Airtight home Humidity issues
Air tight home with high humidity fluctuations.
Brief/thorough run down…total home renovation, lifted house 8′ in the air on CMU block, slab underneath whole home, 1/2 cathedral ceiling, half 2-story, 1800 square feet total. 2-zone Bosch 18 SEER 3-ton, set to 2.5 ton, was at 3 but thought that was part of the humidity issue so we sized down to try to pull more humidity out, EV200 ERV set at 30% run time (lowest while still maintaining good air quality). Zone 3A.
Wall detail, Hardie Z10 siding, 7/16″ Zip (taped all seams, liquid flash all field penetrations), 1/2 RMAX polyisoprene board (R-3.2) installed so seams do not line up with Zip, R-15 Rockwool, 1/2 drywall, paint.
3″ closed-cell under house, attic and gable walls, and band board between 1st and 2nd floor.
Blew a 1.2 ACH/50 pre-drwall, touched up leaks installed drywall, had humidity issues and had blower door done again, blew a 0.9 ACH/50.
Upstairs is always higher by 5-10%. Only 1 air return in the home at top of stairs. Have not been able to get the relative humidity under 55% even with a floor dehumidifier running 16 hours a day for a week with exterior air between 70-90% humidity. Exterior humidity dropped for a while 40-55% and home stayed below 50% RH, even down to 40% during that time. Raining now and home RH is back to mid 60’s….what is going on? Do I need to install a dehum? Going on 6 months in the home so material should not be giving off moisture anymore right?
Greatly appreciate the insight and help, used a lot of articles here to help develop my remodel and be sure that I executed an airtight home correctly and am thankful for that from everyone!
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Replies
Wood can buffer a surprising amount of moisture, and with such high humidity levels, you are limited in the amount of natural drying that can occur. I think your best option is probably a few more humidifiers to better handle the load. You should gradually see improvement.
Bill
Seems to me you need to set the EVR to run 100% of the time in the winter months until you can dry out the house. I would run the dehumidifier 24-7 also.
I have to ask the silly questions, Indoor pool, spa, hot tub, pot farm, aquariums, more than a few house plants, more than 3 showers a day, storing firewood indoors, green wood use for framing?
It may take 18 months to dry out normal construction lumber.
Note what gage are you using to measure the humidity many are way off. Have you checked it with a wet and dry bulb thermometer test?
PS Your air sealing number is absolutely amazing for a retrofit of an existing story and a half building.
Walta
Yeah, humidity control seems to be the next important piece of the tight-home puzzle.
Zephyr7 could be right in the short term.
By your description, though, this issue is 6 months old? I'm generalizing, but it's my understanding that most ERVs (and HRVs) are not designed to keep humidity levels down all year long. Other articles and Q&A's on GBA explain this better than I could.
We live in Wisconsin, Zone 5/6, and have had similar interior humidity issues as you, especially in the spring and fall - even though our renovation is 15 years ago. We have radiant heat, ERV, a mini-split AC, super tight home. Minisplit does not keep up with humidity issues (not really designed to). So, we've decided to install a ducted whole house dehumifier (next month!).
I could almost have written this post from CBJakes. We have a very similar situation. Completed a 2-year gut renovation of 1865 house, including lifting the house & pouring new foundation (2019-2021). All new roof, Marvin windows, insulation (combo of rockwool on 1st & 2nd and foam in basement/attic, etc), all new HVAC (2 zones; Veissmann boiler; ERV in attic), etc. Also a half two-story wood framed home, ~1,900 sq ft on 1st an 2nd floor, additional 1k sqft in fully finished basement with radiant heated concrete floors. We also have a return at the top of our 2nd floor stairs for the upstairs zone and a central floor return on the 1st floor (mostly open floor plan on 1st floor) for the basement/1st floor zone. (Note: the basement has one forced air supply in the bedroom but otherwise radiant is sufficient).
