I live in the San Francisco Bay Area. Low humidity and mild climate year round.

I don't have any problem with mold or humidity in my vented crawlspace, but it is musty and I want to prevent that musty air from getting into the house.

What's the best way to determine how much I need to do? I have looked at a lot of videos and web sites, but I'm left with some questions, like:

  • should I block the vents?
  • do I need a humidifier? guessing not with low humidity in this area, but I don't know if encapsulation might change that

I don't want to create a problem.

adding some details:

  • there is fog, and it can be very wet, but I think the amount of moisture we get crawlspace-wise is minimal

  • drainage around the house is really good in terms of grading and downspouts

  • 55yr old house and I've lived in it for 20 years and the only time it gets a little too wet is when there are heavy rains for a week, and even then, there aren't puddles, just more moist, and then it drys out pretty good

  • we definitely get a fair amount of wind

  • there is absolutely no mold as configured

I think the crawlspace is probably fine as-is, except when the air pressure is such that the crawlspace air is sucked into the house, which is the only reason that I want to do this.

  • What about those times when the bay area is shrouded in fog? Are you affected by that condition?
    – SteveSh
    Jun 17, 2021 at 21:20
  • 1
    And, it all comes down (at least in my mind), to what the dew point temperature is and what's the temperature inside your crawlspace. Anytime the temperature in your crawlspace is lower than the outside dewpoint temperature, you run the risk of moisture condensing on surfaces in your crawlspace. Whether that moisture has a chance to dry out depends on how often that happens and how "dry" your low humid air really is.
    – SteveSh
    Jun 17, 2021 at 21:25
  • @SteveSh I think that's a good point. I feel that for 55 years the house hasn't had a mold or ventilation problem so best to keep vents unblocked and just go with 20 mil plastic over the dirt and monitor for condensation periodically. I wonder if there is a feasible insulation layer that could be put under the plastic to prevent the plastic from getting cold. It's way colder down there even on a really hot day.
    – piton
    Jun 18, 2021 at 17:33
  • 1
    The vapor barrier on the dirt floor is a good thing to do no matter what else you decide to do (or not do). Anything that can reduce the amount of moisture in your crawlspace is a good thing. I've never heard anyone complain about a crawlspace being too dry <wink>.
    – SteveSh
    Jun 18, 2021 at 18:31

2 Answers 2


If you add a small exhaust fan, maybe on a timer, then the air in the crawlspace will be at a lower pressure than air in the house. Air will only migrate from the house to the crawlspace, not the other way.

  • For that to be effective, you have to seal all other outside openings.
    – SteveSh
    Jun 17, 2021 at 22:44

Crawl space ventilation will solve those problems unless, 1) not enough ventilation, and/or 2) moisture in the crawl space.

  1. When I think of SF, I think of year round wind. Unless your building is blocked from the wind, you probably have plenty of ventilation openings.

The Code requires “The minimum net area of ventilation openings shall be not less than 1 square foot for each 150 square feet of crawl space AND one opening shall be within 3’ of each corner.” (See ICC R408.1)

  1. Likewise, the code requires the crawl space “finished grade” shall be higher than the outside surrounding grade, if there’s evidence that the area is prone to be wet or flooding. (See ICC R408.5 and R408.6)

I’d check downspouts, finish grades, etc. to make sure you’re not contributing to a wet environment. In your area I’m sure you have plenty of wind. Id just check to make sure all vents are not blocked.

  • +1 to grading/downspouts etc. (And fyi, the Bay Area is a bizarre clutter of microclimates -- wind isn't a given.) Jun 18, 2021 at 0:02

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