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Crawl space rehabilitation using vapor barrier, rigid foam board and cellular concrete

eaglecraft | Posted in GBA Pro Help on

Whoever coined the phrase “Do it Right the First Time,” had it correct.

I have a technical question concerning how to properly rehabilitate a crawlspace using the correct materials and procedures.

We have a 2000 square foot, single story, ranch style home built atop a vented crawl space. More correctly, one might call it a “slither space.” The home was built in 1962. While some homes in our area do have full basements, ours does not because the home was built atop discontinuous lava rock. There are places in our front yard where lava surfaces. Water and sewer lines were blasted to reach the well and septic tank.

We live in the “high desert” country (about 4,700 feet) of Idaho Falls, Idaho. We receive only about 10 inches of rainfall a year. Consequently, the crawl space is bone dry. I think that the crawlspace vents serve no purpose at all. The vents do provide unintended consequences, however. They provide an access for vermin and cold air in the winter. Mice, voles and snakes are a real issue in our area. This is a county setting. The crawl-space vents are hardly air tight when closed. Minus 20 degrees F are not uncommon during winters in Idaho Falls. The vents were placed as part of pouring the concrete foundation pony wall. Rehabilitating the vents alone would be time-consuming, expensive and not very productive, in my view.

The floor of the crawl space is covered with 6 mil poly sheeting, torn in several places. Whenever I go into the crawl space to check/repair plumbing, I come out covered in “caliche” dust typically found in our area. It’s a fun trip that I really look forward to.

All of our plumbing runs, some of our electrical runs and all of our forced air heating (natural gas) ducts run in the crawl space. I think that the only reason our water pipes haven’t frozen is due to heat losses from the duct work. The spaces between the floor joists are insulated with fiberglass. You can imagine the poor condition of the fiberglass.

I want to rehabilitate the crawlspace by removing the fiberglass insulation (torn and degraded), removing the 6mil poly, and removing the flexible heating ducting.

Then I plan to put down a rugged (resistant to tears) vapor barrier, followed by 2 inches of rigid foam board atop the vapor barrier. On top of the foam board, I want to pump in cellular concrete to a depth of about 2 inches to provide a good, solid “slither” surface. The cellular concrete is self-leveling and strong enough to easily support the weight of one man. There is no room for concrete finishing tools in the crawl space. If I left the foam boards exposed, I think that the foam would be easily damaged from sliding around on top of them.

When the concrete work is done and cured, I want to replace the flex ducting with mostly rigid ducting – I think the flex ducting runs are way too long. When the ducting is in place I want to spray foam the voids between the floor joists, the duct work, and also the inside of the concrete “pony” wall that supports the building stud walls.

I am not planning to provide any ventilation for this “sealed” crawl space. If I do, it would only be one small exhaust fan blowing to the outside air. I should add that access to the crawl space is from inside my heated garage. It has a small door that I will have to upgrade to make it “sealable.”

Please comment on my proposed upgrades my crawl space. Are there specific books, papers or building codes which address the proper specifications or products which I should use for this project? You help and guidance would be appreciated.

TC Trible
Idaho Falls, Idaho

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  1. GBA Editor
    Martin Holladay | | #1

    T.C. Trible,
    I advise you to close the vents permanently and to follow the recommendations in this article: Building an Unvented Crawl Space.

    In many areas of the country, it's hard to order cellular concrete. I don't think it's necessary. A simple rat slab made with soupy concrete is probably all you need. But if you have a source for cellular concrete, it will probably work.

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