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Community and Q&A

Mitigating high indoor humidity

Travis Canfield | Posted in General Questions on

I’ve been battling indoor humidity issues for some time now and need more advice.

Here are some quick stats:

–Location: Western Washington

–1600 sq ft one story rambler built in 1987

–Two adult occupants and two indoor cats

–Main heating is supplied by a free standing propane stove that vents to the outside. Supplemental cadet wall heaters

I have an average of 65% humidity inside the house and have great difficulty getting it down. I contracted a green energy auditor in January and his solution was to replace my current loud, 80cfm bathroom fan (which vents outside) with a 100cfm quiet fan tied to a humidity switch. He posited that this would be sufficient to vent the excess moisture from the whole house. However, from what I’ve been researching about house ventilation, the air that is exiting the structure has to be replaced with incoming air (coming in through air leaks). The air outside in western WA is very high humidity (most of the winter it is high. Today it was in the 80s %). So from what I understand, running the bathroom fan is simply going to draw in moist air from outside. I’ve tested his theory to some degree by running the 80cfm fan that is already installed for hours and it makes a negligible difference in the RH in the bathroom.

As it stands, I’m stumped as to what I should do. I can see mold growing in parts of the bathroom. I’m thinking of buying a dehumdifier for the the whole house, but I want to do that as a last-ditch option.

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  1. GBA Editor
    Martin Holladay | | #1

    When the weather is cold enough to operate your heating system, operating your bathroom exhaust fan will lower your indoor humidity levels. The reason: when outdoor air is cold, it can't hold much moisture. (This is physics; for more information, see How to Use the Psychrometric Chart.)

    When the outdoor air has a relative humidity of 85%, remember that this level is relative. It means that the cold outdoor air is holding 85% of the maximum amount of moisture that cold air can hold, which isn't much moisture.

    During hot, humid weather in the summer, the situation is different. Under those conditions, operating a ventilation fan can make things worse. Fortunately for you, you don't get much hot humid weather in Washington state -- especially when compared to somewhere like Georgia or Florida. In general, you want to keep your ventilation rate as low as possible when outdoor dew points are above about 55 degrees F.

    To learn more about ways to reduce high indoor humidity levels, see this article: Preventing Water Entry Into a Home.

  2. Expert Member
    Dana Dorsett | | #2

    Contrary to popular perception, the air in the PNW is actually quite dry, with the humidity of the prevailng onshore winds moderated by condensation into the cool northern Pacific. The Pacific off the WA coast averages in the high 40sF, peaking at about 50F in summer. Even if the air mass starts out much more more humid in the Philippines or China, it gives up it's moisture to the cooler ocean on it's way, and the dew point drops, approaching that of the ocean temperature.

    As a result, in western WA nearly every hour of every day of the year one can lower the indoor humidity to less than 65%RH @ 70F with ventilation. The dew point of 70F/65% RH air is about 58F, and even during the peak summer humidity outdoor dew points for say, Seattle are in the low 50s.

    On those rare days (a handful per decade) when outdoor dew points exceed 60F it becomes a newsworthy event, covered by print & television media alike.

    [edited to add]

    During the heating season, dehumidifying with a room dehumidifier is reasonably efficient (~50% more efficient than your electric cadet heaters), since it's delivers the latent heat of vaporization recovered when converting the water vapor into liquid water as sensible heat (==warmer air coming out of the dehumidifier.)

  3. Jeremy K | | #3

    Is your vent for the stove working properly. Burning 1 gallon of propane produces just under a gallon of water if I remember correctly?

  4. Travis Canfield | | #4

    The vent for my stove is working correctly as far as I can tell. It was also checked by infrared camera back in January and didn't show any leaking.

    I've been running the vent fan more with the window cracked and it helps some, but the humidity continues to rise when I stop doing that. For instance, I had it down to about 62% in the bathroom last night when I went to sleep (after messing around with it throughout the day) and when I got up this morning it was at 70%.

  5. GBA Editor
    Martin Holladay | | #5

    My advice is to inspect your house for possible sources of moisture. More details are provided in my article, Preventing Water Entry Into a Home.

