Why Is It So Humid In Here?
An Ohio homeowner wonders why his heat-recovery ventilator is doing such a lousy job of controlling the relative humidity in his new house
From the sound of it, Andy Chappell-Dick has left no stone unturned in his quest to keep the air inside his house comfortably dry.
His extremely tight new house in northern Ohio (Climate Zone 5) is built with structural insulated panels, and heated and cooled with a pair of ductless minisplit heat pumps. For ventilation, Chappell-Dick has a Venmar Kubix heat-recovery ventilator that pulls exhaust air from two small bathrooms and supplies fresh air to two upstairs bedrooms with a flow rate of between 40 and 80 cubic feet per minute (cfm).
"All my plumbing traps are good," he writes in a Q&A post at GreenBuildingAdvisor. "We hardly take any showers. We cook some, not a lot. No dishwasher. No aquarium. No dryer — we dry clothes offsite. I figure all the entrained construction moisture is long gone."
And yet the interior air isn't really very dry. Chappell-Dick has seven hygrometers around the house to measure relative humidity, and none of them seems to drop below 47 percent, typically reading about 60 percent.
Does he have a bum HRV?
"Even in a worst-case scenario of compromised or short-circuiting airflow, say 25 cfm, shouldn't this unit be capable of doing several total air changes per day? It was a damp December, but we had some cold, dry days that didn't move the needles much.
"In such a tiny tight house, does the humidity produced by just the two of us require a dehumidifier in addition to the HRV? Is there some moisture source we're overlooking? Or is there typically a long lag in getting the house's contents — tons of drywall and flooring and interior framing and everything — to dry out so it's no longer the source?"
Chappell-Dick's battle with high indoor humidity is the topic for this Q&A Spotlight.
Give it some more time
Don't hit the panic button yet, says GBAGreenBuildingAdvisor.com senior editor Martin Holladay. It may take a couple of months of winter ventilation before interior moisture levels start to fall.
"Summertime ventilation doesn't lower the humidity much, and in many U.S. climates actually raises the indoor humidity," he writes. "So you need to have a few months of winter ventilation before you can be sure what is going on."
But Holladay, like others responding to the post, wonder about the possible role the concrete in Chappell-Dick's basement may be playing. Concrete contains a lot of water that must evaporate after it cures and, Holladay adds, is it possible that the contractor forgot to put a layer of polyethylene plastic or rigid foam insulation beneath the basement slab to block moisture from migrating upward.
GBA reader Flitch Plate agrees that a single year might not be long enough for all of the moisture accumulated during construction to dissipate.
"I would not count on all the construction moisture to be fully evaporated," he says. "I would monitor but not make any decisions until you have another year under your belt."
Try a portable dehumidifier
While he gives the house a chance to dry out on its own, both David Meiland and Jin Kazama suggest Chappell-Dick try a portable dehumidifier.
Meiland wonders whether running a dehumidifier for a short period of time might knock the relative humidity down. He suggests running it in the basement, then moving it upstairs to see what happens.
Also, Meiland adds, if Chappell-Dick has separate fans in the bathrooms, he might try running those for extended periods of time to see what happens.
"Seems like winter air in Ohio should be fairly dry, so it should make a difference," he says.
And there's no major energy penalty for running the dehumidifier inside during the winter, Kazama adds, because the heat it produces won't be wasted.
"That said," he adds, "I do not know the conditions at your place, but as soon as we move in heating territory here (mid Quebec), it's hard to get anything above 40 percent RH... This calls for serious investigation."
What about fans in the bathroom?
Aaron Becker wonders whether ventilation in the bathrooms and kitchen has something to do with it.
Is the kitchen exhaust fan, for example, a recirculating type or vented to the outside? And is all of the moisture generated in the bathroom supposed to be handled solely by the HRV?
"How about some simple tests?" Becker says. "Run the showers (one at a time) and place a humidity gauge in the outdoor exhaust stream vent to see if there is a difference before and after. Do the same thing with the kitchen exhaust venting."
