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Community and Q&A

Design for sealing open dirt floor crawl space

ylekyote | Posted in Energy Efficiency and Durability on

Hi I have a 28′ x 30′ crawl space that has a dirt floor and CMU walls. Ceiling of it is exposed with pipes and wires running around everywhere. I live in western Colorado so it’s dry most of the year. I get some ground dampness, but no flooding, during a few months of summer when my hillside gets saturated in places from a natural spring in the rock ridge above me. I have pipes outside of the crawlspace that provide drainage and I keep the soil sloped away from the home walls. My adjacent slab walkout basement has 4″ gravel, 2″ sand, and vapor barrier under the slab. It seems to drain well also.

I have radon gas coming up from the dirt so I’m having a contractor install the basic radon system, and then planning on sealing my crawl space floor, walls, vents and doors, but leaving its ceiling plain for now.

I want to block and pump out the radon gas as well as block out moisture that sometimes has made my wood joists get damp and grow small surface mold.

This is my plan, does it should good?

1. pay radon mitigation company $1,800 to install adjacent slab and crawlspace system to direct the radon upwards to roof. Confirm it’s working with retests.

2. Install a 26′ x 36′ plastic (non organic) underlayment on top of the radon vapor barrier, terminating it at the floor/wall corners. Unknown type or best kind at this time. Suggestions? The radon vapor sheet is 4 or 5mil string reinforced plastic sheet.

3. Install single sheet of 30′ x 40′ 45mil EDPM pond liner on top of the underlayment material. Seal it above the radon vapor barrier about 12″ – 18″ up the crawl space walls with a double bead of construction adhesive (2 to 3″ apart).

4. Install foil backed 1″ to 2″ foam insulation to the crawlspace walls from 3″ below the top to where the wall meets the ground, overlapping the pond liner by 12′ – 18″. Kind and type suggested? I was going to use Rmax Thermasheath-3, 1 in. x 4 ft. x 8 ft. R-6 Polyisocyanurate Rigid Foam Insulation Board. It’s $18.50 with tax per sheet. I have about 500 SF of wall to cover.

5. Cover this insulation with foil sandwiched bubble insulation to help reflect the heat that my crawl space emits from the floor above it (which has my wood stove on it), radiator oil heater (put on a timer), or future heat duct. This piece will be adhered evenly to the foam board insulation dimensions.

6. I was then going to cut about a 8″ wide piece of 6 or 10 mil clear plastic and glue it from the insulation top edge over the CMU wall top edge and terminate it just short of the sill plate. This maybe overkill. I don’t have termites here and don’t expect them, but it can be removed if necessary for such inspections without messing with the rest of the encapsulation materials.

I’m trying to lock out as much moisture, radon, dirty air, etc from the crawl space sources outside while locking in any heat that it collects from the sun (faces south and west sun) and household heat sources. I am going to use construction glue to adhere all materials together, or to the CMU walls, which I’ll dust a couple of times before applying. Should I use glue as planned on the insulation foam board or use masonry nails or some other mechanical hanging?

The pond liner and underlayment is priced at about $1,200 delivered. The insulation will be about $600 for both kinds together. And misc materials glue and hardware about $150. So about $2,000 for materials. Am I planning anything wrong?

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  1. GBA Editor
    Martin Holladay | | #1

    The foil-faced bubble wrap is a waste of money; don't buy it.

    Here are links to two articles that should answer your questions:

    Building an Unvented Crawl Space

    Stay Away from Foil-Faced Bubble Wrap

  2. ylekyote | | #2

    Ok thank you. I'll look over those. I read articles online that declared XPS poly foam is better than EPS foam board as well as ISO board in cold and damp settings. Is this true? I was getting ready to buy ISO but now I don't know. This is for a closed crawl space and I live in western CO so it does get very cold for a couple of months each year. I was going to use the sandwiched bubble wrap as a vapor barrier on top of the foam board, whatever I use. It's not much more expensive than heavy mil plastic.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "I read articles online that declared XPS polystyrene foam is better than EPS foam board as well as polyiso board in cold and damp settings. Is this true?"

    A. Either XPS or EPS can be installed in damp locations; soil contact is fine for these types of rigid foam. (I would avoid using the cheapest, least dense varieties of EPS if I were you -- the crumbly stuff. EPS is available in different densities, and the dense types of EPS are easier to work with, have a higher R-value per inch, and perform better in a damp environment than the low-cost crumbly stuff.)

    Polyisocyanurate (polyiso) tends to absorb moisture, and should never be used in contact with soil.

    The ability of XPS and EPS to resist the flow of heat improves as the temperature drops. Polyiso, on the other hand, performs better at high temperatures than low temperatures. For that reason, polyiso is preferred by builders in hot climates. Cold-climate builders who use polyiso often assume that it will perform at R-4.5 per inch or R-5 per inch, rather than the R-value on the product label (often R-5.7 or R-6 per inch).

    Green builders avoid the use of XPS, because it is manufactured with a blowing agent that has a high global warming potential.

  4. GBA Editor
    Martin Holladay | | #4

    Polyiso will perform well if it is installed on the interior side of a crawl space wall. If I were you, I would assume that it has an R-value of about R-5 per inch in this application.

    There is no need to install a vapor barrier over rigid foam. All rigid foams are vapor retarders, and rigid foams with a foil facing (like foil-faced polyiso) are vapor barriers.

  5. ylekyote | | #5

    Another idea was given to me today by a radon contractor. He said I could lay plastic on the CS ground and adhere 12" up the walls, and then hang 6mil clear plastic on the walls, followed by plastic-faced fiberglass insulation, and then another layer of hanged 6mil plastic (to protect from dirt and possible water leaks. Is that ok to do? Is the R value for 6" of fiber glass insulation with double plastic face higher than 1" of EPS (which was what I could find and afford locally)? I thought I could hang the plastic and the insulation from the top of the wall and leave it hanging freely from the bottom half down. Securing the top half with nails and plastic washers. This would allow it to breath some and any condensation would flow out the bottom onto my floor sheet. I may have to staple the bottom of the insulation together to keep it from falling out of it's own plastic case though.

  6. GBA Editor
    Martin Holladay | | #6

    You don't want to install any fiberglass insulation in a crawl space, because fiberglass insulation is air-permeable (allowing humid air to reach the cold crawl space walls), and because fiberglass insulation becomes a moldy mess when it gets damp.

    Here is a link to an article that you should read -- an article with advice that you should follow: Building an Unvented Crawl Space.

  7. ohioandy | | #7

    Note that your plan's steps 2 and 3 are way overkill. Rather than a 5-mil poly sheet followed by 45-mil EPDM (man, that would be HEAVY to lug in there!) just go with the 15- or 20-mil fiber-reinforced product made specifically for this purpose: vapor barrier, radon catcher, and durable crawling surface all in one. Can be ordered online and shipped quite inexpensively, along with the tape used to seal edges.

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