Crawlspace Encapsulation vs Sub-Slab Depressurization

Two different jobs

Crawlspace encapsulation covers exposed soil with a sealed membrane, primarily to control moisture. Sub-slab depressurization uses a fan to hold the space beneath a slab — or beneath a membrane — at negative pressure so soil gas is drawn out and vented above the roof. Encapsulation slows radon entry. Depressurization removes radon before it can enter. Used alone, a membrane usually does not bring an elevated home under the action level.

Side by side

How they combine in a crawlspace home

1. 1Grade and clear the soil floor.
2. 2Lay perforated suction pipe, often in a gravel bed, across the soil.
3. 3Cover the entire floor with a sealed polyethylene membrane, lapped and fastened to walls and piers.
4. 4Connect the pipe to an inline fan mounted outside conditioned space.
5. 5Exhaust above the roofline and seal the rim joist and access hatch.
6. 6Verify with a 48-hour test after 24 hours of runtime.

This combined approach is called sub-membrane depressurization. The membrane is what makes the fan effective: without it, the fan would simply pull air from the crawlspace instead of from the soil.

Homes with both a slab and a crawlspace

Mixed foundations are common where an addition was built onto an original structure. Each foundation area typically needs its own suction point, but both can usually be manifolded into a single pipe run and a single fan — provided the fan is sized for the combined resistance. Treating only one half of the house is the most common design failure in these homes.