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Community and Q&A

Moisture/mold in attic

walshmfx | Posted in General Questions on

I have a gambrel style roof. Constantly battling mold. Live in update ny. The roof is almost vertical in the front. I doubt there is any soffitt ventilation that gets up to the attic. Small overhang in the back that is opened up. Ridge vent on top. I haven’t turned the humidifier on for a number of years and have been able to keep the mold under control. Turned humidifier on this year and went up a month ago and saw lots of mold. I sealed the electrical and vent pipe openings in the attic before blowing in insulation several years ago. Roofers suggest testing off plywood, replacing with OSB and adding edge vents to front and back. I also added a positive seal to the hatch going into the attic. Is there anyway to really prevent moisture from the house from getting into the attic? I’m hoping the added edge vents will provide enough ventilation. Also the top of the roof is very shallow and I think this frustrates the chimney effect to get air moving. Comments? Thanks.

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  1. Expert Member
    BILL WICHERS | | #1

    I’m not so sure replacing plywood with OSB will make any difference. In many ways, plywood is actually the better product.

    If you’ve airsealed your ceilings, and that is more than just electrical and plumbing penetrations (ducts and top plates are big offenders too), then you’ll probably get the most benefit from adding soffit vents. It’s usually recommended that you have a little bit more intake venting (soffit vents) than outlet venting (ridge vents) so that the attic is under a slight positive pressure. If you have a ridge vent right now, and no real intake venting in the attic, then the ridge vent is putting the attic under a slight negative pressure. A negative pressure in the attic is going to pull whatever air/moisture it can up from your house to try to equalize the pressure. Basically having a ridge vent and no soffit vents is likely a major part of your attic moisture problems.


  2. PAUL KUENN | | #2

    Could we get a photo? Is the mold consistent across the whole roof in every rafter bay? Mold loves OSB! If you didn't seal above all interior walls, that can allow much more air transfer than electrical and vent raceways. Good start though. You definitely need more in-flowing air at bottom of roof.

  3. Expert Member
    Dana Dorsett | | #3

    >"Turned humidifier on this year and went up a month ago and saw lots of mold."

    If you're actively adding humidity you're actively increasing the risk.

    How high were you keeping the indoor relative humidity (and why)?

    Also, where are you? Upstate NY ranges from US climate zone 4 through US climate zone 6.

  4. seabornman | | #4

    Is it a balloon-framed house?

  5. GBA Editor
    Martin Holladay | | #5

    We need more information. Does the roof profile sit on top of the conditioned space of your house? Or does the roof profile enclose two floors (perhaps your second floor and your third floor)?

    Do you have an unconditioned attic or a conditioned attic? What rooms are finished?

    One thing's for sure: If you get mold problems after turning on a humidifier, the obvious culprit is the humidifier. For goodness' sake, turn off the humidifier.

    Homeowners who operate humidifiers usually live in leaky houses. If your indoor air is dry during the winter, the best remedy is to hire a home-performance contractor to perform blower-door-directed air sealing. For more information, see "Blower-Door-Directed Air Sealing."

  6. Jon_R | | #6

    +1 on blower door directed air sealing. Then do whatever you can to increase outside airflow through the attic (as long as it doesn't create negative pressure).

  7. walshmfx | | #7

    Thank you all for your responses and questions. I've attached some pictures of my house, the hatch into my attic and my attic resplendent with mold.

    The house faces the West and there are tall trees in the back yard on the East side of the house. The mold in the attic has always been primarily on the East side. The garage has the same roof profile but never has a mold issue. I have a roof fan on top of the house which failed this winter. I live near Rochester, NY which is in Zone 6. The house was built in 1972. We moved in 1998 and the first thing I did was to add 6" of pink insulation to the 3 that were there. I have since blown in more insulation bringing the depth to about 20". I added some 2x4's across the trusses 23" above the ceiling and put melamine shelving on top of that. I get up on that and slide along to get from one end of the attic to the other. Not enough space above to crawl. Really sucks!

    I don't think there was any mold issue prior to us moving in. So why is it now such a problem?

    There are two issues that I deal with 1) ice damming and 2) attic mold. I have figured out a solution for the ice damming but not for attic mold.

    Ice damming occurs when the outside air is cold and the air in the attic is warm enough to melt the snow on the roof I installed 2 thermostats in line with the power to the roof fan. If the outside air is below freezing and the inside air is above freezing the switches in both thermostats close and the fan turns on pulling cold air inside the attic. This temperature differential occurs when there is snow on the roof and the ridge vent is blocked. Once the attic cools down below freezing the fan turns off. We had very little snow this winter so even though my fan isn't working there wasn't any ice damming.

