crawlspace venting... 11 gallons a day?

Here in Colorado, a pretty dry climate, they say on the average 11 gallons a day evaporates out of the soil and into the crawlspace. If the crawl is vented, much goes out, the rest must either be absorbed into the surrounding materials, leave the crawl by diffusion through the walls and floor or be carried through the walls and floor by air movement through cracks.
In a tight, unvented crawlspace, much of that moisture must be absorbed by surrounding material. Is it really possible for joists, subfloor etc. to absorb that much water contiuously without reaching the level at which decay fungi become active?


The 11 gallons (where ever that came from) does not apply to a closed CS.

If the CS is not vented, it will reach a saturation point (Relative Humidity of 100%), at which point no more moisture will evaporate into the air.

The absorption, diffusion rate you discuss will continue and an amount commensurate with it’s rate will continue to draw moisture from the space.

That’s why ground vapor barriers are so important. :wink:

That is why I use this product Larry.

Marcel :slight_smile: :slight_smile:

Cool, nice stuff. Thanks for the info.:wink:

“Click to Enlarge”

The Building Science site is down right now but some of these other links are informative:,HPIB:2006-13,HPIB:en&q=unvented+crawl+spaces

When installed correctly I can’t imagine why builders would do it any other way with today’s sealed structures.

Joe Lstibureck from the Building Science Corp. is my hero.
I was trying to come up with an article on how to handle your crawlspace for a local newsletter and in researching and writing the article it became apparent that you can’t really offer information on how to configure a crawlspace that will work across all the climates and site conditions. The best you can do is to try to educate people on how moisture and temperature can effect their home. Building science, made as simple as possible.

I can’t remember were I got the 11 gallons a day number, but of course evaporation rates would depend on crawlspace humidity and temperature. Seems like it was from a pretty good source.