Crawlspace Radon Mitigation: How Sub-Membrane Depressurization Works

Why Crawlspaces Need a Different Approach

You cannot core a suction pit through a dirt floor, so crawlspaces require a different method called sub-membrane depressurization. This technique creates a physical barrier to trap and vent the soil gas safely outdoors.

Our crawlspace radon systems are designed precisely to solve this unique structural challenge.

A lot of homes across the US feature exposed dirt crawlspaces rather than full basement slabs. Radon gas moves upward through this unsealed earth and enters your living space quite easily.

We follow current EPA guidelines that advise taking immediate action when tests show levels at or above 4.0 pCi/L. Recent 2026 industry data confirms the effectiveness of this approach for several reasons:

• Direct Source Control: The membrane traps gas before it reaches the living space.
• Measurable Results: Properly sealed systems often reduce radon levels by 50 to 99 percent.
• Permanent Fix: A professional installation offers lasting protection for your family.

How Sub-Membrane Depressurization Works

The idea is identical to a slab system, creating a slight vacuum under a sealed surface so soil gas flows into a pipe instead of your home. In a crawlspace, the sealed surface is a heavy plastic membrane.

We use an approach called active soil depressurization to create this vacuum effect. A specialized fan runs continuously to pull the gas from beneath the plastic sheet. This pressure difference ensures the dangerous air vents safely above your roofline. Our installation process follows strict ANSI and AARST national standards for safety and effectiveness.

Step 1: Prepare the crawlspace

Preparation involves removing sharp rocks, debris, and old materials that could puncture the new plastic. You need a smooth surface to ensure the system remains airtight.

We clear the area thoroughly before laying down any new material. If there is old or torn plastic on the floor, the crew either removes it or leaves it as a protective underlayer. Taking the time to prep the floor prevents future tears and keeps the vacuum strong.

Step 2: Lay the membrane

Installers lay a heavy, cross-laminated polyethylene plastic sheet over the entire soil floor to trap the gas. Standard residential building codes generally recommend materials between 12-mil and 20-mil thickness for proper durability.

Our team prefers these thicker reinforced materials because they provide several distinct advantages over basic 6-mil plastic:

• Puncture Resistance: Thick plastic withstands crawling and equipment weight.
• Moisture Blocking: Heavier materials act as a superior vapor barrier.
• Longevity: A robust 20-mil sheet lasts for decades without degrading.

The seams must overlap by several inches to maintain a tight boundary. Installers then use specialized polyurethane adhesives or construction tape to lock every seam together permanently.

Step 3: Seal edges, walls and piers

The membrane must run up the foundation walls and seal directly to the concrete to stop edge leaks. Edges are the most common failure point for DIY projects, so this step takes professional care.

We use mechanical fasteners and heavy-duty adhesives to bond the plastic to the walls. The plastic is also wrapped and sealed around support piers, plumbing stacks, and any other pipes that come through the floor. Creating a perfect seal guarantees the suction fan can do its job efficiently.

Step 4: Install the suction point

A perforated PVC pipe goes directly under the sealed membrane to act as the primary suction point. This pipe connects directly to the exterior fan system.

Our technicians strategically place these collection fittings to maximize the vacuum effect across the entire floor. The PVC piping then routes the trapped soil gas out of the foundation area.

Step 5: Route the pipe and mount the fan

The vent pipe runs out of the crawlspace and connects to a continuous-duty radon fan mounted outside the living space. This fan safely discharges the gas high above the roofline.

We typically install a 90-watt fan capable of moving roughly 100 cubic feet of air per minute. A U-tube manometer installed on the pipe provides a visual indicator that the system is pulling suction. You can easily check this gauge anytime to confirm the equipment is working.

Key point

The membrane isn’t the fix by itself. The suction under it does the work. A vapor barrier alone usually leaks enough radon around seams and edges to leave levels high.

Mixed Basement and Crawlspace Homes

Homes with a basement under one part and a crawlspace under another require a combined mitigation strategy. Testing the basement might show a high number, but the crawlspace often serves as the main entry point for the gas.

Our designs for these mixed foundations address both areas simultaneously to guarantee safety. The 2026 US housing data shows these split foundations are very common in homes built with later additions. Treating only one section usually leaves the home unprotected.

For these homes, the design usually includes:

• A suction point drilled under the main basement slab (standard sub-slab depressurization)
• A sealed membrane in the crawlspace with its own dedicated suction point
• Pipe runs from both areas joined into one fan when distance allows
• Sealing any openings between the crawlspace and the basement

Access and Storage After Mitigation

You can still use your crawlspace for storage or plumbing access, provided you take a few simple precautions. The mitigation system is durable, but the plastic membrane does require some basic care to stay airtight.

We recommend following these specific guidelines to protect your investment. A small puncture can drop the system pressure and allow gas to leak back in.

• Choose a durable membrane. Thicker 20-mil cross-laminated material holds up better to crawling and heavy storage boxes.
• Plan a clear path. A walkway of extra membrane or rubber matting where people move most often prevents wear and tear.
• Keep sharp items off the floor. Do not drag metal toolboxes or heavy equipment across the plastic.
• Report any damage immediately. A tear lets air leak in and weakens the suction, but patches are simple if you catch them early.
• Keep the access cover closed. The door is a crucial part of keeping the space completely sealed.

Moisture Benefits

A sealed membrane cuts down on ground moisture rising into the crawlspace, which significantly lowers indoor humidity. While radon reduction is the primary goal, this moisture control is a massive secondary benefit.

Our clients frequently report less dampness and fewer musty odors shortly after installation. By blocking the damp soil, the heavy plastic provides excellent protection for your foundation:

• Prevents Mold Growth: Dry air stops mold spores from taking hold on wood surfaces.
• Protects Wood Joists: Lower humidity prevents wood rot and structural decay.
• Discourages Pests: Insects and rodents avoid dry, sealed environments.

You gain a drier, healthier foundation as a direct result of the mitigation process.

Testing a Crawlspace Home

If your home lacks a basement, you must place the radon test on the lowest livable level to get an accurate reading. You should never place the test device in the crawlspace itself.

We advise following the EPA protocol, which means placing the kit on the main floor directly above the crawlspace. The detailed guide on testing homes without basements explains exact placement rules, and the test must run under closed-house conditions to provide a valid baseline number.

After mitigation, a post-installation test confirms your new indoor level. This follow-up test proves the sub-membrane depressurization is working correctly.

Our company adjusts the system under a standard 5-year warranty if the levels do not drop below the target threshold. Getting a clean final test provides complete peace of mind.

Getting a Price

Crawlspace jobs vary significantly by size, ceiling height, and current soil conditions, so professional quotes require specific details. National averages for a crawlspace mitigation project in 2026 range from $1,500 to $3,000, depending on the complexity of the encapsulation.

We provide a flat-rate price through a free virtual estimate before anyone even has to crawl under your house. Clear photos make a huge difference in planning the pipe route and calculating material costs.

You simply need to send pictures of the access opening, the foundation walls, and the outside of the house, along with your test result. Submit your photos today to receive a free custom design plan and secure a healthier home environment.