We're located in the Boston area, and whenever the temps drop below 30F, we have terrible window condensation in every window on 1st and 2nd floor and through our no-draft/double slat mail-slot on front door (water literally drips down the door). We run our ERV at 90% and still can't get humidity under 42%... oftentimes hovers around 46% despite frigid temps. Last summer, couldn't get under 55%. Like CBJake, always 5-10% higher on 1st floor compared to 2nd. We have exhaust fans in all the bathrooms, range hood, etc that we use religiously. No saunas/hot tubs/etc and usually ~2-3 showers/day. When we're out of town for a few days, and there's no activity in the house, we still can't get humidity under 40-45% in dead of winter.
The basement has a standalone dehumidifier with its own supply & return vents/ducting and so far humidity has not been an issue down there - always in the 30-40% range, and no condensation. We have a standalone plug-in dehumidifier on 1st floor that we run all night and sometimes during day. Collects ~4-5 gallons/24-hour period (my estimate). A few weeks ago we went ahead and installed a ducted Aprilaire dehumidifier in the attic that is integrated into our HVAC system up there. Has helped the 2nd floor get down to 38%, but 1st floor still ~42-46% (despite a very wide/open staircase & 1st floor open floor plan). On really cold days/nights (under 25), we still get quite a bit of sweat in the windows (although not as much, especially on 2nd floor).
Our contractor and HVAC team are at a loss. They can't figure out the source of all the excess moisture. The house was finished in Jan 2021 so it's been over a year. It's a really vexing issue after spending so much $$/time/thought to building a tight house with a deliberate focus on healthy indoor air (CARB-II compliant plywood used in custom kitchen cabinets & casework, no VOC paint throughout, carefully selected drywall, etc). I relied on this forum for so many of our choices - thank you!! Any & all help from this wonderful community is greatly appreciated...
Wanted to be sure and follow up seeing as we seem to have the same situation.
I unfortunately did not save the articles to repost here but I found 2 that really spelled out the issue, one being from GBA. Basically tight home + well insulated = little load and therefore higher humidity. ERV's and HRV's are not dehumidifiers and even if they bring down the levels, if they're high to start with they won't get them to optimal. 90% humidity will still at best become 70% when it gets to your home. The only solution is a whole home dehum. My HVAC contractor quoted me $3500 for equipment and install. We're going to intercept the ERV's fresh air intake and dehumidify the air there before it even gets to the ERV itself. In my home one particular room has very little load and that room is particularly well insulated and tight and is also my most humid. Good news is we did a good job on the tightness and insulation of our homes, bad news is it just does not have enough load to bring humidity levels down and even then most brilliant building science minds don't seem to account for this yet. Hope this helps
Note than an ERV can only lower indoor humidity levels when the outdoor air is LOWER in terms of humidity than the indoor air. If your outdoor air is more humid than your indoor air, the ERV won't help, and might actually increase indoor humidity levels. ERVs are basically heat exchangers, they're not active systems (no compressor or other heat pump type of system inside), so they can't force things against the natural gradients. What this means is that at best, the ERV can equalize humidity levels between indoors and outdoors. As the humidity levels of the indoor and outdoor air get closer and closer, the ability of the ERV to equalize things in a given period of time is reduced (smaller differentials mean less efficiency in this case).
Note that you have to allow for temperature differences too. High %RH of very cold outdoor air will be a low %RH for warmer indoor air. If the temperatures of the indoor and outdoor air are similar, then all of what I said above is easier to follow.
If you are in an area with high average outdoor humidity levels, an ERV isn't going to be able to help much with bringing down indoor humidity levels, and you'll need to consider an active system such as a central dehumidifier.
Bill
That's an important point. A way to use readily available data to figure it out is to look at the outdoor dew point in a weather app or online weather site, and then compare to the indoor dew point, which you can find from the indoor %RH and temperature with a dew point calculator. If the outdoor dewpoint is lower, more ventilation helps; if the outdoor dewpoint is higher, more ventilation makes things worse.
http://www.dpcalc.org/ is a nice simple dew point calculator.
Isn't an HRV better for dehumidification than an ERV? Instead of adding another device to power I'd start with switching to an HRV. I'm in zone 6a with a similar blower door result. The HRV handles the humidity in the winter in my place. In the summer the minisplits handle the humidity somewhat, not really enough though. I've considered a heat pump water heater instead of the resistance electric I have now as a means of reducing humidity and power consumption, but I fear I'd end up heating my basement in the summer as it barely gets above about 22°C in the summer now. I really don't want to see what that bill would look like if I'm trying to heat my basement while I cool my main floor.
