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Humidity levels

jumanji | Posted in General Questions on

Can someone shed light on what is an acceptable humidity level inside a very tight, energy efficient home. I am located outside of Philadelphia, Pa. an area that is fairly humid in warm months.

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  1. Expert Member
    Dana Dorsett | | #1

    Keeping it under 60% RH @ 75F is usually healthy and comfortable for most people. At 65% and above risk of fungal infections of the skin & lungs start to pick up, and 70% isn't very comfortable for most people.

    Those with dust might allergies would need to hold the line at 50% RH, a humidity level below which dust mites can't reproduce.

    Dew point is one measure of absolute (rather than relative) humidity. Air that's 60% RH @ 75F corresponds has a dew point of about 60F, and 50% RH @ 75F air has a dew point of 55F. So any time the outdoor air has a dew point north of those marks the ventilation air can be adding to your indoor humidity.

  2. GBA Editor
    Brian Pontolilo | | #2


    I was just looking into this recently, and the most trusted answers I got was, it depends. A lot of articles will suggest 45% to 50% RH as a target. That can be difficult to achieve in certain climates and in certain homes. 50% to 60% may be more realistic. As Dana pointed out, though, too much higher than that can be unhealthy. Keep in mind that your tight, energy-efficient home may be more difficult to dehumidify in the summer if your A/C doesn't need to run much. If you want to keep RH down in June, July, and August in PA, you may need to run a dehumidifier. Here's a good article about that:

    1. Expert Member
      Dana Dorsett | | #4

      45-50% is indeed a good target, kind of the sweet spot for health and comfort.

      But the question was "...what is an acceptable humidity level...", which is why I only suggested upper bounds. If you don't have a dust mite issue, 60% is still fine for the majority of people.

      If the ventilation rates are low mid summer humidity isn't usually an issue in air conditioned houses. The peak problem is usually in the shoulder seasons when the sensible cooling loads are low and the air conditioning rarely /never runs. Only in houses with very low solar gain or locations higher average summertime dew points than Philly (think "Gulf Coast States") would an air conditioned house need a separate dehumidifier.

      In May Philadelphia's outdoor dew points are above 60F only about 25% of the time. In Galveston TX the outdoor dew points are above 60F more than half the time even in March.

  3. Jon_R | | #3

    Don't assume that the humidity you measure at one point represents the humidity everywhere in the house.

  4. jumanji | | #5

    Thanks for the information. I am wondering if a possibility is the ERV contributes to the problem.
    In order to keep at a reasonable (less than 55 rh) I set the thermostat at 70 degrees. Preferably, we would set it at 72 degrees but the a/c system works less time which allows an increase in humidity. I think a professional other than the HVAC company who installed the system would be a help. C

  5. Expert Member
    Dana Dorsett | | #6

    >" I am wondering if a possibility is the ERV contributes to the problem."

    Any active ventilation running in background will contribute to the problem when outdoor dew points are high.

    Running bath & kitchen fans to purge super-humid air when needed will reduce the problem, even though it's drawing humid (but less humid) outdoor air in.

    Some AC systems are set up to draw in ventilation air from the outdoors whenever the air handler is running. It's usually a 6-8" duct on the return plenum going off to an exterior wall. That will reduce the effectiveness of the AC at removing humidity, since it's bringing some humidity in on the ventilation duct. They are usually adjustable, often with automated damper controls.

  6. Peter Yost | | #7

    1. mold, mildew and dust mites don't grow in thin air; it's the moisture activity level near/on surfaces and IN some surfaces like carpets that matters. I would never recommend an ambient relative humidity at 70F above 50%.

    2. The better the energy performance of the home, the longer the shoulder seasons (when there is no call for space heating or cooling), and the greater the need for some form of whole-house dehumidification. Energy efficiency, ventilation, and dehumidification create tension in terms of performance. A good resource for dehumidification:


    1. Expert Member
      MALCOLM TAYLOR | | #10


      I use my insulated (and heated when necessary) shed as a proxy for ambient indoor humidity levels independent of occupant activity. It closely mirrors the levels in my house. Here in the PNW, in the winter they sit around 50%, in the summer 60%.

