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Community and Q&A

Is it poor ventilation or heat getting into attic causing mold in attic?

Annette Finkelstine | Posted in GBA Pro Help on

We had a lot of mold in the attic. Dilapidated plywood. Ice damns. Drywall damage. Insurance wouldn’t pay, said poor ventilation. We replaced the roof (it was only 9 years old), replaced all the ply, had edge vents installed everywhere as well as new ridge vents.(not enough over hang to add soffit vents across the front of the house). Replaced all the rolled insulation (it was soaking wet) and added fiberglass blow in on top of rolled to R49. Bathroom vents were then vented out the roof. 1 1/2 years later we have huge icicles so we climbed up in the attic to see why and found mold all over the place again. Mold company came back out and said poor ventilation. Originally he only baffled every other joist along the edge vent and we asked him to baffle every joist. Roofer came out and said once he baffles every joist the ventilation should be good because he cut down intake in 1/2 by doing every other. Mold guy won’t eradicate until we figure out what the problem is.He said his baffling is not the problem. Brought a licensed builder with him who also said ventilation. He suggested widening ridge vent (he said currently it’s only 1inch wide and also said maybe Ventsure (OC) may not be a good ridge vent.
1-What do you think about ridge opening?
When you climb into attic there is a plywood inner wall nailed to the joists covering about 1/2 the front of the house. The attic in front of this wall is clean. That area also has it’s own ridge and edge vents.
2-Could that angled plywood wall be cutting off any circulation into the attic? All parties dismissed this idea. We have not air sealed and we don’t have a lot of things (can lights, electrical, duct work) running through the attic.
3-Can heat still be getting into attic with all the additional insulation?We are going crazy trying to solve this expensive puzzle. Also, we turned off the furnace humidifier. House is 3000 sf and this is the upper attic only over 4 bedrooms and 2 baths, not over the family room. Any advice?

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  1. GBA Editor
    Martin Holladay | | #1

    The most likely source of the moisture in your attic is indoor air. There is a high likelihood that you have a leaky ceiling (or an undiscovered chase that delivers warm, humid indoor air into your attic).

    To diagnose this problem, you need to hire a home performance contractor or energy rater with a blower door. Tell this person that you want to find out whether there are ceiling leaks or thermal bypasses that connect your attic with the interior of your home.

    In many cases, problems like yours can be traced to a chase that connects a damp crawl space (or a damp basement) to the attic. If my hunch is correct, you'll want to seal these air leaks once they are discovered.

  2. Charlie Sullivan | | #2

    I agree with Martin's "hunch" and advice.

    Turning off your humidifier was a good idea. That made the indoor air that is leaking into the attic a worse problem. But having the indoor air leaking into the attic is a bad thing anyway.

  3. Jon R | | #3

    I'd also install a wireless humidity sensor.

  4. Annette Finkelstine | | #4

    Thanks for the responses. I just ordered a wireless humiditytemp. sensor. I would like to see if Owen Corning has a rep that can come out to see if the edge venting and ridge venting was installed properly. I need an unbiased opinion from someone whose main goal isn't to sell me something. Air sealing sounds like an expensive job especially since we just had the batt insulation and blow in on top of that done last year. I will call a home performance contractor in my area.

  5. Annette Finkelstine | | #5

    This is the kind of answers I'm getting from a licensed builder. He thinks the ridge vent is the problem to ventilation. Original roofer used OC ventsure ridgevent. ">> I believe there are two main factors that detract from your ridge vent's performance: one, the net free area (NFA, i.e. size) is only 12.5 and, two, its edges do not have baffles. Without baffles, outside air will flow up the angle of your roof and actually go in instead of out of your attic. This appears to be happening in your situation. In addition, Shinglevent ll not only has baffles but its NFA is 18. If you go to, you can see an illustration of what effect baffles have.
    >> I will send a quote for proper installation of this product shortly." Does this make sense to anyone? Thanks.

  6. GBA Editor
    Martin Holladay | | #6

    Your contractor is wrong. This is not a ventilation problem. (In other words, it has nothing to do with soffit vents, ventilation baffles, or your ridge vent.)

    There is no way that your attic can have the type of moisture, mold, and rot that you are describing unless there is a moisture source. Somehow, large amounts of water are entering your attic. That isn't normal -- most attics are dry.

    I doubt if you have a roof leak, so the chances are that the moisture is entering your attic through an enormous hole (or holes) in your ceiling. There is also an excellent chance that the indoor relative humidity in your house is very high.

    If you can locate and seal the air leaks, your attic will be dry -- whether it has any ventilation channels (soffit vents and ridge vent) or not. The ventilation channels are mostly irrelevant.

    If you installed new insulation on your attic floor without performing air sealing work, you made a mistake. You put the cart before the horse. Air sealing work must always precede insulation work.

  7. GBA Editor
    Martin Holladay | | #7

    One more point -- I forgot to answer the question that you used as the title of this thread.

    Q. "Is it poor ventilation or heat getting into attic causing mold in attic?"

    A. No and no. It can't be heat -- because heat is your friend. (A hot attic is a dry attic.) When the sun comes out and bakes your attic, your attic becomes dryer.

    If you had heat in your attic, that would be good. What you don't want is moisture. If you have air leaks that connect the heated sections of your home with your attic, the moisture in the air is the problem -- not the heat.

  8. Annette Finkelstine | | #8

    Thanks for the responses. I installed a wireless temp/humidistat for attic.
    the outdoors is 10 degrees, 76% humidity. The indoors is 70 degrees with 29% humidity. The ATTIC is 21 degrees with 93% humidity! My husband and the contractor found some areas along the walls that you can see down to first floor and they want to add insulation there as well as build up more insulation to r60. The roofer is coming out to take out the Ventsure ridge vent (He agreed the 1 inch total opening was not sufficient in the plywood and he will cut the ridge opening larger and install Shinglevent 2 because it has built in baffles to avoid intake (downdraft) and the NFA is 18 vs. Ventsure with NFA of 12. Nobody seems to think air sealing is neccessary. Also the roof nails have frost on them. Is all this going to help if we don't air seal?

  9. GBA Editor
    Martin Holladay | | #9

    My advice is unchanged. You need to evaluate air leakage between your attic and the conditioned space. It is unwise to try to address this problem before you have determined the source of the moisture.

  10. Alan B | | #10

    Nobody thinks air sealing is necessary but they can't solve the problem otherwise despite their best efforts and lots of expense.
    I say you ditch those people and take the advice of better experts like Martin Holladay. Warm air holds much more moisture then cold air so when you cool that air down the humidity goes up substantially (somebody who can read a psychometric chart can translate), and if you can see the first floor you have a major air leakage problem. The blower door test and air sealing is the correct advice.

  11. GBA Editor
    Martin Holladay | | #11

    Can you describe your basement or crawl space?

  12. Annette Finkelstine | | #12

    It's a full basement over the entire house. Partially finished with drop ceiling squares (easy access) and drywall. Carpeted in finished area. Furnace in non finished area. I found MEECA which is Michigan Engergy efficiency Contractors Assoc with a list of contractors. I was going to call one of these companies to do a blower test and tell them what you said in your first response. But they also provide all the services. I'd rather find a "rater" that doesn't do the contract work themselves so I don't get a company selling me on a multi thousand dollar job.

  13. Annette Finkelstine | | #13

    here's the link if you're interested to see what I'm looking at.

  14. Jon R | | #14

    I would think that a thermal camera would also be useful in finding the air leaks into your attic.

  15. Steve Knapp CZ 3A Georgia | | #15

    Annette. Consider checking out the RESNET site ( You should be able to find someone in your area who can give you an objective opinion.

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