Closed Crawlspaces

Hey all. I'm putting this out there to get your opinions on closed crawlspaces. What to do? What not to do?

My current plan follows Design Sample 2 from Advance Energy.

It is designed for the south-west, which is where I live. The crawl will be slightly sloped to one corner, where water can drain out if/when needed (capped when not needed). A small supply-side leak will provide a bit of pressure to the whole space. Walls insulated, good 10mm plastic vapor barrier. 

Your opinions are greatly appreciated!


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  • Check out Joe Lstiburek's Building Science Insight: 

    BSI-115: Crawlspaces - Either In or Out

    BSI-115: Crawlspaces - Either In or Out
    • Thanks. Interesting. Seems to consider balanced flow a lot but I've always considered it useful to have a positive pressure in the closed crawl. I'll have to look more into that.

  • Scott

    A number of years ago, David Hales (noted building science guy in the NW) took a long look at this subject. I hope you will find this document useful:



  • A worthy topic!

    Here are some observations on my DIY retrofit, based upon absolutely no actual science.

    For reasons that could not be satisfactorily explained, the builder of my tract home insisted on a 6" supply duct, from the attic to the crawl space. The duct was discovered during an unsuccessful search for a way to drop a TV cable inside the home. I confirmed air flow by having my patient wife open and close the access hatch and with the central fan running, air motion was obvious when the hatch was open.

    A figurative light bulb came on over my head and after closing and sealing all crawlspace vents, checking foundation wall insulation for soundness, installing a vent in the access hatch and rearranging the somewhat crumpled vapor barrier, I now am the proud owner of a considerably warmer floor and I presume, lower heat loss from through that area.

    We are in the high, dry desert of Northern Nevada so moisture is not an issue.

    While this application, as so professionally presented above, is an important addition to the new-construction toolbox, application to the massive existing home population also deserves consideration.

    Now, how do we easily get that pesky supply duct into the crawlspace?

  • I build raised conditioned floors in the southwest and cut the pad, place 4-6" of gravel with radon venting then R10 EPS. I then place a 20 mill vapor barrier across the whole surface with a mixed soil cement like dry pack to protect the surfaces this gives a excellent work surface. Water concerns are driven by the water table with wells at 150-450 ft water isn't the issue now that said on river bottom land and valleys investigate water table depth prior to build.Common sense and thought to what you build remember wet to dry is how to build. The crawl space is linked to living space and stays within 2 degrees of living space.

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