Moisture Buildup in Wall Assembly
Have a hone we recently built that is giving us some serious anxiety as to what to do. Traditional wood frame construction with Hardie board, zip wall exterior sheathing, we added a layer of hydro gap paper and traditional batt insulation. We have in various random areas throughout the home only on exterior walls extremely high humidity / moisture inside the wall cavity. We are redlining a moisture meter in no less than 50 spots throughout the home. What’s baffling is it’s not consistent. In some areas the meter redlines and in other areas it’s 12%. No water intrusion at all. We’re seeing growth inside the walls on the drywall paper side of the wall assembly and where the drywall paper meets the stud, not seeing growth on the sheathing itself. Humidity in the home is hanging at 60-65% which is high but is it high enough to be parking gallons of water inside the wall assembly? How in the world could the vapor penetrate through the paint/drywall and be presenting itself on the drywall paper but there’s no sign of anything going on in the living area of the home itself? We’re in SC so it’s extremely humid and hot out lately and the problem is rearing it’s head now. Could we have been creating a dew point in the wall during the winter months and that moisture stayed in the wall until now and the warmer weather spurred the growth? Are we maybe creating a dew point on the backside of the drywall with all the hot air outside? Pulling humid air through the zip wall and hydro gap seems like a long shot. Any moisture should have accumulated on the barrier and run down. HVAC system feels like it’s the number one suspect at the moment and we’re adding 2 large dehumidifiers in an effort to pull RH down to 40-45%. Has anyone experienced anything like this in a hot humid climate? If so what was the cause?
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Just curious, you mentioned some spots have higher humidity. Is there anything inside that is creating extra humidity? A kitchen, bathroom, etc. could also be increasing humidity in the wall or maybe it’s just too humid inside. I would think a dehumidifier would fix this problem though
What have exterior dewpoints been like and how do they compare to the interior temperature and dewpoint? Vapor drive is most surely inwards right now so your statement about interior humidity 'parking water in the wall' or vapor penetrating the drywall (from the inside) likely don't apply.
>"Any moisture should have accumulated on the [hydro gap] barrier and run down."
Water resistance and vapor resistance are not the same. Hydro gap is not a vapor barrier.
More questions:
—When you say moisture meters, are you referring to wood moisture content measurements, RH, other?
—What sort of growth are you seeing? (You have the walls opened up to see this growth?) How did you identify you had a problem to begin with?
—When was the house completed? Construction moisture shouldn't be ruled out.
—Is the zip sheathing taped?
—Is it just drywall on the interior with standard latex paint?
—What type of insulation?
—Are the problem areas being made relatively colder due to HVAC registers or other?
> the problem is rearing it’s head now
My guess is outdoor air infiltration through some path. Your outdoor dew points have been high enough to cause condensation. To test the theory, you can run the house at slight positive pressure for awhile. Measure and set the indoor pressure about 2 pascals higher than outside (for a single story house).
Temperatures outside are hanging around 88-92 every day and high humidity. We are using a non destructive moisture meter that when set on drywall is absolutely redlining in many areas throughout the home. Specifically on exterior walls but no real rhyme or reason as to where. Some near windows, some near corners, sometimes just on blank exterior walls. In some cases the moisture can go from one section between 2 studs at 15% to 90% in literally the next section between studs. We’re seeing significant mold growth on the back side of the drywall and in many cases where a stud or stud pack meets the drywall board. We smelled a problem in the master bedroom initially and then after poking around with a moisture meter realized the problem is actually systemic throughout the entire home on exterior walls. House was completed in February of this year, only been in for 5-6 months. Zip wall is taped and there’s hydrogap on top of the zip wall. Open cell foam in roof and floor system underneath and batt in the walls. Normal drywall and latex paint. Problem areas are random and not necessarily next to HVAC registers or anything like that. The negative pressure idea makes sense in that we could be pulling warm humid air from the outside in through the weak points in the wall assembly and when the warm air contacts the drywall we’re creating a condensation point but we’re not seeing any clear signs of negative pressure with doors slamming or anything like that. Don’t even feel any kind of sucking effect when you open a door. House “feels” balanced as far as pressure goes. We’re chasing a ghost that we can’t seem to catch. The proposed plan right now is to positive pressure the house with fresh air supply that is dumped into a large dehu first and then dumped into the HVAC ststem.
Any update on solution? I'm having the exact same issue in my home which is in a similar climate. The house is 5 years old so not construction moisture, I have pulled some of the dry wall and it is dry but there is a musty smell at some points in the house so im assuming there is growth somewhere. Is a colder or hotter interior in terms of AC better for controlling humidity in the walls? I was assuming the colder the better but it sounds like that might create more condensation? Thanks.