Another place to check: if there is a basement sump, it should have an airtight sump lid, to make sure that isn't the source of moisture in the basement.
The kitchen fans theory, as it turns out, was a dead end. Chappell-Dick replies there is no kitchen exhaust fan, and no separate bathroom exhaust fans, either.
Refocusing attention on the basement
Jill Fussell turns the conversation back to the basement, based on her own experience in a house of similar size built in the Austin, Texas, area during a wet winter a few years back.
"The slab had been poured and only allowed to dry a day or so before framing started, which I attribute to the high relative humidity (60 percent or more)," she writes. "Luckily I had a portable dehumidifier that I ran the first year, and finally the RH dropped into the 40-45 percent range."
She asked an architect friend to computer how much water the basement slab would give up during the first year of occupancy, and his answer was 600 gallons.
"So, based on the size of your house and probably the size of the basement associated with it, I would venture a guess that the curing of the basement concrete is the reason you have high RH," Fussell says. "I would suggest running a portable dehumidifier in the basement for at least a full year as the concrete cures. "
One last step would be to check on how often the HRV runs. In Fussell's case, the HRV was running on factory settings that made it cycle too frequently, "bringing in too much relatively moist air for the rather efficient HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. to condense and dispose of (I don't have a dehumidifier on the HVAC)."
Our expert's opinion
Here's what GBA technical director Peter Yost has to say:
Andy Chappell-Dick and I had quite a few e-mail exchanges to try and figure out why his interior relative humidity (IRH) seems higher than it should be, given all the conditions of his structure and mechanicals. Here is what we know, in addition to what is covered above:
- Andy is an admitted “tech nerd” and is a very engaged owner, keeping very good track of how his home operates and manually engaging mechanical equipment for optimal performance.
- All of Andy’s hygrometers are “el cheapo,” eBay purchases of about $4 each. While their precision seems to be about ±3 percent, I know from experience that this type of inexpensive hygrometer can read as much as 10 percent different than actual RH (so lack of accuracy despite apparent precision). The window condensation tells us the indoor RH is higher than what we should expect, but we don’t know really how much higher it is running. (There is a big difference between 40% and 60%).
- The Venmar HRV seems to be operating properly. Without an interior source of moisture, Andy is operating his system in such a way that wintertime IRH should not be running at 40 percent and above.
- We could not determine any other source of moisture besides moisture of construction. Andy’s home construction started in April 2013 and they moved in in December 2013.
So, the question is: almost two years into a home’s life, can the concrete’s vapor emission rate still be enough to cause higher indoor RH than we would expect?
First, there is a GBA reference that helps with this question: Moisture Sources, Relative Humidity, and Mold. And in Andy’s case, we can use the psychrometric chart to calculate how much water in his volume of air results in changes in indoor RH: Andy’s home measures about 10,500 cubic feet, with a dry air weight of about 788 pounds (1 cubic foot of dry air @ 70°F = 0.075 pounds). That means that to move from 20 percent to 40 percent IRH, adding only about 2 pounds (or pints) of water is needed, and to move from 40 percent to 60 percent only about 2.4 pounds of additional water is required.
In general, concrete continues to emit measurable amounts of water vapor (with no added source of wetting) for at least 3 years and in the range of the amounts of water cited above (see Figure 2 in Moisture in Concrete). And if you add to that the admittedly tiny amounts of water generated by occupants, we do have enough moisture to significantly elevate indoor RH in tiny, tight homes or apartments, like Andy’s.
I think that Andy needs to use his dehumidifier as the most efficient way to handle his elevated wintertime indoor RH for at least another year but that over time the vapor emitted as moisture of construction will move down in such a way that optimal use of his HRV will move his average wintertime indoor RH down to a more typical range for his climate and building characteristics.
GBA invites Andy to come back at us (or more rightly, I guess, me) in a year for a “Click and Clack” sort of “Stump the Chump” followup to see if we got this right.
And by the way, Andy’s situation — moisture problem in a very airtight structure — is not an anomaly; even small amounts or sources of moisture can become problematic in really airtight structures.
- Andrew Chappell-Dick
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