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Drywall moisture

user-6957254 | Posted in General Questions on

I was playing around with my moisture meters the other day, and found a few things that I cannot explain, or have me somewhat perplexed.

I have an interior wall, in which the radon pipe runs up from the basement to exit through the roof. On extremely hot days, over 90 degrees, this wall reads high in moisture. Only on hot days. Once the temp drops, the moisture meter reads back at 0% on the drywall setting (pinless meter).

We went up into the attic and installed more insulation over the area, and we checked everything out while up there. All looked good.

I started to play around with other walls, and the bathroom ceilings tend to do the same thing. Again, only on extremely hot and humid days. Once the temp goes down, everything is back at 0%.

I see no signs of any issues, no water damage. I just happened upon this oddity I cannot explain.

What steps would you take moving forward? I am not sure where to start. Or what kind of issue I am dealing with.

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  1. GBA Editor
    Martin Holladay | | #1

    One possibility is that your house is operating at negative pressure (due to the use of exhaust fans or an unbalanced HVAC system with leaky ducts outside of the thermal envelope), along with an imperfect ceiling air barrier.

    (This has nothing to do with the thickness of your insulation.)

    Under the circumstances I described, hot, humid outdoor air can be drawn into the house through ceiling cracks. This could affect the moisture content of the drywall.

  2. user-6957254 | | #2

    Thank you. So the ideal solution would be to address the ceiling cracks.
    I might have to hire someone to help me with this, as I have no experience.

  3. walta100 | | #3

    After spending time reading Martin’s great article “Understanding Dew Point”

    Let’s consider the air flowing in your radon pipe will likely be close to the ground temp say 57°. Is it possible that on the warm moist day the pipe cools the dry wall to the dew point and the drywall absorbs moisture from the air allowing you to detect with your meter. When the air cools off and the dew point is below 55° the moisture in the drywall is absorbed by the air giving you a low reading.

    I do like Martin’s idea if your ductwork in in your attic or crawlspace.


  4. user-6957254 | | #4

    What type of professional would you have come out to help address this?
    I already had one building inspector come out, and he basically dismissed it and said I was "overthinking".

  5. GBA Editor
    Martin Holladay | | #5

    The best type of contractor to hire for this investigation would be a home performance contractor, an energy rater, or a weatherization contractor. You want someone who has a blower door and who is experienced at pressure diagnostics.

    To learn more about this field of expertise, see this article: "An Introduction to Pressure Diagnostics."

  6. Jon_R | | #6

    If you run AC, then stack effect alone creates a negative pressure at the ceiling. You can reduce infiltrating air/moisture by sealing air leaks - but it would take positive building pressure (or negative attic pressure) to eliminate it.

  7. user-6957254 | | #7

    Where can I learn more about negative attic pressure?

  8. user-6957254 | | #8

    Or how to achieve positive building/negative attic pressure?

  9. Jon_R | | #9

    Unfortunately, the references I've seen cover only excessive negative attic pressure, far beyond what is needed to stop ceiling infiltration. This creates unneeded AC load.

    Some info here:

    While it's all interesting, I'm not convinced that you have a problem that you need to address.

  10. user-6957254 | | #10

    Jon, do you mean my situation could be typical?

    I have to be honest, I don't know much about building science, but I love to learn.

    Hence all the tools.

  11. Jon_R | | #11

    Yes, I think that many buildings have air leaks, dew points and pressure differentials that cause some localized partition moisture increase. I expect that most of them dry before there is a mold problem.

  12. GBA Editor
    Martin Holladay | | #12

    Jon is correct that you can use ventilation fans to either pressurize or depressurize your house or your attic. Doing this will change the pressure dynamics, and may be used to change an air-entry leak into an air-exit leak.

    But there are three problems with this approach: (1) fans require electricity, so your energy bills will rise due to your higher electricity bill, and (2) fans can increase the air leakage rate of your home, so your energy bills will rise due to increased infiltration, which increases heating bills in winter and air conditioning bills in summer, and (3) you'll still be living in a leaky house.

