Drive through the older parts of Huntsville, such as Five Points, Blossomwood, and the mid-century neighborhoods around Lowe Mill, or out toward Gurley, New Market, and the lake-country homes near the Tennessee River, and you'll see plenty of houses sitting a couple of feet off the ground on block foundations. Those crawl space homes can pick up radon just as easily as basements, and they need a different kind of mitigation system. This page explains the options.
Why crawl spaces let radon in
A crawl space is exposed soil sitting directly under your floor. Even with foundation vents, the stack effect, warm air rising and escaping through the upper house, pulls air from the crawl space up through gaps around plumbing, ductwork, wiring, and the subfloor. Return-air ducts that run through the crawl space and leak are an especially strong pathway because the HVAC system can actively draw crawl space air into the house.
Open vents dilute some radon in mild weather, but in winter and on still days the crawl space and the rooms above can still build up gas. And in north Alabama's humid summers, open vents also bring in moisture, which is why many homeowners are already thinking about sealing or encapsulating their crawl space anyway.
The main fix: sub-membrane depressurization
The method the EPA and certified mitigators use most for crawl spaces is sub-membrane depressurization (SMD):
- Prep. Debris, old plastic, and sharp rocks are cleared from the dirt floor.
- Membrane. A heavy polyethylene sheet, much thicker than the thin plastic often left by builders, is laid over the whole soil floor. Seams overlap and are sealed, and the edges are sealed to the foundation walls and around piers.
- Suction. One or more perforated pipes or a suction mat go under the membrane, connected to a PVC riser.
- Fan and vent. The riser runs to an inline fan outside the living space, then vents above the roof, just like a slab system.
- Test. A post-mitigation test confirms the result.
Because the fan pulls from under the sealed plastic, radon is captured before it ever reaches crawl space air. SMD is generally the most effective crawl space approach and usually achieves larger reductions than ventilation alone.
Encapsulation versus a radon membrane
People use "encapsulation" to mean different things, so it's worth being precise:
- A radon membrane is a sealed barrier over the soil that a fan draws beneath. Its job is radon control.
- Full encapsulation usually means a barrier on the floor and up the walls, sealed or closed vents, and often a dehumidifier or conditioned air. Its main job is moisture and comfort control.
The two can be combined. If you're already planning to encapsulate for moisture, ask for a design that includes a sub-membrane suction port. Adding radon suction to an encapsulation job usually costs much less than doing the two separately. Be careful, though: sealing a crawl space without radon suction can sometimes increase radon in the house above, because there's no longer any outside air diluting it. Test after any crawl space sealing project.
Other crawl space options
- Crawl space depressurization. In a well-sealed crawl space, a fan can pull on the crawl space air itself. This works in some homes, but it can draw conditioned air from the house and may cause backdrafting of fuel-burning appliances, so it calls for careful evaluation.
- Natural ventilation. Keeping vents open and unblocked lowers radon somewhat, but results are inconsistent and it's rarely enough when levels are well above 4 pCi/L.
- Sealing. Sealing ducts and penetrations helps every system work better but is seldom enough on its own.
Mixed foundations
Older Huntsville houses often have more than one foundation type: a crawl space under the original house, a slab under a den or garage addition, sometimes a partial basement. Radon can come in through every part. Good installers test-map or inspect each section and may tie a membrane suction point and a slab suction point into a single fan. If your house is like this, mention it when you call. It changes the assessment.
What crawl space mitigation costs
Crawl space jobs vary more than slab jobs because the membrane is priced by area and the work is done on hands and knees in a tight space. For reference, Angi's 2026 national data puts typical active radon systems at about $805 to $1,326, while larger homes, crawl spaces that need full membranes, and combined foundations can run higher. Encapsulation for moisture control is a separate, larger project with its own pricing. These are national ranges for context only; local pros quote each house individually after an inspection.
Questions to ask about a crawl space system
- What membrane thickness will you use, and how will seams and edges be sealed?
- How many suction points do you expect, and where will the pipe exit the crawl space?
- Will the membrane hold up if a plumber or HVAC tech has to crawl across it?
- Can the plan be combined with moisture control or a dehumidifier?
- Do I need to keep the crawl space vents open or closed after the install?
- Are you NRPP or NRSB certified? Alabama has no state license, so certification is the best quality check.
Before you start
If you haven't tested yet, start with a test in the lowest lived-in room above the crawl space. See radon testing. If you've already tested high, call and describe the crawl space: height, whether it's vented, any existing plastic, and whether the HVAC runs through it. That's enough for a professional to plan an assessment.