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Community and Q&A

High Relative Humidity in New Construction House

pNJRiLZxNH | Posted in General Questions on

I live in Seattle (currently 60 degrees, 87%RH) and have just moved into my new construction house – RH 70%. Through the construction process, I have introduced a lot of moisture into the home – damp cellulose insulation, suspended concrete slabs main and second floor, full basement (properly waterproofed with exterior/interior drainage/sump with upslope curtain drain and tightlined downspouts), textured drywall and paint.

Reading this site, I read M Holidays article which suggested I may be adding more moisture into the house by my use of a HRV- since the RH outside is higher than inside.

My current strategy to lower the RH is to keep spot fans running in all bathrooms, run a dehumidifier in the basement, run the HRV on low…and limit the kids and dog to … marginal mayhem.

What other strategies are recommended? Also, are there quality dehumidifiers where I can tackle high RH while minimizing an energy penalty?

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  1. Doug McEvers | | #1

    As the outdoor average temperature falls so will the indoor relative humidity, I would stick with the HRV on low and some dehumidification until you are satisfied with the indoor humidity level.

  2. GBA Editor
    Martin Holladay | | #2

    Doug is right. Winter is your friend.

    The colder the outdoor temperature, the less moisture can be held in the air, and the more ventilation benefits your house.

    Even if the outdoor RH is high, it makes sense to ventilate with outdoor air -- as long as the outdoor temperature is significantly colder than the indoor temperature.

  3. Riversong | | #3


    It sounds like you've done everything possible to maximize the moisture in the construction of your home. A typical new wood-framed house will release 300 gallons of water just from the KD framing as it equilibrates to a heated environment. Concrete, damp-spray cellulose and spray texture paints each add considerably more to that load.

    It takes a new house a full year to dry out. A concrete slab typically requires as much as three months before it's dry enough to install floor coverings. Building a house during the humid rainy season can add additional incidental moisture.

    Ventilate and dehumidify as necessary until the house equilibrates, and don't wash the dog indoors.

  4. davidmeiland | | #4

    You might consider renting a Drizair dehumidifier for a while. I typically bring one in as soon as the house is closed in, and use it to varying degrees until the drywall has been finished and primed, depending on the thermo/hygro readings I'm getting. You can collect quite a lot of water with one (or more) of these units--the model I use is the 110. With all the concrete you installed you probably have quite a moisture load.

    Assuming you have wood trim, flooring, cabinets, etc., you need to be careful not to dry things too quickly. Ideally you would have knocked the humidity down early and kept it at an appropriate level thru the completion of the house, making sure at the same time that all of your wood materials coming in were at a good MC% level. At this point I would be monitoring temp and RH as well as MC in all of the finish woodwork (i.e. you need a good moisture meter or two).

  5. Frank | | #5

    Thanks's going to be hard to keep the Labrador dry though!

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