    So either of these solutions is better: (a) Hire a weatherization contractor to perform blower-door-directed air sealing, or (b) Ignore the phenomenon you have noticed because, as Jon notes, there is no mold.

  13. user-6957254 | | #13

    Update.....I had a weatherization contractor come to my home. He did a Blower Door test, results was 1700 for 4000 sq ft home.
    He stated my problem with drywall in certain locations (bathroom ceilings and some walls) reading high moisture on HOT days with high dew points was related to air sealing and lack of insulation in some locations.
    He stated to identify and cure all these issues would take a lot of time and money, and wouldn't yield much in the end.
    He recommended installing an ERV and Whole House Dehumidifier.

    I am trying to wrap my head around how these will help? Our indoor humidity is ok. (40-50% in summer, 20% in winter). The drywall isn't condensating on the interior side, I assume the high reading is coming from the other side of the wall/ceiling. Maybe this is above my head, but I want to make sure I am doing the right thing before I spend almost $6000.

    Any advice welcome.

  14. NormanWB | | #14

    Can you share more about your attic area and access? It seems your simplest solution should be your least costly. That is to add insulation and air seal. You can do a good bit of air sealing from below by removing ceiling fixtures and sealing around the perimeter of the junction boxes (power off first, please.). You can also seal where the wires enter the box. I suggest a good caulk.

    Also seal where pipes enter your walls, such as under vanities and behind shower trim. Spray foam works well here.

    If you can get access to the attic areas missing insulation, then add some.

    This solution has no moving parts and will last for years/decades.

    If you feel certain areas need dehumidification, then a portable dehumidifier ($200 or so) has been show to be as effective as much more expensive options.

  15. user-6957254 | | #15

    The contractor feels the fiberglass in the attic (which we have added a lot but the problem persists), is just a poor air sealer. And the bath exhausts, which have also been air sealed, don't really help the problem because that is where moisture is trying to escape.

    Also, in the walls, he feels some of the spray foam is lacking in some areas. He thinks it would be way too time consuming and costly to remove drywall and add foam.

    I really am thinking maybe I need a second opinion.

    I am trying to do this on my own, with two very small children. I was hoping the contractor would come and fix any areas that needed it. Or would that be hiring a different professional?

  16. Expert Member
    Peter Engle | | #16


    I'm still not convinced that you have identified any actual problems. If you are not getting condensation and/or mold growth and odors, you don't really have a problem. Drywall, like most porous materials, will adsorb and desorb moisture from the air. If you have cool drywall in the house and warm, moist air in the attic, the drywall will adsorb moisture from the air and that will show up as a slight increase in the moisture content of the drywall. You don't start to get worried until the drywall gets wet enough for mold to grow on the paper, but this rarly happens unless you've got vapor barrier materials on the inside and Miami conditions outside.
    When the outside air cools off (and the dewpoint decreases), the drywall will dry out again. No harm, no foul.

    I don't think you've mentioned your location. Outside conditions clearly make a difference here, since this is a hot (humid) behavior.

    As far as the radon pipe chase, air sealing the top and bottom of the chase is more important than insulating it. Pull the insulation back and use spray foam to air seal the chase around the top and bottom of the pipe. Then put the insulation back. See if that changes things.

    From what you've described so far, I would not even consider an ERV and dehumidifier. As recommended above, the first thing to do is air sealing, then insulation. Then wait and see. It is very hard to seal an existing house enough that you need an ERV. Also, in hot/humid weather, and ERV will bring more moisture into the house, not less.

    Does this contractor sell ERV/dehumidification systems, by chance?

  17. user-6957254 | | #17

    Thanks, Peter.
    We are in Climate Zone 5. We get all 4 seasons, and our summers are typically hot and humid.

    Thank you for your comments, I appreciate that. The moisture readings get high enough that I start to get nervous, although I see no real signs of any moisture.

    This particular contractor does not sell them. I had to go through my HVAC company to get the quote.

