I read so many contradicting things online: Some recommend dehumidiers (if so, which?), some advice against. Some recommend fans (which, how many, how/where to install?), some advice against. Some advice so seal the vents, some advice against. And yet others recommend to seal all vents but one.

I assume the reason is it highly depends on location (climate), vented vs unvented, dirt vs non-dirt.

I am located in the Bay Area (California), so for most time of the year the humidity outside is low so vented crawl space seems to be OK (I have been told).

However, I am impacted by this atmospheric river which made water seep into the crawl space (and unfinished basements): The concrete perimeter foundation and sill plates are soaked and in the part with the unfinished basement the floor is wet (little bit of standing water). I see fungus growing.

  • Current measured humidity is 76.4%
  • Total crawl space size: ~1100 sqft
  • 4 vents (each corner)
  • Crawl space has two parts:
    • a small unfinished basement (small standing water, fungus growing on the floor, see attached picture)
    • A larger crawl space (concrete perimeter, sill plates and wood is wet throughout the perimeter, see attached picture)

What is the most effective and quick way to dry this out as soon as possible?

Crawl Space

Unfinished Basement

  • 3
    I can't imagine why anyone would advise against either fans or dehumidifiers. They're both effective if used properly.
    – isherwood
    Jan 16, 2023 at 21:20
  • How is the wood getting wet? Have you used a quantitative moisture reading probe on the sill plates? Jan 16, 2023 at 21:27
  • @isherwood As I said I think it depends on many factors, hence hard for me to know what's best. Example against fans: youtube.com/watch?v=uBMhN3KdgS8. Example against (with dirt crawl space): youtube.com/watch?v=s35Y2ktUHbk
    – divB
    Jan 16, 2023 at 23:31
  • 3
    Instead of "fastest", do the right thing. Which is installing a fan (any type, anywhere... by anyone) that will be on for the rest of the life of the house to keep air moving at all times. That white stuff didn't show up two days ago; this has been an ongoing problem. Make it not a problem anymore, ever, because even if you fix the leak, crawlspaces need to be (power) vented otherwise your house will rot out in 50y. Crawlspaces are gross. Make it not gross.
    – Mazura
    Jan 17, 2023 at 1:35
  • 1
    @Mazura This appears t be a temporary situation of unusual weather patterns, not a "rest of the life of the house" issue from what the OP has said.
    – Ecnerwal
    Jan 17, 2023 at 13:56

4 Answers 4


Fastest, without tearing the house apart:

4 fans sized to your 4 vents - two blow in, two blow out. Less fans will probably work fast enough, but 4 would fit "fastest" as requested. If you have prevailing winds, agreeing with them rather than fighting them will increase effectiveness when choosing fan directions.

Type of fan, judging by the (apparently) fairly small visible vent in one picture: In-line duct booster fan or similar. "Mixed flow axial" might be best. Depends how much you care about efficiency - you presumably won't be running them 24/7/365, just until you get the basement dry. I like the air movement .vs. power of the more efficient "radon fans" or the ones that have flooded the market to serve "indoor plant enthusiasts" which can be had with similar airflow/pressure/power ratings while not being restricted to vertical mounting as most radon fans are. But I was solving a radon issue and then looking at a heat balancing issue with long-term running in mind. The "plant ventilation" ones were something I stumbled across while looking for something else. That may be what I get for the heat balancing job, as they are like half the price without going to direct Asian import junque.

Additionally, any other fan you have handy to stir around the air inside the basement/crawlspace. If you somehow don't have other fans, they do sell "drying fans" specifically, which are a sort of plastic-cased squirrel-cage fan, typically, with an outlet right at floor level, but whether any will be in stock where you can get them is debatable. A plain old box fan or oscillating fan you probably already own will do what you actually need on that front well enough for no extra investment.

