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Community and Q&A

Help PLEASE! How to fix or vent moldy/damp attic

aimee00 | Posted in GBA Pro Help on

Firstly, thank you to folks like Martin, and Peter who have wrote numerous different articles about attics, venting, and mold. I’m writing because after hours of reading, and hearing recommendations from mold remediators, and roofers, I’m left overloaded with information with no definitive, and clear path forward. Help please!

Let’s start first with what I can see is the problem:
1. We visibility see black mold on 85% of the interior attic sheathing, on most of the boards, it’s worst/complete black near the eaves. 
2. At the ridge, I can see droplets of water, which I assume is condensation
3. Front of the attic seems to have less mold than the back of the attic

Recommended fixes (or lack thereof): 
All roofers, and mold inspectors have recommended floor insulation and venting solutions. One roofer said I only needed a ridge vent, another wouldn’t take the job at all unless I agreed to ridge and soffit because that’s the “correct way” to vent attic, yet another recommended solar power fan with ridge.

Based on articles I’ve read on this website, however, it sounds like venting could increase the moisture problem or at best, will only marginally reduce moisture. Air-leaks should be sealed first followed by insulation; air leaks can best be identified through blower door test. Yet, the energy audit cannot be completed without remediating the mold first, and if I remediate the mold first without fixing the moisture problem, I worry the mold might come back before the blower door test can be done. 

What is the most likely cause of the increased moisture in my attic, and what should I fix first? Do I need more vents beyond 2 gables? If so, just ridge? or ridge AND soffit vents? or just sealing air leaks is enough? Or is this one of those cases where I might just need to accept a little mold similar to the Pacific NW homes situation? 

Additional basics of the home:
1. Small ranch built in the 60’s in Southwestern Connecticut, we get all four seasons, and slightly near the long island sound “coast”; electric heating, no HVAC
2. Attic is over the entire living area except the addition, ~1300 sq ft, Gable vents only (no ridge or soffits). Front of attic faces east, back attic faces west.
3. Attic floor is only partially insulated 
4. ~5-6 years ago, we renovated portions of the home, and added recessed round “can” lights, renovator thought it was ok to leave the bathroom vent terminated in the attic
5. Roof was replaced ~15 years ago by prior owner, but although the ridge line was created, vent was not installed
6. Crawlspace is built with concrete, floor has uneven concrete. We had a moisture problem, but fixed the water issue. Found a little bit of mold on the crawlspace insulation

For reference, I’ve attached pictures to show slope of my roof, inside the attic, and mold.

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  1. Jason S. | | #1

    It sounds like you have a host of contributing factors. This one seems to be the 'smoking gun':
    "4. ~5-6 years ago, we renovated portions of the home, and added recessed round “can” lights, renovator thought it was ok to leave the bathroom vent terminated in the attic"

    You have to seal up those air leaks properly and vent the bathroom to the outside. Do that first to stop the bleeding, then the remediation. A full energy audit may or may not be needed; I suspect 90% of the problem can be solved by addressing what's plainly visible once you've had the attic insulation pulled out to inspect.

    Ridge and soffit venting are both needed, otherwise you'll just exacerbate the amount of air leakage from the house with ridge venting only. Avoid the power-vented option for the same reason.

    Best of luck.

  2. Jon R | | #2

    +1 on first fixing the attic floor air sealing and the vent termination. And start monitoring attic (and crawlspace and interior) humidity and temperature - so you know how much effect fixes have had.

    Never conflate powered attic exhaust ventilation with powered attic intake ventilation - they have opposite effects.

  3. Steve Knapp CZ 3A Georgia | | #3

    I agree with Jason. Venting the bath fan into the attic is probably the source of most of the moisture that is condensing on the sheathing, and you should fix that now. To cover the recessed lights, you can use something like this: You can also make your own enclosures.

    After you take these steps and seal the attic, I would wait to see if the mold goes inactive.

    Is there any way to access the ceiling in the addition? You may have issues there as well.

