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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

Posted on Feb 16 2015 by Scott Gibson

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 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.

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Image Credits:

  1. Andrew Chappell-Dick

Feb 17, 2015 11:28 PM ET

We had the same issue
by Chris Gleba

We had the same issue in our house:

.9ACH50 retrofit and we had poured a new slab in the basement (with sealed XPS under it). RH was in the 60s the first winter, 50s the second winter and now that we're in the 3rd winter it is in the 40s.

Like the person with the question I had left virtually no stone unturned in trying to figure out "why"; I could not pin exactly the cause and It bugged me that I could not quantify it. Like in the article, the RH numbers did not lign up with the water contributions from people, concrete, showers and cooking. . . but I saw a long-term average trend of the RH falling (between seasonal cycles) so I figured I would re-visit the issue when the trend bottomed out. . .now that it is no longer an issue my best guess is that it was unaccounted for construction-related moisture (maybe paint, joint compound, water intrusion before the weather barrier went up, etc, etc)?

The other unexpected (and likely related) aspect is I put a coat of poylurethane on the floors during construction and it took nearly a year to clear the VOC smell. During that time I ran the HRV at a higher setting but if it went off for a few hours the smell would be quite strong! On year 3 it is comepletely gone now.

The last interesting moisture-related aspect is that post-DER the house does not creak at all with changes in humidity, Pre-DER it used to sound like we were sailing in a galeon come spring time with all the creaks and craks! Now it is nice and quiet. . .

Feb 18, 2015 11:57 AM ET

Edited Feb 18, 2015 11:59 AM ET.

Data reliability
by Carl Mezoff

If the owner is spending so much effort on resolving this issue, it makes sense for his first step to be obtaining good data. He can buy a simple "sling psychrometer" for about $60 that will give more reliable information than (as Peter Yost notes) cheap electronic devices. The psychrometer has two thermometers built in, one that has a moist "sock" over the tip to provide an evaporative effect (wet bulb temperature). These two thermometers give you a direct readout of wet and dry bulb temperatures from which, with a chart, you can directly read the relative humidity with a lot of confidence.

Feb 18, 2015 1:17 PM ET

Checking things out
by Don Fugler

My suggestion is always to run the HRV at a high speed for the first winter after construction, to accelerate the drying. You can reduce it to a normal level in subsequent winters.

Further suggestions:
1) Check the HRV airflow to ensure it is operating as designed (
2) Check the hygrometer calibration (


Feb 19, 2015 11:05 AM ET

by Tim Johnson

Was there a capillary break installed between the basement footing and the wall? Water will wick up 1000' in concrete, so I am told.

Feb 19, 2015 3:32 PM ET

Edited Feb 19, 2015 3:33 PM ET.

Relative Humidity
by Brooke Guthman

I would also question the stone siding on this house and the structure behind it. Is there an air space behind the stone, is there a moisture barrier behind it? If not, stone/brick can be like a sponge, soaking up moisture. When sun hits the stone, it will drive the water vapor inward. If there is not adequate ventilation or a moisture barrier behind the stone, it can be driven inward into the home.
I built my home 22 years ago with a .10 NACH and as an experiment, did not install the ventilation system the first winter we moved in (March of that year). That winter, we had very high moisture to the fact i also had to use a dehumidifier. I did install a ventilation system the second year, but i also believe the lower RH was due to the fact of the home finally drying out (sheetrock, paint, concrete). So, i too would give it time, but would still be curious as to the construction techniques used behind the stone.

Feb 19, 2015 11:57 PM ET

brief responses to questions posed
by Andy Chappell-Dick

Hi. I'm the guy in the article above. At this point I'm resigned to waiting one more winter, to see if things dry out a little more. Anecdotally, there seems to be precedent for needing multiple drying seasons to get to a baseline humidity.

Carl and others pointed out the need to get better data by using better instruments. Point well taken. FWIW, my cheap hygrometers are analog, not digital. They all agree with each other to within a couple degrees, and with the digital meter on the dehumidifier.

Don suggested running the HRV on high speed the first winter. That's probably a good idea. I'm so reluctant to do that, since it brings in so much cold air. It's efficiency is only rated at something like 70%, so the energy penalty is high. I've tried it a few times for a couple days, and RH does approach 40% but stalls out there.

Don's other suggestion to check airflow is also good, and I did, and it is pushing air as rated.

Tim asked about capillary break at foundation wall/footer. There is none. Oops. Didn't nail that detail at the design stage. How much this draws moisture up, I don't know. It was a question I asked on the original blog post, and nobody has addressed it. My basement is insulated with quite tight 3.5" of XPS on the interior side, though, so the concrete is somewhat sealed off from the interior.

Bryce questioned the stone masonry. Ah yes, a thorny issue. But I'm pretty sure it's not affecting this particular question. There is a minimum 1" well-ventilated air gap behind the stone, and behind that the building structure was thoroughly coated with the StoGuard liquid-applied weather barrier system, then further covered by a Typar-drainage mat sheet.

I'm still hoping someone can comment on two basic questions: is it worth spending all the energy dollars on mechanically reducing my IRH from 45% to 35%?