  6. Travis Canfield | | #6

    Thanks. I've done quite a bit of searching, and haven't found anything that obvious. The only thing I can think of is the vapor barrier in the dirt crawl space. Over 95% of the dirt is covered, but there is about an eight inch patch of bare dirt around the entire perimeter that they didn't extend the plastic over. Is that a critical factor? The vast majority of it is covered.

  7. GBA Editor
    Martin Holladay | | #7

    New house or old house?

    A gap in your polyethylene of 1/8 inch is not your problem.

    Your foundation walls (crawl space walls) could easily be contributing moisture to your house.

  8. Travis Canfield | | #8

    Built 1987. It's 8 inches of gap not 1/8 inch. The foundation is cinder block all the way around with air vents.

  9. Expert Member
    Dana Dorsett | | #9

    How deep are the roof overhangs?

    Any chance that leaking plumbing is dripping on top of the crawlspace vapor barrier?

  10. Brian Knight | | #10

    That adds up to quite a bit of square footage, potential ground contributions in addition to what the CMU blocks are adding. Surprised the auditor didn't address your crawl. Like most humans, they probably don't want to go down there very much.

  11. Travis Canfield | | #11

    Eave side is 30", gable ends are 20".

    I spent quite a bit of time under there over the summer dealing with rodents and never noticed any moisture having dripped down onto the ground plastic. I never ventured my arm up into the insulation, however.

  12. GBA Editor
    Martin Holladay | | #12

    Oops! I misread "eight inch" for "eighth inch." My mistake.

  13. Expert Member
    Dana Dorsett | | #13

    With 20" overhang at the rake and 30" at the eaves you should have reasonable protection from copious direct wetting of the siding dripping onto or draining into the ground vapor barrier below.

    Does the CMU foundation show any significant spalling?

  14. Travis Canfield | | #14

    The siding is dry. There is no obvious spalling on the block. The dirt is wet up to the cinder blocks due to heavy rain right now, but if I kick over a little dirt it is dry just an inch below the surface, so it's not soaking in right around the foundation.

    Where the siding meets the block there was a large gap that went up behind the siding, but I caulked all of that a couple months ago along all of the house where I could get to it.

    Should I go through all the trouble of sealing that perimeter under the house? After all that time under there with mice this summer I really don't want to go back!

  15. Expert Member
    Dana Dorsett | | #15

    The large gap between the siding and sheathing may have been built-in as an intentional "rainscreen", which allows the siding to dry more quickly & evenly (extending the lifespan of the paint), preventing capillary draw of dew or rain wetted siding into deeper layers, and giving the sheathing a better drying path. The gap may have been intentionally established by strips of thin plywood on the exterior of the sheathing at each stud. With some investigation you can probably figure out if that is the case.

    Rainscreen type construction wasn't super-common in WA in 1987, but had been an increasingly common practice in B.C. by that time. (A 10mm minimum rainscreen requirement has since been enshrined in building code north of the 49th parallel on the western slopes.)

  16. Jon R | | #16

    Leave interior doors open and you will find that even an inexpensive portable dehumidifier does a reasonable job.

    Right now, Aberdeen, WA is 60F with 85% RF. Heated to 68F that's 64% RH.

  17. Expert Member
    Dana Dorsett | | #17

    That that would be an unusually high ~60F dew point, high even for foggy-dew Aberdeen (one of the cloudiest locations in WA!) What weather station is that?

    Wunderground is currently reporting a 54F dew point: (57.3F, 91% RH)

    Seasonal averages are lower than that: this time of year, but the mid-summer average dew point for Aberdeen/Hoquiam/ Grays Harbor is about 55F, with the average daily high at peak season of 58F Scroll down to the dew point graph on this page:

  18. Patrick Stuart | | #18

    Kind of late to the party here, but I wanted to add something to the topic of vapor barriers. Working with the DOE/Ohio weatherization program, we require 6-mil polyethylene everywhere, extending at least 6" up foundation walls and on any interior protrusions (pipes, pilasters, etc.). All overlapping seams must be sealed airtight, and edges should be sealed to walls with adhesive or wood strips and mechanical fasteners. An inspection showing the edges stopped short 8" from the walls (as described) would be an automatic failure. Otherwise, it's kind of like leaving one window open 24/7 . . . compared with the square footage of wall space it's not that much, but it makes a huge difference for heating and cooling.

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