  4. Expert Member
    Zephyr7 | | #4

    Your second roofer was correct -- you need BOTH soffit AND ridge vents. The ridge vent is the "out", the soffit vent is the "in". You typically want slightly more soffit vent area than ridge vent area to avoid depressurizing the attic (which increases the draw of air from inside the home to the attic, which is where moisture problems typically come from).

    Vent that bathroom fan to the outdoors. Do that now. Mold grows in moist enviornments, and the purpose of a bathroom fan is to exhaust warm, humid air from the bathroom. That warm, humid air is exactly what you do NOT want in your attic! It is NEVER OK to vent a bathroom fan into an attic.

    Your can lights are all air leaks. It doesn't matter if they are rated "air tight" (AT), that rating just means they leak less. You'll want to seal those all up, there are articles on GBA about how to do that. Using LED inserts will also help, since those tend to help with air sealing.

    Notice how this stuff all has to do with leaks and venting, not insulation. That's because that mold is growing due to warm, moist air leaking from inside your home into the attic, and the attic likely also has insufficient venting. Fix those issues FIRST.

    Second, don't use your attic for storage. Storing stuff in an attic that has no real floor causes problems with the insulation, and risks damaging your ceiling (which means more air leaks). You'll need some baffles out at the eaves when your soffit vents go in, since you don't want the insulation to block the vents. I would probably remove all those random batts from the attic floor, which will expose all the things you need to air seal on the attic floor (can lights, wire and pipe penetrations, pretty much anything that air can get through). Once you've sealed those leaks, put in enough blown cellulose insulation for your climate zone. Blown cellulose is often the cheapest insulation, and it's also one of the best so a win-win.

    The good news is that none of this work is particularly expensive to do. A roofer can install a ridge vent in less than a day, those go quick. Don't let them use one of the foam-style vents that you shingle over -- those don't work well. Use a "real" ridge vent, which will be a sort of plastic assembly with a grille on the edges. Installing soffit vents is more involved, and will take a few days. That gets you your venting.

    You can pull out the old batts yourself if you're careful. I'd recommend a full tyvek suit to cut down on fiberglass itchiness, and a dust mask or respirator since you don't want to breath that dust. Stuff the old batts into big trash bags and get rid of them. That will expose everything. I'd treat the mold with a bleach solution (or hire a remediation contractor, but those guys can be expensive) before doing further work. Once that's done, do your first round of air sealing with some caulk and canned foam -- ideally using a foam gun (lots easier with the gun). After round 1 of air sealing, where you're after holes and gaps, put in some vent baffles out at the eaves to keep your soffit vents open. Next is round two of air sealing, where you'll box over the can lights to seal those up. Be sure that bathroom van gets vented outside somewhere in there too.

    Last step, blow in cellulose. You can do this yourself, it's not difficult, or contract it out -- it's not that expensive. If you do it yourself, get a few yard sticks and tack them up around the attic with the low-numbered end on the floor, then wrap some colored electric tape around them at the depth you want. Now you have reference markers to let you know when you've blown in enough insulation. If you do this yourself, you can usually get a free rental of an insulation blower with a minimum amount of insulation purchase from your favorite box store. Install all that blown insulation, trying for an even layer over the entire floor, using your taped yardsticks as references to make sure you're putting down a thick enough layer.

    When you're done, make sure your attic hatch is insulated with rigid foam and sealing well at the edges. You should find this project solves the mold issue, and will likely cut down on your heating costs as well.


    1. Jon R | | #5

      > more soffit vent area than ridge vent area to avoid depressurizing the attic

      Take a low wind day and analyze the stack effect pressures. The floor of the attic will be negative wrt outdoors. The interior ceiling will be positive wrt outdoors. Net result - the attic IS often depressurized wrt to the interior. Yes, even with more soffit vent area than ridge vent area.

      Which is why some have used powered ventilation to fix the problem.