Yes, HRVs are better able to limit moisture movement due to the construction of the heat exchanger. An ERV is basically a moisture permeable version of an HRV. Neither can really do much to help lower your indoor humidity levels when the outdoor air is MORE humid than your indoor air though.
Bill
If the absolute humidity is higher, no, but if it's relative humidity and the outdoor air is cold... Well, my house can easily sit at 35% RH in a 6a climate zone during the winter.
Solid point. I was think of colder months.
Neither an HRV or ERV can dehumidify (in the active sense).
I know the common thought is that HRVs will help lower indoor humidity more than an ERV, but I have seen data that contradicts that. If the in and outside temps are relatively similar, but outdoor humidity is 80% and indoor humidity is 50%… an HRV will be bringing in air with more humidity than it is replacing; an ERV will still bring in some, but it would be closer to 65% than 80%… so not all generally accepted rules apply to all situations.
Great conversation and thank you all for contributing.
To answer a few of the questions, no plants, or spa or any general source of water, no fireplace/firewood (a cold winter day is 40F), recirculating vent hood. Only 2 roof penetrations , 2 wall for ERV sealed liquid flashing and 4 in the crawl space all sealed with Hilti foam prior to spray foam. Point-of-use ERV switch during showers so that is negated. I found that with the humidity levels outside running it more just increased in-home levels. The home was built in 1979 I did replace/added about 50% of the lumber, though research here including past posts by Martin Holladay lead me to believe that the construction moisture would be relatively low at this point. Going to borrow a pin-prick moisture meter and take some readings and will re-post if I find anything above the accepted 8-9% EMC.
Using an La Crosse system for exterior readings, temp and actual humidty
Honeywell thermostat for interior/ext humidity and temp
Awair for RH and air quality readings
Verified temp and RH levels with Sensor Pushes in all locations of above equipment...maybe I have a building science problem?
Ultimately thinking that the issue is the ERV not being able to handle it and the central air just not running enough to draw out the moisture. I think being Zone 3A, which for us on an island we really think of ourselves as a sub-tropical environment, that I need to add dehumidifier to the fresh air intake. Even in the winter we average 60-70% humidity and summers are 80-100% (without rain!). I'm thinking I need to start researching a dehum and bump the Bosch back up to 3-ton for the efficiency.
Fun note, old 1100 sqft home avg $175/mth energy bills, new remodeled home at 1800 sqft avg $65/mth so that's sweet, no drafts of cold spots super quite comfortable home so believe the building science! But I agree that humidity levels are the weak link of the airtight home currently...attached some photos too
What were your indoor humidity levels prior to the reno when your home was ‘leaky’? With outdoor RH over 60%, it makes me think the humidity issue you currently have was there all along due to the environment. I do agree the solution is active dehumidification. Aprilaire seems to be the golden standard from my research on this subject… planning to build a wood shop and may be seasoning lumber indoors, so active dehumidification is an absolute must.
It’s worth noting that ERVs transfer moisture as well as heat. In a Northern climate in winter, an HRV will lower indoor humidity by bringing in outside air with low relative humidity and exhausting air with higher indoor humidity. So in winter an HRV will tend to dehumidify. An ERV does not do this.
For example Zehnder’s Q350 ERV has a humidity recovery rate of 73%, meaning that the unit recovers 73% of the difference between outside and inside RH. So putting an ERV in boost mode in winter will bring in fresh air but will redistribute most of that shower moisture inside the house rather than exhausting it.
If you combine occupant loads and construction moisture, you can easily end up in the trouble zone at 60-70% RH. On a recent high performance new construction project, the company I work for experienced this, and we swapped the ERV core for an HRV in late Fall in order to mitigate. This immediately lowered indoor RH by 20+% by bringing drier air in from outside.
The problem of course is that either way (ERV or HRV) you are at the mercy of physics. Outside of a dedicated dehumidifier, there’s no way to control humidity.