Summertime Stack Effect and Humid Basements
Hello,
My day job is product development. I’m BPI certified and am responsible for developing procedures for our comfort advisors and field crews to diagnose and ultimately solve problems in homes, not just sell things. We provide HVAC and building envelope solutions. We are in climate zone 4A.
We’ve had a few humid basement calls with the crazy weather this August. The basements were both finished and unfinished. I’ve read the information from GBA and Building Science Corporation regarding how basements can be wet and how to build them properly to avoid this. We are in the retrofit business, so I’m still struggling with a few points about how to keep existing homes dry:
1.) Assuming bulk water has been addressed, how much of the moisture load can be attributed to capillary action and then subsequent diffusion from the slab and foundation walls, relative to air infiltration through the rimjoist? For context, my goal is to be able to focus on rim joist air sealing first rather than adding mechanical dehumidification as a bandaid. If the diffusion is the real concern, dehumidification would appear to be required.
2.) How does basement infiltration occur when summer stack effect states that air should actually be exfiltrating out of the bottom of the home. You could infer that conditioned air is actually being pressed down from above/sucked from below into the basement which would effectively serve to condition it before it exits through the rim joist. In this example, rim joist air sealing wouldn’t seem to provide a benefit. In the context of this question, how else would moisture-laden air get into the basement besides through foundation diffusion?
(Bonus Question)
If summer stack effect is present, how does this factor into the buoyancy of warm air in the home and the fact that the 2nd floor HVAC system’s run times are often much greater than that of the 1st-floor system’s? In other words, when I walk up a set of steps, I can feel it gradually getting warmer. Is this convection from below as heat load enters the building and rises to the upper levels due to the boyancy of warm air or convection from above as warm air is pressed into the building and stagnates due to the summer stack and lack of zoned control and/or poor mixing?
Thanks in advance,
-Grant
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Replies
Interesting questions. I can't answer them decisively; hopefully someone who has measured these things weighs in, but my somewhat-educated opinions:
1. Varies greatly, as some sites are wet and some are dry; foundation damp-proofing, waterproofing and drainage vary; air-sealing varies; conditions change throughout the seasons. I imagine that in most cases diffusion is a larger source of moisture but in many cases it's probably air leakage.
2. If you have seen or done a blower door test, you'll know that the indoor, conditioned air is nearly always connected with the basement air. It's very unusual to have a basement air-sealed well enough from the above-grade spaces that they don't share the same air. Looking only at the basement air, it's usually cooler than the outdoor air but also drier because it shares air with the living spaces, so hot, humid outdoor air will still do its best to get into the basement. The stack effect on its own is relatively weak so I doubt it could overcome the exterior-to-interior vapor pressure differential in the basement.
3. The basement air is cooler but more moist than the indoor air, so it carries at least as much moisture as the living spaces. Therefor the stack effect still applies normally--i.e., warm air will tend to rise. The leakier the house, the more pronounced the temperature differential will be.
Note that moist air also tends to rise, so you have humidity AND temperature gradients in play here.
In my own experience, moisture diffusing through masonry and air leakage contribute similar amounts of moisture to the interior. That's my own "seems like" opionion though, since I haven't made any real tests and tried to control all the variables. The nice part here is that, aside from bulk water issues, you do similar things to control diffusion as you do with air leaks: sealants and barriers. If, for example, you do an ecapsulation project, you're putting up a vapor barrier AND air sealing it, so you end up blocking moisture that is trying to get through the masonry AND you're block moist air that's trying to get in through air leaks. Win-win.
The usual stack effect runs from air leaks coming in around the rim joist area, which is the classic leaky spot down low on the house, then gets out through the attic ceiling for similar reasons. If you go after the low hanging fruit in both areas (rim joist in the basement, ceiling penetrations for lights and access hatches in the upper floor ceiling), you can cut down a lot on total air movement from outdoors to indoors and back, which will help limit how much moisture gets drawn into the home through air leaks. I would always consider an air seal and insulate project for a rim joist as one of the first things to try when looking to seal up a basement for this reason.
Regarding HVAC, you'll probably find the upper and lower systems, if zoned, are tied together more than you think. I've seen plenty of combined air return plenums, and any time the inside of a studbay is used as a "duct", chances are it hasn't been sealed off so it "leaks" into everything else. You'll end up drawing in some basement air regardless. Remember that one of the common complaints is that musty smells from the basement make it up into the living spaces on the upper levels, which is indicative of air rising up from the basement either by stack effect, forced movement, or, most likely, a combination of both.
Bill
This may be obvious, but even 'humid' basements may actually have the same or lower dewpoints as above grade rooms. In which case the cause of the 'problem' is an uninsulated basement. Of course some air exchange too.
I, for the most part, don't run AC, have an uninsulated basement with poorly installed fiberglass in the rim joist, and the dewpoints in thr basement are often similar or lower than upstairs. RH can be higher though. I do have going for me that the basement is pretty dry due to previous owners installing exterior drainage of sorts, and there is a radon fan running 24/7.
2 other thoughts:
Any sort of imbalanced ductwork may very well overpower stack effect. Running kitchen or bath exaust may be enough to bring outdoor air in through an unsealed basement rim (I haven't actually tested this to know, I'm assuming).
The upstairs may be much warmer for reasons that don't have to do with air movement, namely that there is likely more exterior surface area, and additionally that surface area is getting more solar irradiance. I.e. heat gain through the roof.
To expand on Maine Tyler: basement wall and floor surfaces are cooler than ambient air above. So when warmer ambient air carrying a certain amount of moisture encounters cooler surfaces and the temperature drops, the relative humidity increases. So the “source” of higher basement humidity could be lower temperature.
one option is insulation to prevent heat transfer.
Another is dehumidification.