    There is an article on this site "Rethinking Ventilated Attics
    How to stop mold growth in coastal climates"
    By Marcus Dell | September 8, 2015

    A line in that article jumped out at me: "Upon reflection, attics have historically performed well because heat loss from the occupied space warmed the air in the attic, allowing the attic air to absorb and dry any deposited moisture."

    Even with no ventilation issues they saw mold in there experiments. They also found that Kilz reduces mold - at least in there short (2 year) experiment.

    So while I made the house more energy efficient and more comfortable have I turned the attic into a mold making machine? Should I add radiant heaters to the attic that I turn on when there is no snow on the roof to keep the interior surfaces above the dew point?

    I'll respond to the questions/comments you all have posed related to the mold in the attic.

    The roofer I'm working with wants to use OSB. He found when walking the roof that some of the plywood bowed up. He said OSB doesn't do that and it is no more susceptible to mold than plywood. I was thinking that I might have him deliver the sheathing ahead of time and I'll paint the underside with Kilz. I like the comment about increasing the intake ventilation to reduce the draw on the air/moisture from the house.

    The idea of going up into the shallow attic, pulling insulation back and sealing (how should I do that?) the above the interior walls is depressing but something I should probably do.

    I did turn the humidifier off as soon as I saw the mold in mid-February. It is definitely drier up there now.

    It is not a balloon-frame house.

    It is a traditional 2 story colonial with a funky roof. The second floor is finished with 4 bedrooms and two bathrooms whose fans blow out through the walls. The attic is unconditioned. I turned the humidifier on because my skin gets very dry and itchy in the winter. The humidity helps. The humidistat I had until last week wasn't very good. I could turn it on and turn it up and down but it didn't provide any feedback. I have temperature sensors and humidity sensors around the house, outside and in the attic. The RH in the house was about 45% this winter. Outside and in the attic the RH was typically 85-95%. The manufacturer of the sensors says the sensors are good to +/-5% RH. I doubt they are that good. I replaced the humidistat with a Honeywell version that has an outdoor sensor and is preset to keep the humidity at 35%. If use it next year I'll start out turning it down to 30%.

    I need to think more about the Blower-door directed air sealing. I've seen articles about health issues with overly tight houses. They usually recommend an air-to-air heat exchanger with a tight house. So with the dry winter air wouldn't that will dry the interior of the house out as fast a s a leaky house?

  8. walshmfx | | #8

    Here are more pictures...

  9. walshmfx | | #9

    And finally...

  10. Jon_R | | #10

    As you learned, fans can be effective for preventing ice dams. Make sure they don't create negative attic pressure and they can also fix Winter attic humidity problems.

    If you are going to maintain elevated humidity, consider reduced pressure in the house (to protect walls/ceilings).

  11. Expert Member
    BILL WICHERS | | #11

    That is a LOT of mold in those pics!

    OSB is more prone to mold than plywood, but it’s not a huge difference. The general rule is the more processed the wood, the more mold likes to grow on it. OSB does weaken more than plywood does when it gets wet. I’m not a fan of OSB in roof assemblies.

    If you don’t have sufficient inlet venting for your attic, that fan will actually depressurize the attic and draw even more moist air into the attic from your house. Forced venting of attics with fans is not a good idea. You should be able to solve your mold problem by correcting the attic vents (add lots more soffit vents, and make sure they’re clear on the inside and not blocked with insulation), and making sure your attic floor is well air sealed.

    Also, Do you have weather stripping on that attic hatch to help seal it to the ceiling? Those hatches can be pretty leaky.


    1. walshmfx | | #18

      I'll have to push back on my roofer to explain his use of OSB.

      The fan was only used in previous years to prevent ice damming but mold wasn't an issue. It has failed and I don;t think it ever turned on this winter. I do need to revisit sealing the attic floor.

      The hatch does have a seal but should probably be replaced.

  12. Mrglasecki | | #12

    Do you use a attic fan in the summer, great cause of attic mold..
    Is your home ultra sealed with synthetics (plastics/foams) can cause a poor draft, using a fresh air intake from the crawl space in extremely wet areas can help.

    Building with breathable organic fiber fixes all those, some climates are hard for synthetic materials.
    Hemp fiber can build healthy living anywhere on this rock.

