phenomenon of the damp attic by dividing them into two categories: vented unconditioned attics and unvented conditioned attics.

Vented unconditioned attics

A vented unconditioned attic is the old-fashioned kind. You usually enter this kind of attic through a ceiling hatch, although some vented unconditioned attics have stairs. These attics are cold in winter and hot in summer.

How is this type of attic vented? Arrangements vary. In a new house, this type of attic usually has soffit vents and a ridge vent. Some older attics simply have gable vents. And some of these old attics don’t have any obvious vents — they’re just naturally leaky, with outdoor air entering and leaving through random cracks.

Vented unconditioned attics are naturally dry. They tend to get baked by the sun, which beats down on roof slopes facing east, south, and west. This type of solar heating just about guarantees dry conditions.

When a GBA readers tells me, “I have a vented unconditioned attic that is damp. There is black mold on the underside of the sheathing, and water dripping from the pointed ends of the roofing nails,” I’ve got a pretty good idea of what’s going on. In almost all cases, the moisture that is causing these problems is entering the attics through leaks in the ceiling.

Here is the mechanism: In winter, the stack effect causes the indoor air near the ceiling of a building to be pressurized with respect to the attic. If there are any holes in the ceiling — and there usually are —…

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  1. AndyKosick | | #1

    Along with damp crawlspaces and basements, I think it's worth mentioning homeowners who are humidifying their homes to 50 and even 60 %RH in cold weather, intentionally or in some cases without even knowing it. I recently looked at a home that had condensation issues (not in the attic), the humidifier on the furnace was set to 50% and the couple wasn't even aware of it.

    1. GBA Editor
      Martin Holladay | | #2

      You're right, of course. Thanks for the reminder. When the subject of humidifiers comes up here on GBA, I've been known to advise readers to remove the humidifier from their house and to put the equipment in the path of a steam roller. (Assuming that steam rollers still exist -- they probably don't. I guess they call them "asphalt rollers" these days.)

      1. Trevor_Lambert | | #3

        People still call them steam rollers. The general term is road roller. True steam rollers haven't been in even sparing use for about 50 years. That's longer than I've been alive, and yet steam roller is the only term I've ever heard used.

  2. williamfinch | | #4

    Speaking of Florida, Georgia (and AL, MS, NC, SC, TN, LA) and all those other forsaken places where the humidity is continuously high (particularly at night) and where attics are usually vented and where temperatures drop below freezing (and sometimes well below) 10-70 days per year ...are the problems here similar to what you see in PNW? Or are high humidity/cold nights too infrequent in these climates to support damage or concern? Does having a cool roof contribute significantly to the problem in those climates?

    1. GBA Editor
      Martin Holladay | | #5

      I haven't heard any reports from the Southeast of the type of attic mold problems seen in Washington and Oregon. One reason may be that these southeastern states are sunnier, and sun tends to bake attics. The areas that suffer from mold due to outdoor air tend to be cloudier and cooler than the states you mention.

      All of that said, I'm interested in reports from GBA readers -- especially reports from those who live in the states you mention.

  3. Expert Member
    Peter Engle | | #6

    I just came back to the office from a house in NJ that is having mold problems in the attic related to exterior moisture, not interior. There has been dampness throughout the attic, but it is clearly worse down near the vented soffits and it decreases as you go higher (warmer) in the attic. There are some partial cathedral ceilings with insulation in the cavities and plastic chutes for ventilation space. There's mold above the chutes. Dead-end cavities at the hip rafters have less mold than full-length cavities vented at the soffits. The house doesn't have any of the typical indoor sources for moisture that cause attic problems. The only thing I can think of is that it's outside moisture, condensing under cold shingles. FWIW, the house is located on a river in coastal NJ. Not a lot of clear/cold skies when it is (relatively) warm and humid outside, but it does happen.

    1. GBA Editor
      Martin Holladay | | #7

      It's possible you're right. It's also possible that the reason that the full-length ventilation cavities above the cathedral ceilings have more mold than the dead-end cavities is because the full-length ventilation cavities have a stronger air flow rate -- strong enough to pull interior air into the cavities through ceiling leaks. If the ridge vent has lots of free area, and the soffit vents are anemic or imperfect, that type of short-circuit happens all the time.

      You might want to use a blower door to check the airtightness of the ceiling.

  4. Expert Member
    Peter Engle | | #8

    The ceiling is certainly not airtight - Far from it. But the house doesn't have any of the most common sources of indoor moisture that gets into attics. FWIW, the ridge vent is much smaller than the soffit vents. And the cavities and areas with the strongest staining are low down near the soffits, not near any particular interior air leaks. Also, the stains are worse on the south side than the north side. With cold weather condensation, that's reversed. I'm pretty sure the moisture is coming from outside.

  5. StephenInCalgary | | #9

    Martin, I am thinking about a new build in climate zone 6/7 . I wonder about adding a layer of actual floor trusses to the top of the top storey of the house and laying down a subfloor, so the attic has a nice, smooth, easy to seal surface. Maybe a fluid applied wrb on the attic floor. Then roof trusses installed on top of the attic floor.I would insulated from inside between the joists, then later spray in cellulose to the attic. I think this should make the air sealing easier and negate issues around tops of walls and holes for wiring. Your thoughts?

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