      During the summer we always have windows open. So given that the humidity levels are driven by those outside. how do you suggest we maintain less than 50%? Are you saying houses in this climate should stay sealed up and use dehumidification?

  7. jumanji | | #8

    I am still discussing the issue with the HVAC contractor who put the system in the house. He is scheduled to inspect the system in two weeks. One comment from him has me concerned as he said when a home is so tight it can cause humidity issues. This does not seem to make sense to me even though I am lost on the HVAC subject matter. What level of humidity should be expected to be in the air coming from the ductwork when the A/C is on? Any help and advice is appreciated.

    1. Jon_R | | #9

      > What level of humidity should be expected to be in the air coming from the ductwork

      Don't worry about that number. But do have the AC checked for the proper CFM/ton (or the related delta-T). Wrong values result in poor (or even no) dehumidification.

    2. Expert Member
      Dana Dorsett | | #11

      >"What level of humidity should be expected to be in the air coming from the ductwork when the A/C is on?"

      With most split-system air conditioning when the AC is on the air coming of the registers will have a dew point of 50F or less, and after it's been running awhile can even hit 40F in some cases.

      When warmed to room temperature a 50F dew point translates into:

      35% RH @ 80F

      41% RH @ 75F

      49% RH @ 70F

      Air with a 40F dew point translates to:

      24% RH @ 80F

      28% RH @ 75F

      34% RH @ 70F

      So with a right-sized air conditioner the average room humidity will be between those numbers on days where it's running a high duty cycle (like when it's actually hot outside.) When the outdoor temps are low it can run higher than that but is usually only an issue in high-performance low solar gain newer homes, and some amount of dehumidifier use may be required.

      With very high SEER systems (or many super-efficient mini-splits) the humidity of the air coming out of the ducts will usually be higher.

  8. jameshowison | | #12

    I doubt that you'll get useful discussion from your HVAC guy. If he didn't think it through when he installed things, then he'll just argue in ways that justify the equipment choices he did make. He'll wield his more practiced jargon until you yield :) There will be confusion about climate zones, seasons, ERV vs HRV, and dewpoint vs relative humidity.

    If at all possible to install a whole house dehumidifier then you decouple the whole discussion, it all becomes so much simpler since if feels too humid you just change the setting, you don't have to worry about when/whether the AC runs. Check out UltraAire or AprilAire. Ballpark is $2500 installed.

    For monitoring (especially uneven levels throughout the house) I've had really good luck with the sensor network from I find their site a bit overwhelming but settled on these guys: combined with the needed base station (only $29).

    So you can wireless monitor 4 different rooms for ~$120 (sensors are ~$29, base station also $29). Fun pics over time :) I particularly enjoy knowing when the heat pump water heater turns on in the garage (can see in both temperature and dewpoint time series).

    1. Expert Member
      Dana Dorsett | | #13

      >"If at all possible to install a whole house dehumidifier then you decouple the whole discussion, it all becomes so much simpler since if feels too humid you just change the setting, you don't have to worry about when/whether the AC runs"

      That would only be need on really tight high performance houses in Philadelphia, not typical code-min construction. Simply sizing the AC right would get it right 95% of the time in a Philly type climate, and for the other 5% of the time a room dehumidifier would handle it. A whole house dehumidifier is a bit like swatting at flies with sledgehammers in that climate.

      Whole house dehumidifiers start to make more sense in US climate zone 1A, 2A and parts of 3A, locations where outdoor dew points are north of 60F for most of the year even when it's not so hot outside. In Philadelphia outdoor dew points only average north of 60F for about 5 weeks of the year:

      Compare that to Savannah GA, where the dew points average above 60F for about 5 MONTHS out of the year:

      Or Houston TX, where dew points average above 60F for about 7 months out of the year:

      In Miami FL the dew points average above 60F all year round:

      So in Miami mechanical dehumidification will be necessary to maintain comfortable healthy indoor humidity levels even on not-so-hot days when the AC is barely moving which is a large fraction of the year. That isn't going to be the case in Philadelphia, where most of the time ventilation and normal air conditioning covers the latent load.

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