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Community and Q&A

Indoor relative humidity

Michael Cicero | Posted in General Questions on

I have been reading on here and one concern for moisture in the home is indoor relative humidity and controlling it.  We live in Maine and do have central cooling, but most days we like to have windows open and enjoy the fresh air when its not oppressively hot.  Today for example, its warm outside but the temperature inside is still feels very comfortable at 75F but the thermostat is reading 73% RH inside.

I recently have discovered that our fairly new house (2 years old), though stays fairly cool or warm without much input of the geothermal system, isn’t insulated optimally or too code and can run the risk of having moisture issues as a result.  Should I be worried about the high humidity levels inside while enjoying the fresh air?

House details:  2×6 walls with fiberglass insulation and 3/4″ exterior foam on the exterior of the plywood sheathing and wrapped with tyvek paper with vinyl siding.  Interior drywall is painted with vapor barrier sealant paint.  Roof is Zip system with shingles directly on it, fiberglass insulated in the 2×12 rafter bays with soffit/ridge vent channels (not sure what size).   The attic area though isn’t conditioned or fully finished yet like the house, but was just prepped for future bonus attic room.

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    In summer, if your family enjoys leaving the windows open (and keeping the air conditioner off), that's fine -- as long as you don't notice problems like mold on your drywall. Everyone in the U.S. lived that way until 1950 or 1960, so it's not a problem.

    If you see mold, or if your windows have lots of condensation in winter, you may have elevated indoor relative humidity. Otherwise, don't worry.

  2. User avatar
    Jon R | | #2

    +1 on what Martin said. And note that mold (and mites) can be stopped with an occasional good (< 50% RH) drying (using some combination of natural weather variations, AC and dehumidifier). How occasional depends (do you have a cool slab?) - perhaps once per week and adjust as needed.

  3. kjmass1 | | #3

    Nice little calculator here that you can play around with

    It would take almost 2 months at those levels for mold growth, I wouldn't worry to much about it. Maine certainly will get a mix of weather allowing you to take advantage of drier times, or your AC.

  4. Michael Cicero | | #4

    Thanks for all your replies. I was especially concerned after reading this article by you Martin.

    I have very thin exterior foam, 3/4" and no internal poly vapor barrier. We have only spent one winter in the house and the relative humidity averaged 25-35% with some days I noticed it being higher.

    I will keep an eye on things more closely now and address as necessary. If I do need to take the siding off and add extra exterior foam what is the best way to wrap around the windows?

    1. User avatar
      Jon R | | #5

      > If I do need to take the siding off and add extra exterior foam

      Review your interior side and exterior side vapor permeability and compare to the recommendations here (Table 3). You might need vapor barrier interior paint - but too little foil faced exterior foam is an expensive to fix issue.

      1. Michael Cicero | | #6

        What should I review specifically? Is there something to measure? Is it just the total sum of the materials used? I do know that vapor barrier paint was used in painting the house, but as a class III is it enough?

        1. User avatar
          Jon R | | #7

          Tell us more about the foam - unfaced XPS, foil faced? And are you sure the interior paint isn't Class II? Air sealing was verified to be very good with a blower door?

          1. Michael Cicero | | #8

            I am not sure if the paint is class III or II. I thought all vapor retarding paints were class III. The paint used was Sherman Williams B72W1 primer. The foam is blue unfaced rigid foam board. I believe XPS. I have not had blower door test done yet. I am planning one in the near future but guys are quite backed up.

  5. Matt F | | #9

    Where in Maine are you, climate varies a bit across the state?

    While, maybe not ideal, your foam should have a perm rating more than 1 and should allow reasonable drying under most conditions.

    1. Michael Cicero | | #10

      That is ture that the climate varies quite a bit. I am in southern Maine in York.

  6. Brad Cook | | #11

    It is all about the dew point, which is that temperature for a given volume of air at a given temperature and Relative Humidity (RH) cannot hold any more moisture, which starts to condense to a liquid. A given volume of air has a given number of water molecules. As that air is heated up, the volume gets larger, so the RH goes down (same number of water molecules in a larger volume). That also means that warmer air can hold more moisture. As that same volume of air cools down, the volume gets smaller and the RH goes up, until it hits the dew point. In the summer, when the outside air is hot and humid, the moisture in the air may find a surface at the dew point. For example, 80 degF air at 80% RH has a dew point of 74 degF. If you leave windows open and that air contacts an un-insulated basement wall or the floor of an uninsulated slab, it could condense to water. Leave a pile of clothes on a concrete floor and you could have mold growing on them. The clothes are insulating the concrete from the warm house, keeping it colder, but the warm moist air will permeate the clothes and condense.
    In the winter, if that warm moist air can find a surface at the dew point, it will condense. Windows are often that surface.
    Your RH in winter is at a fairly healthy level. The comfort level is about 30-50% RH. It should be kept below about 50% RH, which will be above the dew point for exterior surfaces in the average home. When outside temperature is in the single digits, you may have to lower the interior RH to <20% to avoid condensation on the windows (depends on their U-value).
    As for vapor retarders, keep in mind that you will move more moisture into a wall by the air leaking through the gap around a single outlet box, than a 20 ft. long Living Room wall that is sheetrocked with no vapor barrier. In your case, the XPS on the exterior is a Class II vapor retarder (almost a Class I). I would do nothing to your house as it has been described.
    Everyone should monitor their Temperature and RH, using a digital hygrometer. I give my clients a meter from Acurite. It costs about $12 retail. I buy a bunch at a time and set them up in my Living Room, side-by-side. I find that at least 95% of them read temperature within about a degree of each other and RH within about 2-3%, which is quite good for that price.

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