  18. Jon_R | | #18

    Unless your house is quite leaky (it isn't) you should have mechanical ventilation (preferably an HRV or ERV, but you could use supply only in Summer and exhaust only in the Winter). If your house has too high humidity (apparently it doesn't), you should have a dehumidifier. Portables work and are much less expensive.

    If you really want dryer drywall in Summer, then you need to prevent outside air from reaching it.

  19. user-36575 | | #19

    Others have said the same things, I’ll just say them with different emphasis.

    1. You don’t seem to have a real problem with moisture.
    2. I like all of Norman and Peter’s recommendations, but just to be explicit, you need to do the air sealing first, and then add insulation second if you want to improve thermal performance.
    3. Fiberglass is not an air sealing insulation. Fiberglass is what you use for air filters in your forced air furnace. Do your air sealing separately, then add insulation.

    IMO, I would air seal in the attic first. After that, I’d air seal the basement rim joist area. If you’ve got money left over, add some blown cellulose over the top of the existing fiberglass insulation. Cellulose, unlike fiberglass, does slow down air movement, so “capping” your existing fiberglass insulation with (say) 4-5” of cellulose will make it more effective.
    Aside: I’ve done this in three different houses that I’ve owned. Another benefit of adding cellulose to your attic is that it may make your house quieter, depending on the rest of the building’s construction and the windows. In one house, the decrease in exterior noise was dramatic.

    You've gotten more than one quote for air sealing and insulation, right? I think you'll be fine.

  20. Expert Member
    Dana Dorsett | | #20

    This might be simpler than it looks.

    Is the high moisture part just the chase surrounding the radon stack?

    Is that chase reasonably air sealed at both the top and bottom?

    The temperature of the radon stack pipe will be approximately the temperature of the slab, which is probably below the outdoor dew point temp on hot/very hot days. If attic air(essentially outdoor air) is being pulled down the stack chase there will be some condensation on the outside of the stack on those hot days, and possibly some direct adsorption into the wallboard if the wallboard's temperature is lower than the outdoor dew point.

    Cracks/leaks in a depressurized slab will pull some air in from the basement, depressurizing the basement, and potentially pulling some attic air down into the radon stack's chase.

    It's true that fiberglass insulation doesn't even come close to being an air barrier, and should never be thought of as such. But if the high moisture levels are extremely localized it's probably pretty easy to figure out where the biggest leaks are and fix them.

    Sealing any cracks sump seals, seams in the slab could mitigate active depressurization of the basement, and improve the radon purging function of the fan system. Sealing the where the stack goes through the basement ceiling might also be pretty easy if it's an unfinished basement. Whether it's also easy to seal that chase at the top depends.

  21. user-6957254 | | #21

    Thank you everyone. I am trying to find someone local that specializes in air sealing. I don't have any quotes yet. We live pretty remote (about two hours from St. Louis), so I am hoping I can find someone that will travel.

  22. D_i_v | | #22

    I'm new here but signed up to comment. Curveballs nobody thought of. I too live in the midwest and also had a midigation system installed. Not for radon, but I tried it to reduce summer humidity in my basement. In my basement I have interior drainage tiles, "slotted flex pipe" runing the perimeter of the property to an open sump pump pit in the basement. They drilled into my basement floor directly into the flex pipe, then sealed my sump pump pit with clear plexiglas and silicone. All that sump water was evaporating into the basement, and the rise and fall of the water level in the pit can even cause ground water odors. I have no idea if you have a sump pit or not, but if your ground gets wet your radon fan draws and forces humid air out thru the exhaust. When it rains hard, water even enters the top of the pipe and runs back under your foundation that way. Point is, your radon pipe is also loaded with moisture! Is it possible that the installer was careless or forgot to seal PVC joints in the wall like sometimes happens with careless plumbers with sewer vent stacks? Maybe when it's hot out it dries your top soil but also raises water table under your basement? Sealing the radon pipe in from the attic and basement in that case can only make it even worse possibly? Im not sure? Perhaps run a camera scope from attic or basement and make sure nobody pierced it hanging a picture or towel bar ect... Just a thought.

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