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Low winter humidity in a “pretty good house”

r_designer | Posted in Pretty Good House on

Greetings!  I hope that the folks here can help me find the cause(s) for the very low winter humidity in my new “Pretty Good House”.  This past summer we moved into a newly built home designed for good energy efficiency in zone 5a.  The house is designed with double stud walls, dense-packed fiberglass insulation (R40 walls, R60 in the roof), efficient triple pane windows and doors, smart moisture barrier on the interior with seams taped, and high-efficiency heat pump heating/cooling.  We have an ERV set to 107 CFM (measured).  The independent energy audit reported 757 CFM50 (1.06 ACH50) for air infiltration. Most of the time there are two people living in the home.  We have a 400CFM vent hood in the kitchen, but it is not used very often.  We also have an unused dryer vent that is filled with insulation but not moisture sealed.  If more information is needed, please let me know.

This winter the interior humidity levels have been pretty low (about 25%RH).  We keep the house at 66F when we are home and awake and lower the thermostat to 64 when we are sleeping or away.  It takes a while to drop off, as it should.   The humidity level has become uncomfortable, even compared to our previous 1980’s average house in the same region.  We started hearing noises of wood popping and crackling and decided that we needed to run a humidifier.  Some of that might just be new house settling, but either way, it seemed like good idea.  

We have been running the humidifier for about a week now, and have gotten the interior humidity level up to a pretty steady 40%RH.  It varies with temperature, as would be expected.  We are adding about 5 gallons of water to the house interior each day.  I did the quick math and that is about equivalent to the total moisture contained in the air inside the house.  So it raised the question:  where is all of that moisture going every day?

From reading on this site, people have indicated the opposite problem is more common for tighter, energy efficient houses:  high humidity in the winter.  My understanding was that the ERV should help keep the moisture in the house by putting some of it back into the incoming airstream.  I do understand that it will only be so efficient, though.  

So I am looking for possible reasons that we have such low humidity.  I recognize that we might not be able to address some of the possible reasons now that the house is finished, but any thoughts are welcome.  Thank you!

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Replies

  1. nynick | | #1

    We recently finished a very similar project completed in Coastal Zone 5. Our ACH is .88 and we have ducted HP as HVAC and an ERV, triple glass windows etc. Sounds similar to yours. You're correct the opposite problem is more often the case in tight homes.

    Our RH hovers around 45-50%. Very little to no condensation anywhere. I believe I heard the HVAC guy said he set the ERV to 52 but am not positive about that. There's 2 of us living in the home. We run shower fans when we shower and the cooking exhaust fan as well. We also have a fireplace that burns wood with an operable cap on top. Not the greatest seal but better than nothing. Hardly ever use it.

    What is the RH outdoors? I'd try turning the ERV off for a day or so and see what happens, while monitoring your air quality levels. I have a feeling your ERV is removing all your warm moist air and replacing it with dry outdoor air. Maybe adjusting it could raise your RH?
    I'm no expert but I think it's worth a shot.
    Good luck
    Nick in CT

  2. Expert Member
    Michael Maines | | #2

    ERVs retain more moisture in the house than an HRV, but there is still a net loss.

    Your numbers indicate that your house is very large, around 4,500 to 5,000 sq.ft., depending on ceiling height. If you had a 1500 sq.ft. house I doubt you would have any issues. The two of you are only putting so much moisture into the air.

    I would lower the ventilation rate and use a CO2 monitor along with your humidity sensor to determine an effective ventilation rate that works for you. Or get an integrated CO2 sensor if you prefer.

    Most relative humidity sensors are normalized for 68°F, so their readings at lower temperatures are not as accurate. And inexpensive ones aren't particularly accurate anyway.

    My house is smallish (1,400 sq.ft.) but not air-sealed (yet), and my home office is currently 64°F and reading 25% RH. I find that to be comfortable, but I'm used to it.

    1. r_designer | | #4

      Michael,
      You are correct. The volume of the house would be typical for a 4500-5000Sq-ft house. We have cathedral ceilings throughout and a very tall great room, so a lot more volume for the actual square footage.

