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Community and Q&A

High humidity in house after insulation

hotandhumid | Posted in General Questions on


I am hoping someone might be able to shed some light on a humidity mystery. I had my attic insulated with blown cellulose and air sealed ( I hope). Since that time my home has gone from around 45-48 % RH to 52-56% RH humidity ( 73-74 degrees). Also we have an encapsulated conditioned ( dehumidifier and supply vent) crawl with a radon system. The roof has radiant barrier sheathing installed when the house was built 8 years ago.

We have several knee walls ( facing south and east) which were spray foamed in addition to 13″ of cellulose. I did notice the original batt insulation was left in the slope portion of the roof ( just above knee wall) with a twisted baffle left in place to act as a damn for the blown cellulose. On the other knee wall, spray foam continues all the way up, blocking the top of the knee wall ( no air circulation or baffels)…which is in another small separated portion of the attic.

I am wondering if I should remove the batt insulation for better ventilation, or something else could be causing the high humidity that I should check.


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  1. GBA Editor
    Martin Holladay | | #1

    The most likely explanation has nothing to do with insulation; it has to do with air sealing.

    If your house was formerly leaky, and was recently made more airtight, existing moisture sources (including normal human activities like taking a shower, mopping the floor, and cooking pasta) are no longer being diluted by high rates of infiltration and exfiltration.

    If my guess is correct, you'll need to learn how to operate your existing exhaust fans (your bathroom exhaust fans and your range hood fan) more regularly. If your house is quite tight, you may need a mechanical ventilation system. For more information, see Designing a Good Ventilation System.

  2. hotandhumid | | #2

    Thanks Martin. I understand the need for ventilation, however I am the only one at home...and not cooking currently :) Only source of humidity would be daily shower...not mopping either ( retired mom :). Could there be any other reasons for the high humidity??

  3. user-2310254 | | #3


    Where do you live? You interior humidity levels aren't that high, especially if you live in an area where the outside conditions tend to be hot and sticky. I have whole house ventilation and dehumidification, and my humidity stays close to 50 percent throughout the cooling season.

  4. hotandhumid | | #4

    I live in Nashville. I know it's humid here. What I do not understand is why the humidity has gotten noticeably worse since the insulation was done. We had a much more unpleasant summer last year ( high heat and humidity), and the interior humidity was lower than it is now. I still think it may have something to do with blocking air and moisture in the soffit area with batts.

    1. Meriwether | | #12

      I am in the same area - south of Memphis. I had the exact same problem but fairly new HVAC (1 year, so first summer for that insullation) and my relative humidity soared up to 65-73% after blown in insulation. Temp outside also was up to 97 deg. Floors buckled, damp air = big problem. If attic insulation is not the problem, then what is? I ordered whole house dehumidifiers because I don't know what to do? Would SO appreciate some advise.

      1. Expert Member
        Akos | | #13

        The problem is that with more insulation, your have much less heat gain, thus your AC runs less. Since the AC runs less, it removes less moisture from the air thus the high humidity.

        A separate dehumidifier is usually the best fix. The AC+dehumidifer combined will still cost much less to operate than the house before insulation upgrades.

  5. GBA Editor
    Martin Holladay | | #5

    Steve makes an important point. In your climate, at this time of year, your indoor relative humidity (RH) appears to be normal.

    Do you operate an air conditioner?

    For more information on sources of humidity, see this article: Preventing Water Entry Into a Home.

    By the way, your soffit vents (whether they are open or blocked) have nothing to do with regulating your home's indoor relative humidity.

  6. Expert Member
    Dana Dorsett | | #6

    It most likely became noticeably more humid because the house is more air tight, as Martin suggests, and has nothing to do with pre-existing batts.

    Spray foaming the kneewalls improves the air tightness of notoriously leaky type of construction detail, and the other air sealing done prior to the cellulose fixed some air leaks too.

    At the same time the new insulation and air tightness likely lowered the sensible cooling load, so the AC is operating at a lower duty cycle than previously, but the latent-load (humidity) probably wasn't lowered in the same proportion, thus the AC isn't removing as much moisture as it did previously. Latent load handling is a common problem with higher performance homes, which can be often dealt with by adding a room dehumidifier near a main AC return register.

    Anything under 60% RH is pretty healthy & comfortable for most people. Under 50% is recommended for those with dust mite allergies.

  7. hotandhumid | | #7

    Thanks Martin. Yes, I do have an air conditioner ( several) ducted Mitsubishi Citi Multi. The ducts were sealed on the units prior to insulation work-I did some homework before having the work done. On the soffit vent issue-I thought they had to be open so that moisture and hot air could move up towards roof vent. We have an odd attic-2 stories with several knee walls ( now I know better).

  8. hotandhumid | | #8

    Thanks Dana,

    The HVAC units run for quite a long time ( sometimes for hours, minimum is 30 minutes). They are ducted Mitsubishi Citi Multi units-was told they are used for small offices and condos ( they're not the residential wall units Mitsubishi is typically know for).
    The reason for concern is because the humidity increased quite a bit-very uncomfortable at night. My daughter who visits frequently has asthma, and I have allergies, so humidity levels are a big concern.

  9. GBA Editor
    Martin Holladay | | #9

    Q. "On the soffit vent issue: I thought they had to be open so that moisture and hot air could move up towards roof vent."

    A. The air that enters a soffit vent is exterior air, so this ventilation system has nothing to do with the air inside your house, or the relative humidity inside your house.

    Soffit vents work with ridge vents to ventilate unconditioned attics. They don't act to ventilate a home.

    The idea that unconditioned attics need ventilation is not particularly scientific, but this practice is now enshrined in most building codes. For more information on attic ventilation, see All About Attic Venting.

    If your house needs better ventilation, your ventilation inlets won't be anywhere near your soffits. You'll have special ventilation inlets that connect to ductwork -- generally leading to an HRV, an ERV, or the return plenum of your furnace.

  10. hotandhumid | | #10


    Thank you so much for all the info. My thoughts about the soffit vent blockage and the relationship to moisture and attic ventilation relate to the interior only if the air sealing wasn't complete. I don't think they air sealed the top plates on the exterior walls ( I was told they couldn't access it well enough). I was wondering if that could be causing the moisture issue: soffits blocked; attic humidity, which could be introduced to the interior if the wall plates on the exterior walls were not air sealed. I apologize for not being more clear. Thanks!

  11. Expert Member
    Dana Dorsett | | #11

    All Mitsubishi indoor cassettes used with the City Multi VRF systems have a "DRY" cooling mode that can be selected, which increases the latent/sensible cooling ratio over the normal operation. Operating it in DRY mode most (or all) of the time is totally appropriate in your climate, especially for occupants with asthma & /or mold allergy issues. Since your system is capable of operating in a more effective latent-load handling mode, give that mode a shot!

    The as-used SEER efficiency of the system goes down a bit when operated in DRY mode, but the poor latent load handling of very high SEER systems is a well known "sucker trap" in a southeastern US climate, since it results in lower comfort due to the higher humidity.

    If your system has one of the dedicated outdoor air (DOAS) implementations, adjusting the ventilation rates up or down may fix the high humidity problem too. (I'm not at all familiar with how Mitsubishi's DOAS systems are normally set up or adjusted, only aware that they exist in the City Multi product line.)

    1. Deleted | | #14


  12. robotman2009 | | #15

    Chances are your ac or heat doesnt have to come as frequently due to more insulation. That reduces the amount of time air can get dehunidified either by ac or by heater.

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