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What is the desired RH in a house to prevent the woodwork from cracking and splitting and what is best way to humidify the air?

Joseph Samsonowitz | Posted in General Questions on

I was recently hired as an owner’s rep for a 7,000 sq new construction house that is being heated with hydro air and radiant heating. For the past year they have been working on the molding, doors and wall panels around the house that is starting to show cracking and splitting in various areas. Although I have never tested the humidity levels in the house, the temperature is constantly set to 70 degrees. The painter is claiming that the cracking is due to low humidity levels in the house. If that is true, what is the minimum humidity level and how is the best way to increase the humidity? I should mention that they had installed humidifiers to the HVAC blowers. I am reluctant to turn these humidifiers on due to all the potential problems it can cause as mentioned previously in this blog. What is your take on this situation?

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  1. Joseph Samsonowitz | | #1

    I just want to add one more point. I have been in the field for many years and I understand that due to expansion and contraction of wood some small amount of cracking is normal but this is more than the normal amount that is ordinarily seen under regular conditions.

  2. David Meiland | | #2

    What region is this?

    If there is a lot of air leakage, that could easily be the problem, and it should be dealt with for at least a couple of reasons. Tell the owners that you want to have a blower door test done, and fine out how leaky (and where) the house is. You might also need duct leakage testing.

    If workers are leaving doors and windows open because 70 is too high for their comfort, close the windows and doors and turn down the heat.

    If the forced air ductwork has an outside air intake, or if there is a HRV/ERV connected to the system, that may be the problem.

    Get yourself a good hygrometer and try to maintain 68 degrees and around 35% RH. The cheap hardware store versions could be way off and lead you into trouble.

  3. GBA Editor
    Martin Holladay | | #3

    David gave you good advice.

    1. You need to use an accurate hygrometer to determine the interior RH.

    2. If the interior air is dry during the winter, the best way to fix the problem is to seal the air leaks in the home's envelope.

  4. Expert Member
    Dana Dorsett | | #4

    Humidifiers are a bad idea- good call on leaving them off. Most new construction has quite a bit of residual moisture from the concrete and wood that has to be PURGED prior to finishing(!).

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