And since we mostly keep the windows open for 6-7 months a year, will it take all winter EVERY year to dry out the moisture accumulated in the wood and concrete of the house's interior?

Feb 20, 2015 12:48 AM ET

I'm confused by Holladay comment
by Kevin Hogan

"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."

How is it that putting a layer of polyethylene plastic or rigid foam insulation beneath the basement slab would serve to block slab moisture from migrating upward? That doesn't make sense to me.

The purpose/role of the polyethylene or rigid foam is to reduce/prevent ground moisture from continually migrating upward into the slab. When polyethylene or rigid foam insulation is placed under the basement slab, it causes/forces the moisture, associated with the slab, to evaporate to the interior of the home. After all, that is the desired effect/rationale for doing so.

Halladay must have been referring to using polyethylene or rigid foam insulation beneath the basement slab to block ground moisture from migrating upward or he was confused because using polyethylene or rigid foam insulation beneath the basement slab does not serve to block moisture migrating from the slab to the interior.

It is obviously too late to employ a capillary break installed between the basement footing and the foundation wall as a legitimate concern mentioned by Tim Johnson, but it's not too late to install a sub-slab ventilation system (a/k/a active radon mitigation system) if there is a sump crock present in this home. By depressurizing the space below the slab, moisture will be removed to the outdoor atmosphere 24/7, all year long. As a result, the basement will experience lower relative humidity levels as will the entire home. Most of these systems utilize a low wattage in-line fan that can be controlled with a variable speed control that the savvy homeowner can "dial down" as the home's relative humidity is reduced over time. This strategy is also less expensive to operate vs. mechanical dehumidification. Another obvious side benefit is radon mitigation. Operating the HRV in conjunction can provide a reliable source of "background" fresh air ventilation provided the HRV airflow is operating as designed (as mentioned by Don Fugler).

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Feb 20, 2015 5:48 AM ET

Response to Kevin Hogan
by Martin Holladay

Here's what I actually wrote (as opposed to the summary of my comments provided by Scott Gibson): "One possible source of indoor moisture is your basement. Of course, there is construction moisture in your curing concrete that has to evaporate. There is also the possibility that your contractor forgot to install 6 mil polyethylene (or horizontal rigid foam) under your basement slab."

You wrote: "Holladay... was confused because using polyethylene or rigid foam insulation beneath the basement slab does not serve to block moisture migrating from the slab to the interior."

I was not confused. I was referring to two different phenomena: (1) Moisture that enters basement air from curing concrete, and (2) Ground moisture that diffuses through a slab that was installed without a layer of polyethylene or rigid foam underneath it.

Feb 20, 2015 3:56 PM ET

I appreciate the explanation
by Kevin Hogan

Thanks for your prompt reply. After reading what you actually wrote as opposed to the summary of your comments provided by Scott Gibson, it casts an entirely different perspective for me.To be clear, I am in complete agreement with you.

Please see the subject line in my original post to know where my thought process was to begin with.

Feb 23, 2015 2:49 PM ET

Simple test
by Steve P

Would it be worthwhile to tape a square (as large as feasible) of clear poly down against the basement floor (or flooring) and see what (if any) moisture condenses on the bottom?

Apr 20, 2015 10:43 PM ET

Simple test
by Andrew Bennett

If there is an uncovered bit of concrete I think SteveP's suggestion could be quite compelling. It doesn't provide a metric, but it does give a good indication of vapor permeation.

Feb 24, 2016 7:26 PM ET

Update on the original Spotlight
by Andy Chappell-Dick

Allison Bailes' 2/24/2016 article on relative humidity ( prompts this update. I am the subject of the above Spotlight. This winter is our third in the house. RH remains constant at around 50%, or just below. I am now measuring with a sling hygrometer for more accuracy. Difference in RH throughout the house, between the finished basement floor and the second floor ceiling, never deviates more than a couple points. Heating is via two minisplits.

The plastic-sheet-taped-to-the-basement-floor test revealed NO moisture beneath.

So I'm still at a loss as to why I can't mechanically vent my place down to below 40%. The construction moisture explanation seems to be moot by now.

However, Allison seems to suggest I probably shouldn't worry. I was meticulous in SIP sealing, inside and out, and my 0.6 ACH50 seems to confirm that. The risk of interstitial condensation is extremely low. I do worry about point defects, though.

Feb 25, 2016 6:30 AM ET

Response to Andy Chappell-Dick
by Martin Holladay

Are you ventilating your house at 40 cfm or at 80 cfm?

What happens when you crank up your HRV to maximum, to increase the ventilation rate -- especially when the weather is very cold?

Feb 16, 2017 11:48 AM ET

Air coming in dry?
by Tyler Keniston

I see this is a bit old, but am curious if it ever got solved.
I am wondering if a humidity reading was ever taken of the incoming air, just in case some odd source of moisture was finding its way into the intake.
Lots of plants in the house?

Feb 16, 2017 12:14 PM ET

Response to Tyler Keniston
by Martin Holladay

I have no idea whether Andy Chappell-Dick ever solved this problem from 2015. Perhaps Andy will see your comment and post a response.

In any case, you may be interested in reading a relevant article that I wrote after this Q&A Spotlight was published: Preventing Water Entry Into a Home.

-- Martin Holladay

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