Sub-Membrane Crawlspace Systems

Crawlspaces don't have a concrete slab to draw suction through, so they get a different approach: sub-membrane depressurization (SMD). Instead of drilling through concrete, the contractor creates the "slab" out of a heavy sealed vapor barrier laid across the entire crawlspace floor, then pulls suction from beneath it.

This page covers how SMD works, the specific vapor barrier and sealing requirements, why crawlspace systems cost more than basement systems, and how SMD interacts with crawlspace encapsulation.

A correctly installed crawlspace mitigation system has these elements:

1. Site prep. Debris removal, addressing moisture if present, clearing the floor to the dirt or coarse gravel base.
2. Vapor barrier laid across the entire crawlspace floor. Current best practice favors a heavier reinforced membrane (10–20 mil) over the older 6-mil minimum. The barrier overlaps any seams by at least 12 inches.
3. Perimeter sealing. The membrane is attached to the foundation walls or footings and sealed with mastic or specialized tape so soil gas can't bypass it at the edge.
4. Seam and penetration sealing. Lap seams between sheets are sealed. Plumbing, HVAC ducts, and any structural penetrations are sealed where they pass through the membrane.
5. Suction point drawing air from beneath the membrane, connected to a PVC pipe.
6. Pipe routing from the suction point, through the rim joist or up an exterior wall, to the fan and then to an exhaust point above the roofline.
7. Fan sized for the crawlspace area and Colorado altitude.
8. Manometer at the suction point, accessible from the crawlspace hatch or wherever the system is visible.
9. Post-mitigation test verifying indoor radon is below 4.0 pCi/L.^[1]

Older AARST standards allowed 6-mil polyethylene as the minimum vapor barrier thickness. The industry has largely moved past that:

• 6-mil tears easily when crawlspaces see foot traffic for future HVAC or plumbing work.
• 10–20 mil reinforced membranes are more puncture-resistant and seal better at the seams.
• Some installers now use 20-mil string-reinforced material as a standard option for permanent installs.

If your crawlspace mitigation quote calls for 6-mil for a permanent system, ask why. There may be a valid reason (very small space, no future foot traffic expected, cost constraint, or a very dry space) — or the contractor may be using outdated specs. The heavier barrier costs more upfront and saves headaches later.^[2]

SMD only works if the vapor barrier is genuinely sealed. Unsealed seams, gaps at the perimeter, and unsealed penetrations all let soil gas bypass the membrane and re-enter the crawlspace air above it.

Three sealing scopes a quality contractor itemizes:

1. Perimeter sealing — the membrane attached to the foundation wall or footings with butyl tape, mastic, or polyurethane sealant.
2. Lap and seam sealing — wherever two sheets of membrane meet, they overlap by at least 12 inches and are sealed at both edges.
3. Penetration sealing — plumbing pipes, HVAC ducts, structural posts, and any other element passing through the membrane is sealed with a boot or sealant.

The system inside a basement is installed in standing height with good lighting. Inside a crawlspace, the installer is often on knees, stomach, and elbows. Three conditions push the labor cost up:

• Low headroom. Anything under 30 inches makes every motion slower.
• Debris and moisture. Old insulation scraps, construction debris, and damp dirt all need clearing before the barrier goes down.
• Footing complexity. Stone, post-and-pier, or stepped foundations are detailed work to seal against.

This is most of why crawlspace mitigation costs $1,800–$4,000 in Colorado Springs vs $900–$1,900 for a basement install. Full crawlspace cost breakdown →

Crawlspace encapsulation is a moisture and air quality treatment. A vapor barrier covers the floor (and often walls), sealed and sometimes paired with a dehumidifier. The goal is dry crawlspace air.

Crawlspace radon mitigation uses similar materials — a sealed vapor barrier — but adds an active depressurization fan beneath the membrane. The goal is reducing indoor radon below 4.0 pCi/L, verified by a post-mitigation test.

Encapsulation without active depressurization may incidentally reduce radon, but it isn't designed to and isn't verified. If your crawlspace has elevated radon, the system needs to be a true SMD with a fan and a post-mitigation test — not just encapsulation.

Many Colorado contractors bundle the two services. If yours does, ask whether the bundled price includes the active fan and the post-mit test, or only the membrane and dehumidifier.