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Community and Q&A

Neutral pressure plane?

AlanB4 | Posted in Energy Efficiency and Durability on

Background, my house has about 15ACH50, much of it coming from a very shallow crawlspace i cannot access (never mind fix, i hope to dig it out enough to access the entire perimeter of the house and encapsulate it someday)

My hallway storm door gets covered with humidity very quickly (less then 30 seconds) when i open the foam core door, and this is rather unique to this house, i could never explain why but this doesn’t happen in about a dozen other places i have lived in the area. When i found out about the massive air leakage last year i assumed that even though the storm door is rather airtight (during the blower door test) perhaps its a kind of stack effect going on.

Another door at the same level has a bad wood frame, i’ve had to tear out parts of it and am working on repairing it (its a crummy old wood door i would like to replace someday. With parts of the trim off its storm door is dripping water. From opening windows at this level i assumed the lack of airflow except during wind is because of the neutral pressure plane that has been mentioned, but could it instead be caused by exfiltration?

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  1. GBA Editor
    Martin Holladay | | #1

    If you want to determine the cause of these problems, the next step is to buy a hygrometer and measure you indoor relative humidity. My guess is that your inaccessible crawl space is introducing large amounts of moisture into your home.

  2. AlanB4 | | #2

    The basement is at 40% and the main floor 55%, the door to the basement i usually keep closed to reduce dehumidifier cost in the summer (though its nowhere near airtight from basement to first floor, single story house). Currently 21ºC indoors and 1ºC outdoors

  3. GBA Editor
    Martin Holladay | | #3

    So your main problem is high indoor humidity. The most likely sources of moisture are your basement and crawl space. Other possibilities are: the use of a humidifier (using a humidifier is always risky); storing firewood indoors; unusual hobbies like raising tropical fish or lots of houseplants; or a family that takes frequent showers without remembering to turn on the bathroom exhaust fan.

    If the problem is your basement, you'll probably want to read this article: Fixing a Wet Basement.

  4. Dana1 | | #4

    It fogs up wheh you open the insulated door because A: it's cold and B: it's air-tight, with little outdoor air dilution.

  5. AlanB4 | | #5

    @ Martin When its warm out the basement hits 100% humidity very fast, 700sqft basement, the dehumidifier was taking out 20 pints a day on average. I actually like the humidity now, before the high efficiency furnace 20-30% humidity all winter was what i had, now it only falls below 30% below -20ºC.
    I don't use a humidifier, no firewood or unusual water source hobbies.
    My house is a 2x4 loose fill cellulose century old home. I have actually wondered when the vinyl siding wears out (its a decade old now) how i should insulate it on the exterior, would 2in EPS R10 foam be dangerous considering the old plaster and lath probably has half a dozen layers of leaded oil and latex paint on it? The wall stackup is interior paint layers, plaster, lath, plaster again, 1/1/2 inch barn board, true 2x4, wood siding (no sheathing), fabric blanket and vinyl siding. I would propose removing the vinyl siding, blanket and wood siding add sheathing Tyvek, foam and new siding on top.

    @ Dana but this house is unique that the condensation develops so quickly, friends and other houses i have lived in do not develop so fast (the condensation on the storm door past the foam core door, i am less surprised at the dripping condensation for the trim removed storm door. I suppose when i fix the crawlspace i will see if the fast condensation problem is affected.

  6. GBA Editor
    Martin Holladay | | #6

    You've explained to us that your house has high indoor humidity levels -- so high that cold surfaces in contact with indoor air "get covered with humidity very quickly," and that these surfaces are "dripping water."

    You also say, "I like the humidity now."

    OK -- so you're happy. It's your house.

  7. AlanB4 | | #7

    I meant i like the humidity indoors (at 20-25% my hands were cracking from dryness in previous winters, not fun), but i don't like it dripping on storm doors when i open the regular door or the summertime basement humidity.

    What normal humidity level should be aiming for since mine is very high?

  8. GBA Editor
    Martin Holladay | | #8

    Your indoor RH is at 55%. Most experts recommend that you aim for about 30% indoor RH during the winter. Some people prefer to let the indoor RH creep up to 40%. But 55% is much too high.

    But -- as I said -- it's your house. The dripping storm door is a tell-tale sign. You may have condensation or moisture accumulation problems in hidden parts of your wall assemblies. That's one of the risks you have to accept once you get up to 55% indoor RH.

  9. AlanB4 | | #9

    I can live with 40%, in fact it will drop on its own as we get below freezing, we have had 5-10C for the last month or so, well above seasonal.

  10. Expert Member
    Dana Dorsett | | #10

    At 55% RH you're above the human-healthy range for folks with dust-mite allergies. Most of the medical community advice is to keep it between 30%-50%.

    For the wintertime keeping it on the low end of that range is better, for controlling condensation issues or moisture build-up in susceptible building materials.

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