My house was built in 1950, and in my crawlspace, I have vents covered with mesh wire that circulate air to the outside.

I've been reading online that its now best practice to seal the vents in the winter, and open them up in the summer, but I wanted to get peoples feedback on this, given the environment my house resides in.

I live in a typical Northern California climate that experiences moderate weather all year around. In the winter, occasional rain storms, with the temperature approaching, but rarely hitting freezing, and in the summer, temperatures between 80-90, rarely hitting the 100s.

Throughout the year, unless its raining or foggy, humidity is never very high.

In addition, I have a sump pump in my crawlspace, as I experience a good rush of water from the hill I'm on during a downpour. The crawlspace is rat-proofed, and over time, I do need to epoxy paint over the cracks that develop that leak water to the unfinished floor (its just epoxyed-over concrete) at the end of each rainy season. This water that may escape through the cracks that develop finds its way to a drain in the concrete floor that drains to the same reservoir that the sump-pump feeds off.

Given these conditions, is it advised to regularly open/close the vents, seasonally or otherwise, for insulation or other reasons?

  • Sorry I can't answer your questions, but I do wonder, how did you "rat-proof" your crawlspace. Secondly, where in NorCal do you reside in and is there insulation in your crawlspace?
    – josephnvu
    Jan 15, 2018 at 9:03
  • Sealed at the foundation with epoxy-coated concrete floor. Or at least thats what they tell me :)
    – Geremy
    Jan 15, 2018 at 23:49

2 Answers 2


I grew up in Sonoma county, we would close our vents about thanksgiving and open them in March. Other than those few months it rarely got cold enough to freeze pipes but we were closer to the coast (Sebastopol). If they are closed year round things under the house will rot including the wood most homes are made of.

  • Thanks Ed. What agents are responsible for the rot if closed year-round? Hot outside, cool inside, cool inside, hot outside? Condensation forming inside because its warmer outside, etc?
    – Geremy
    Jan 15, 2018 at 23:48
  • The ground is damp even under your home, both moisture and gasses trying to escape are trapped causing higher humidity , high humidity wood and a warm floor are the mildew conditions dreamed about.
    – Ed Beal
    Jan 16, 2018 at 0:05

If you seal the crawl space the humidity can rot wood and cause corrosion of metals. I had a similar house in northern IL, I guess it may have had more humidity. When I bought it in the 70's I added vents because the joists were black in places. There was also corrosion in the galvanized electric boxes in the house. My crawl space "floor" was just soil.

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