What is the rule of thumb for crawlspace vents?

1cs1267
by 1cs1267

My house was built 30 years ago on a crawlspace. The crawl space has about a dozen vents in the brick foundation. I live in central Arkansas, and it is very humid in our state. I have had conflicting advice on leaving the vents open vs. close. What is the rule of thumb on this matter? We have a tendency to get some standing water under the house during certain rainy spells.


  9 answers
  • Kathy Gunter Law Kathy Gunter Law on Jan 27, 2020

    You should have 1 square foot of vent for every 500 square feet of crawl space. You might want to invest in a dehumidifier or a small pump. If they are the kind of vents that can be closed, perhaps close them when you are expecting rain - especially blowing rain. However, I couldn't do that because I'm not getting in a crawl space. I hyperventilate. But if you are able to go under there, perhaps that's a working solution.

  • Annie Annie on Jan 27, 2020

    Can you call your local building department and ask them? They know your area and what would be best.


    I wouldn't be closing the vents if you have a water problem down there, but that's just me


    Do your neighbours have standing water under their houses?

  • Sharon Sharon on Jan 27, 2020

    I have a 1940s house with crawlspace and quite a few vents, I close my vents off in the dead of winter, and then take off the covers in the spring. I have a rubber liner on the floor of the crawlspace as many in Oregon do due to our wet climate.

    If you have standing water, you may want to get a few clip on fans to put in the vents to get the air to circulate to dry it out and prevent odors.

  • Laura Cooper Laura Cooper on Jan 27, 2020

    I have vents that automatically open and close based on the temperature of the crawl space. They are about $25 each and don't require electricity.

  • 1cs1267 1cs1267 on Jan 27, 2020

    Where did you purchase them? Or, at least, what is the brand?

  • Lifestyles Homes Lifestyles Homes on Jan 27, 2020

    Since you have a history of standing water in your Crawlspace, this is unhealthy for you & your home.

    Rule Of Thumb is not enough in this case. Negative Air Pressure is what is needed.


    I have two of these Tjerlund Crawlspace fans that have adjustable settings for both humidity & temperature.


    Since I only had one exterior vent and my calcs dictated a second fan, I had a hole bored in my rim joists for the second one.


    I set mine to run 24/7 3 years ago and they’re working like champs.

    3 amps total, for both of them.

  • William William on Jan 28, 2020

    These vents allow outside air to circulate under the floor in summer to prevent the moisture buildup that encourages mildew and rot. In winter, when the air is drier, the vents are closed to reduce the chance that the pipes in the crawl space might freeze.

  • Agnes Chrzanowska Agnes Chrzanowska on Jan 31, 2020

    I would ask professionals about it for sure

  • You might check with your town's building department or a structural engineer, they'll best be able to guide you with fact based best construction practices.