    Hope this helps, Pray it doesn't hurt..

  13. Expert Member
    Dana Dorsett | | #13

    "I live near Rochester, NY which is in Zone 6."

    Rochester, Monroe county and all abutting counties are zone 5. That matters in this discussion only if you're building an unvented roof assembly to deal with the problem.

    >"The RH in the house was about 45% this winter."

    The maximum recommended indoor humidity from a mold perspective would be 40% RH @ 70F, and even that's pushing your luck (big time) if there is any air leakage between the interior and the roof deck. At 45% even vapor diffusion through latex ceiling paint is enough to be problematic.

    1. walshmfx | | #15

      Yes. I plan to keep the humidity down next year.

  14. GBA Editor
    Martin Holladay | | #14

    You've made lots of mistakes here.

    1. The attic fan was a mistake; to learn why, see this article: "Fans in the Attic: Do They Help or Do They Hurt?"

    2. The humidifier was a mistake.

    3. Adding insulation to your attic floor without air sealing first was a mistake. (I may be wrong on this point -- perhaps you did perform air sealing work -- but if you did, you never mentioned it.) For more information on this issue, see "Air Sealing an Attic."

    1. walshmfx | | #16

      I did mention that I sealed around vent pipes and electrical boxes. I have not sealed above walls.

      The fan is only used during conditions that were conducive to ice damming in previous years without leading to mold growth. The fan failed this year and so is not a factor in the recent mold growth.

      The humidifier is the main difference and therefore the culprit here.

      1. GBA Editor
        Martin Holladay | | #17

        While I agree that the humidifier is the main culprit here, the humidifier would not be contributing to attic mold unless you have fairly massive holes in your ceiling (allowing the humid indoor air to reach the attic).

        Step one is blower-door-directed air sealing.

        1. walshmfx | | #19

          So you're saying to use the blower-door-directed air sealing to find leaks going from the living space into the attic? That makes sense.

          1. GBA Editor
            Martin Holladay | | #20

            Yes, that's the recommendation I made back in Comment #5. High moisture levels in unconditioned attics are almost always due to air leaks in the ceiling separating the conditioned space below from the unconditioned space above.

            Once again, here is the link to the relevant article: "Blower-Door-Directed Air Sealing."

  15. walshmfx | | #21

    Thanks, Martin! I misunderstood what you were saying in Comment 5. I'm new to this site and expected that most of the responses would be from hacks like myself. I can't tell you how much I appreciate the comments from people who really seem to know what they're talking about.

  16. PAUL KUENN | | #22

    "expected that most of the responses would be from hacks like myself."

    On the contrary, most on this site are well meaning professionals working with people in all climates trying to keep there houses upright because 99% are all built poorly. We're very busy because of that.

    1. walshmfx | | #25

      I totally agree. I am surprised at the high level of expertise shown in these responses.

  17. PAUL KUENN | | #23

    Just so you know, I'm working in an attic today, They had blown cellulose in at 14" without air sealing 15 years ago. Now lots of issues and a horribly tight hip roof. Get ready! First move as much out of the way so you can work. Then pay for an industrial vacuum to get rid of most of it. Then seal up all along the exterior top wall plates, move inward to seal everything else. Once it is blower door tested again, then re-insulate. Or like Martin said, take everything out of the house or seal it under plastic, then bombard the air with blower door sealing. Have fun with that.

    1. GBA Editor
      Martin Holladay | | #24

      You wrote, "like Martin said, take everything out of the house or seal it under plastic, then bombard the air with blower door sealing."

      I think you are confusing the use of a commercial system called the Aeroseal AeroBarrier system with blower-door-directed air sealing.

      I recommended the latter, not the former.

  18. walshmfx | | #26

    Someone came out yesterday to discuss blower-door-directed air sealing. After seeing the quantity of mold in the attic he said he would not put the house under negative pressure for fear of pulling mold into the attic. He eventually recommended that the insulation be removed from the attic floor, the mold killed with Moldex and the spray foam insulation applied to the attic "ceiling" (underside of sheathing and rafters) thereby closing off all ventilation and making the attic conditioned space.

    My understanding is that here in Rochester NY we should have R38-50 attic insulation. He recommended 4" of R7 spray foam insulation.

    My questions are: 1) is R28 spray foam closed cell insulation equal to R38-50 of fiberglass/cellulose insulation? and 2) is there any reason to remove the existing insulation other than to allow easier access to spray the foam?


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