I have a conventional 2x8 floor that is 40 inches off the ground. The crawlspace has 6"x12" vents on all sides of the house, spaced every 12-15 feet. We live in a humid area.

Can I add plastic moisture barrier under my conventional floors without further damaging wood floor joists?

  • Do you have crawl space vents? Size? Spacing?
    – Lee Sam
    Dec 25, 2021 at 23:43
  • Do you have floor insulation? Do you have a vapor barrier on the warm side of the insulation?
    – Lee Sam
    Dec 25, 2021 at 23:49
  • I do have vents on all sides of house. I don’t have insulation Dec 26, 2021 at 12:46
  • Vents are 6in x 12in spaced every 12-15 feet Dec 26, 2021 at 12:47
  • 1
    Where are you? Accepted and typical building practices vary wildly. Knowing your location could give you a better answer.
    – longneck
    Dec 28, 2021 at 23:15

1 Answer 1


Humidity is confusing. Too much can cause moisture on inside of windows, mold , etc. , too little can reduce moisture in wood and cause it to dry out and twist boards.

Here is an article that explains it:


You indicate you live in a high humidity area when you say, will adding a moisture barrier create “further damage to your wood joists”. Therefore, I’m not concerned about drying the wood out to the point it will twist and crack.

I don’t think venting is the answer. If you increase the venting, you are merely exchanging old humid air with new humid air.

A moisture barrier on the ground could trap extra moisture and create a place for bugs and mold.

I think a dehumidifier is a better choice than trapping moisture in the ground under your house, but you’ll need to monitor it closely so it doesn’t dry the joists out too much.

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