      1. aimee00 | | #7

        Thanks Jon. What would you recommend to do this analysis correctly, and make sure we've got the appropriate soffits to ridge to avoid depressurizing the attic?

        1. Jon R | | #8

          My advice is to fix the the vent termination (easy) and better seal the attic floor (depends). If that isn't sufficient/practical, consider a fan blowing outside air into the attic, pushing out moisture and reducing the amount of moisture entering the attic in Winter (easy).

    2. aimee00 | | #6

      Thank you Bill, Jason, and Jon. I've purchased and placed couple of hygrometers in the attic to get a baseline of moisture levels and temperature. Once the bathroom vent is taken care of, I'll be able to know how much it helped. The bathroom vent has not been used since discovering the mold but I expect the moisture in the bathroom from general use will still rise to the attic regardless of the exhaust being on or off.

      My attic has flooring so it's not as straight forward as pulling out insulation to put a cover over the recessed lights. I read on other GBA air sealing articles that a smoke pen is ideal for existing homes to at least "validate" the air leak. Would you recommend proceeding using a smoke pen from the attic space or in the living space near the recessed light coverings to identify the leaks? Or have other ideas?

      Based on responses, I'm now thinking:
      1. immediate steps: fix bathroom vent and at least confirm air leaks near the recessed lights
      2. measure humidity level after bathroom vent is fixed to see progress
      3. remediate mold
      4. Put in new insulation, seal up attic stairs entrance as recommended, and measure humidity level again to validate need for additional ridge and soffit
      5. Install ridge and soffit

  5. aimee00 | | #9

    In addition to fixing the bathroom vent so it vents outside, I've purchased several halo led retrofits and replaced all of my ceiling lights. I've got two indoor hygrometer and have been monitoring the humidity level prior to, and after these fixes.

    It's been several days now since these fixes, and I have not seen significant drop in the humidity level; in fact, it might've even gone up (although could be +/- variance with the device itself).

    What could it mean that I'm not seeing improvements? That my problem is now the need for more ride+soffit ventilation?

    1. Jason S. | | #10

      To bring the current humidity levels down, yes, you'll need soffit and ridge venting, and/or dehumidifier and/or some warmer sunny days to drive that moisture out.

      Don't be discouraged, you're on the right track!

  6. Steve Knapp CZ 3A Georgia | | #11

    People with new builds often post about indoor humidity issues. The general advice is that reducing levels can takes several weeks or months. (Houses can hold a lot of moisture.) This is a different situation, but I think you have to adjust your expectations. I'd keep monitoring the attic while also keeping an eye on outdoor conditions.

  7. Expert Member
    Dana Dorsett | | #12

    Not that you asked but:

    >"1. Small ranch built in the 60’s in Southwestern Connecticut, we get all four seasons, and slightly near the long island sound “coast”; electric heating, no HVAC"

    At CT type electricity rates a cold climate heat pump (or really any heat pump) would pay for itself in relatively short years. If ducted (probably should be), installing it in the crawlspace rather than the attic is highly recommended.

    Air sealing and insulating the crawl space will also improve your attic moisture problem. A vented crawlspace allows stack effect to drive air flow upward into the attic, pulling cold dry outdoor air (in winter) or warm humid outdoor air (in mid summer) to be pulled in to the crawl space, creating wintertime comfort issues in the first floor. Since there is a full attic floor it's going to be easier to air seal the BOTTOM (crawl space) of the "stack" than the top of the stack (first floor ceiling), but do the best you can with both.

    Simply opening up the venting in the attic may mitigate against mold in the attic, but it increases the 24/365 stack effect air flows. Sometimes the increase in stack effect flows overwhelms the capacity of the venting to fully dry the attic, leading to a few cold-wet spots near leak points.

    Replacing the leaky can lights with surface mount LED fixtures makes those penetrations easier to air-seal from below. But look around for other penetrations too, such as chimneys or plumbing stacks, wiring coming up through top plates of studwalls (sometimes hard to spot or fix without ripping up the attic floor), etc.

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