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Best way to reduce interior humidity in tight house?

mtndrew | Posted in Green Building Techniques on

I have been building and refining my “best practice” for 20+ years. I have built a house with walls wrapped in XPS foam (sheetrock–>2×6 with fiberglass batt–> osb–> XPS–> rain screen–> cladding). All XPS joints are taped. The roof is TJI cathedral with blown in open cell foam under plywood, under shingles. The sheetrock finishers had turned up the heat inside to 71 degrees and the temperature dropped last night to the upper 20’s. I’m building in zone 4A. There is a conventional HVAC system with fresh air intake motorized damper.

The symptom I noticed this morning was a drastic amount of condensation around all window sash perimeters. Obviously, I need to reduce the interior humidity. ERV, HRV, or whole house dehumidifier?

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  1. user-2310254 | | #1

    I am going to start this process by asking a few questions and pointing you to three relevant GBA articles. I'm sure other members will chime in as well.

    How much XPS do you have the exterior? You need at least R-3.75. See this article for additional details:

    Air permeable insulation in a cathedral ceiling is risky. Do you have R-10 of rigid foam on the exterior? Or did you opt to install ventilation channels to promote promote drying on the interior side of the sheathing? See this article for more detail:

    Are all the seams on your OSB and foam layers taped?

    If you are building a tight home, you need a ventilation strategy. See this article for more information:

  2. GBA Editor
    Martin Holladay | | #2

    You've got construction moisture. It will go away in a few months. Turn on all your bathroom exhaust fans and leave them on for 24 hours per day, and leave a few windows open a crack. Be patient.

    -- Martin Holladay

  3. GBA Editor
    Martin Holladay | | #3

    Steve Knapp's advice is flawed.

    For a 2x6 wall in Climate Zone 4A, there is no minimum R-value for exterior rigid foam on walls. You can install any thickness of foam you want. More information here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    And the fact that you are insulating your cathedral ceiling with open-cell spray foam may be concerning, but not for the reason that Steve gave. Open-cell spray foam isn't air-permeable, so you should ignore Steve's comment.

    The concern about open-cell spray foam has nothing to do with air permeance; it has to do with vapor permeance. More information here: Open-Cell Spray Foam and Damp Roof Sheathing.

    -- Martin Holladay

  4. user-2310254 | | #4

    Martin. Thanks for correcting my post. I needed to read the "minimum thickness" article more carefully to understand that R-3.75 applied only to Marine Zone 4. On the cathedral ceiling, I somehow read "open cell foam" as cellulose.

  5. Expert Member
    Dana Dorsett | | #5

    A motorized damper for ventilation air on the hot air furnace is a lousy way to ventilate any house, since the ventilation rate rises & falls with the heat load/duty cycle of the heating system, whereas ventilation is needed all the time. Once the construction moisture dissipates it can result in over-ventilating with the ultra-dry air when outdoor temps are coldest, and under-ventilating during the shoulder seasons. Building tight and using a different ventilation strategy is almost always going to be better for comfort & health.

    Buy a few cheapo humidity monitors ($10 AccuRites are good enough) and a room dehumidifier plugged in near one of the bigger return registers to the furnace, and set it as low as it can go. If you can get the humidity down to under 40%RH @ 70F everywhere the risks of mold growth in the wall assemblies in spring will go down, along with the excess window condensation.

    Comfortable & healthy ~35% RH @70F air has a dew point of ~41F, and the interior surface temps of even a code-min window would be warmer than that 100% of the time in a zone 4A climate.

  6. mtndrew | | #6

    Thank you gentlemen for your thoughtful responses. I have wired the bath fans to run 24/7 and left the HVAC system with the fan on to bring in fresh air. Exterior walls have 1" XPS and, to be clear, CDX plywood was installed as roof sheathing, not OSB.

    Martin, assuming the humidity levels out after the construction moisture has evacuated the house, what is my best course of action to quell the danger of having open cell foam installed under plywood roof deck over cathedral ceiling? Should I recommend leaving the HVAC fan on so it constantly brings in fresh air through the winter months, regardless of heat demand? Would a whole-house dehumidifier be recommended should the humidity levels remain high? What is my best course of action moving forward?

  7. GBA Editor
    Martin Holladay | | #7

    Your insulated cathedral ceilings will probably be OK. You can reduce the chance of moisture accumulation in the roof sheathing by installing an interior smart retarder like MemBrain, or, if the ceiling drywall is already up, by installing vapor-retarder paint on the drywall ceiling.

    Every house needs a good mechanical ventilation system. Most homes don't need a dehumidifier. As Dana told you, the motorized damper on your fresh air intake needs to be controlled by an AirCycler control (also called a FanCycler), and the ventilation system needs to be commissioned to be sure that the ventilation airflow meets ASHRAE 62.2 guidelines. It's easy for this type of ventilation system to be incorrectly commissioned, resulting in either overventilation or underventilation. Both extremes are undesirable.

    For more information on these issues, see these articles:

    Designing a Good Ventilation System

    All About Dehumidifiers

    Preventing Water Entry Into a Home

    -- Martin Holladay

  8. mtndrew | | #8

    How do I treat the wood burning masonry fireplace? Should I consider installing a combustion air intake at the chimney location?

  9. GBA Editor
    Martin Holladay | | #9

    Q. "How do I treat the wood burning masonry fireplace?"

    A. The answer depends on how often you use it. If you use it once or twice a year, don't worry. (You'll probably want to invest in a Chimney Balloon, however, to stop air from rising up your flue.)

    If you use the fireplace often, the next question is: Do you have backdrafting problems? If so, you may need to provide ducted outdoor combustion air -- or just crack open a window when you are lighting your fireplace.

    -- Martin Holladay

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