  • I agree, but would add that if you happen to have fewer fans than vents, blow them all out. That's much more effective. Most effective, though, is a fan blowing right on the moist area. There's rarely 100% humidity in the air, so evaporation will occur quickly, and it'll find its way out of the vents naturally.
    – isherwood
    Jan 16, 2023 at 21:22
  • @Ecnerwal Thanks. A friend just lent me a device like this: snipboard.io/HZ9fjT.jpg -- an "evaporative cooler" but can just function as a fan. It seems the air comes out through the round hole and is sucked in from the back (there is a filter). Would I lean the round side against one crawl space vent or vice versa?
    – divB
    Jan 16, 2023 at 23:34
  • 1
    Output to the vent, blowing outwards, yes.
    – Ecnerwal
    Jan 17, 2023 at 1:11
  • 3
    If the environment is relatively dry most of the time (as you indicated), then fans are a good solution. If you go that route, I would make a provision to turn them off when the bay area fog develops. If you're trying to dry out a crawl space, you don't want to pull in or circulate air that is at or near 100% relative humidity. That wold just contribute to the problem.
    – SteveSh
    Jan 17, 2023 at 12:33
  • "indoor plant enthusiasts" lols!
    – SiHa
    Jan 17, 2023 at 12:36

For standing/very wet surfaces fans will be the best(besides mopping/sponging/pumping).

You want a good breeze blowing in from one end and out the other.

Amount of fans will depend on area.

  • Thank you. I edited post with more details and pictures of crawl space walls and unfinished basements and area (~1100 sqft). Would you be able to add more details?
    – divB
    Jan 16, 2023 at 20:16
  • @divB One or two fans working should be enough. Will need vents/openings at each end. Oscillating fan would be helpful to move the air around.
    – crip659
    Jan 16, 2023 at 20:43

In a pinch a portable or window air conditioner may be some help. Just haul it in there and let it run so that the hot and cold side are both there in the crawlspace. It's not a purpose-built humidifier, but it could:

  • warm the air under the crawl space, increasing its capacity to carry moisture
  • if the evaporator coil can get cold enough it'll actually de-humidify the air. Leave a pan or bucket or something to catch the liquid water coming off the air conditioner before it drips back into the soil.

I wouldn't be so confident in this approach as to go out and buy a new air conditioner, but if I already owned, could easily borrow, or could inexpensively buy a used one, I'd give it a shot.

  • This is actually an amazing hack I’ll try. As a matter of fact, there are 3 old in-window Fedder ACs left by the previous owners exactly where the water is …
    – divB
    Jan 16, 2023 at 22:10
  • 1
    Just make sure the water drains are routed outside the house, or you'll be funding an infinite loop! Jan 16, 2023 at 23:19

Natural ventilation is totally preferable, if your local atmosferic conditions are favourable. Forced ventilation might help, but it won't do a magic.

Good dehumidifiers can definetely resolve the problem (or bridge period of time), if ventilation doesn't help. You want hi-performance unit, one with integrated heat pump. Some 30 to 50L/day (~10 gallons/day) would definitely help quickly. They require about 600W at full power, when you need to remove humidity agressively. They're not cheap, starting around $400 I guess, but in many places they can also be rented.

Based on your description, I assume you're in building process and you will eventually take care about the source of the problem - which should be the ultimate aim. Simply put, humid warm air gets to colder surfaces, causing condensations and this has to be prevented.

  • Pretty sure it's a fully built house and the unusual amount of rain they had recently is to blame.
    – Ecnerwal
    Jan 17, 2023 at 13:23
  • 2
    Your assumption is incorrect. The pics indicate an old house - 1900-1950s construction, I'd guess. This is not a "during construction" issue at all. Also, the San Fransisco Bay Area (of California) is usually quite dry, as the OP noted, so this is a short-term issue to be solved due to an unusual weather situation.
    – FreeMan
    Jan 17, 2023 at 13:47
  • @FreeMan He said "unfinished", that's why I thought he's in process of renovating. I didn't say "new". If there's unusual weather situation that caused the condensation, than borrowing dehumidifier would be the best option, since the same condition will prevent to deal with the problem while it remains. But I don't think this looks like one-time situation. Jan 17, 2023 at 15:09
  • 1
    @Oak_3260548 An "unfinished basement" is a basement/crawlspace with bare concrete/brick walls, with no "finishing" like drywall and flooring to make it a livable space.
    – shoover
    Jan 17, 2023 at 19:34

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