Was a blower door test ever conducted?
You could look for air movement with a smoke pen or some incense. Could tape plastic in a window to test for differentials.
There is obviously a reason, even if it doesn't rhyme. Air leaks kinda fit that bill. I wonder if the latent construction moisture gave it a head start. Certainly sounds odd given taped zip, but I've seen that stuff hanging off when applied poorly.
We did a blower door test in the house and it rated at 2.1 which is actually pretty tight. This is where I’m struggling the most with the idea that we are pulling in humid air through the envelope. All of the seams were taped so I’m not seeing how we could be pulling in that much humid air at so many random spots on the home.
Hopefully someone more familiar with the norms of construction in your climate (I'm in Maine) can shed some light. A last few thoughts on investigation techniques:
You could open up a section of wall (already done?) where you're reading high MC and check the area/sheathing for any possible air leaks or other strange sources of moisture. Depressurize the house during testing with large fan etc.
If a wall is open, I would try to get readings of stud moisture content with a reliable (perhaps pin style) moisture meter to get a better picture of where exactly the moisture is accumulating. Sorption can happen on wood at temps higher than dewpoint (pre condensation) so it makes sense you would see a concentration of moisture and mold around the studs. Take a reading of wood moisture content close to the sheathing and close to the drywall separately. In theory, the MC should be higher closer to the drywall where it's colder, if our hypotheses are correct.
And Is the moisture meter you are using now designed to work on drywall? It is possible for meters to give false readings, but since you are seeing and smelling mold, obviously there is some moisture. But going from 12% MC to redline (which is what? what's the cut off?) does seem odd.
If it all continues to be elusive, don't discount getting an experienced contractor in to do some sleuthing. Maybe a thermal imaging camera could also shed some light. Best of luck!
I'd also just double check your exterior for soundness against bulk water intrusion behind cladding. Even though it's a new house, poorly installed gutters, inadequate overhangs with poor flashing, etc. could exist.
This sounds a lot like construction moisture to me. The taped ZIP is a low-perm wall, and if the house was finished in winter, there could have been a fair amount of moisture in the studs. There would have been close to zero drying in winter. If you had a short spring in your area and went straight from heating to cooling, that moisture is now trying to dry to the interior. If your drywall is chilled below the dewpoint of the air in the stud cavities, you'll get condensation (or at least near-condensation) on the backside of the drywall, and mold. The enhanced mold growth near the studs tends to support this theory more than exterior air leakage.
60%-65% interior humidity won't "park" any moisture in the walls in summer (vapor drive is toward the interior, not the exterior in summer), but it sure would pump a lot of moisture into the walls in winter if it was that high. You should keep interior humidity around 35%-45% in winter in coastal SC, closer to 35% in the mountains in winter.
The good news is that if this is construction moisture, it will probably dry out this winter if you keep interior humidity low. After that, it won't happen again. The bad news is that any mold already inside the walls isn't going away by itself. If it's just an odor issue, that probably won't recur. But if anyone is sensitive you could still get some "leakage" into your living spaces.
For now, I would dehumidify the interior air as you propose, run at a slight positive pressure if you can, and set your AC temperature as high as you can tolerate to minimize chilling of the drywall. Hope for an early and dry Fall.
Jason,
2.1 ACH is not bad, but it is pretty leaky for a house with taped sheathing. This means you have some uglies buried in the structure such as leaky foundation connections, unsealed wall top plates or ducts in the attic.
If you combine this with the HVAC system mounted in the unconditioned attic so loved in warm climates, chances are the system is depressurizing the house and drawing in outside air through these leaks.
This makes sense from the moisture issues you are seeing, as only the leaky stud bays have problems.
Your solution to positive pressure the house should help. You can get a budget digital differential manometer and measure the pressure difference between the house and outdoors to see if this is the issue and how much positive pressure you actually need.
Running a dehumidifier combined with the AC would let you get the house air dryer without overcooling the place. Most people find that dry but warmish air is more comfortable than cold but humid air. Combing the AC with the dehumidifier might let you get away with running the thermostat above outdoor dewpoint temperature, which will minimize the amount of moisture deposited in the walls.
Any update on solution? I'm having the exact same issue in my home which is in a similar climate. The house is 5 years old so not construction moisture, I have pulled some of the dry wall and it is dry but there is a musty smell at some points in the house so im assuming there is growth somewhere. Is a colder or hotter interior in terms of AC better for controlling humidity in the walls? I was assuming the colder the better but it sounds like that might create more condensation? Thanks.