      I will find a CO2 sensor and see what that tells us then adjust the ERV duty cycle a little. I have definitely seen that the RH sensors we have are not particularly accurate we have a range of 10% over the three different sensors we have. We have been able to get a 10% increase across all sensors with the humidifier, though, so that is at least an improvement.

  3. r_designer | | #3

    Thanks, for the comments nynick. I am in NH, so outside has been pretty cold and dry lately. RH outside has been 40-80%, but when its only 10-25F in temperature, there is very little water in the air. Like you, we boost the ERV for usually about 40 minutes when we shower, but that is about it.

    After I posted I thought about the possibility that the ERV might be set too high. Indoor air quality has been fine, but I need to run some estimates on how much moisture is being sent outside by the ERV based on the current flow rate. It is currently set for about 5 exchanges a day. So even if 80% of the moisture gets put back into the incoming air, we would be losing the equivalent of all the moisture in the house each day. That would be approximately what we are having to put back in with the humidifier.

    So is 5 exchanges a day about right? I've seen numbers indicating 0.35 exchanges per hour is suggested, so that would be 8 exchanges a day. Air quality has generally been fine, and I don't have a problem bumping up the ERV for a little while when we have guests or cook with a lot of onions.

    I guess the other comment would be that we don't use a ton of water. Relatively short showers, high efficiency washers, etc. We also have a heat pump dryer so laundry water mostly goes down the drain.

    1. Expert Member
      Michael Maines | | #7

      Like everything prescriptive, they are trying to cover the majority of typical situations, but there are always outliers. Your situation is likely outside the range that the standard formulas cover. The point of ventilation is to have good indoor air quality, including a healthy and safe indoor RH, so that should rule over a formula you find in a book. If you don't have one already, I recommend getting an Awair Element or Airthings View Plus--both are pretty affordable, reasonably accurate and give you useful data.

      The building likes the air to be dry, other than potential wood movement issues. In most cases, the healthiest range for people is 40-60%. Over 50% gets into potential risk for mold, especially if there are thermal bridges. (i.e., in a Passive House you could push the RH higher.) The experts I have learned from generally recommend that a good balance is around 40-45%.

      1. r_designer | | #8

        Thanks, Mike. I have an air quality meter on order. Another clue is that we had a hard time getting the humidity down last summer after we moved in. It never got above 60%RH but even running AC at about 75F it never improved. That might also point to too much ventilation. We are sensitive to mold, so I was aware of the 50%RH suggested limit to help limit mold. I'm also OK with making some measurements to fine tune the ERV for this house.

        1. Expert Member
          Michael Maines | | #9

          Just as an ERV still removes some moisture in cold weather, it also increases humidity in summer, just not as quickly as an HRV would. The more you run it, the more closely the interior humidity will get to the outdoor humidity.

          Air conditioners are now so efficient at cooling, especially in a well-insulated, airtight house, that they don't remove as much moisture as older, less efficient air conditioners do.

          If you need to control humidity, a dedicated dehumidifier is best.

          1. r_designer | | #10

            That makes sense. I had a similar thought. The current plan is to figure out how low we can turn down the ERV, based on CO2 levels, then see how the humidity does. I doubt we will get away from a humidifier in the winter, but we might be able to lower the summer humidity to a more comfortable level. Might take some trial and error, but I love figuring stuff like this out. People have figured out a lot about energy efficient building, but every house is a little different. With so many different building techniques, there are bound to be cases where the "typical" doesn't work. It's pretty common in engineering...build something, then figure out exactly what it is that you built.

  4. nynick | | #5

    I have no idea. Michael is the expert here, not me. Still, it's worth a shot lowering or shutting off the ERV to test the theory. Just monitor the air quality.

    Personally, I can't wait for Spring and Summer so I can just open the windows...lol.

    1. r_designer | | #6

      I agree...bring on Spring! I'll report back when I can do some testing with the ERV. Might take a few weeks to get the results. Outside conditions will have an effect also.

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