A dirt-floor crawl space is the most direct connection between your house and the ground underneath it. Sub-membrane depressurization covers that soil and pulls the gas out before it ever reaches your living space.
If your home has an exposed dirt or gravel crawl space, there is nothing between the soil and the air inside your house. No slab, no barrier, no resistance at all. Every square foot of that floor is an open surface breathing soil gas into the space above it, and the stack effect carries it up through the floor system into the rooms you actually use.
The fix for a crawl space is a different system than the one used on a basement slab. It is called sub-membrane depressurization, or SMD, and it is the standard approach for any crawl space with exposed earth. Cost in Minnesota falls inside the same $1,500 to $3,000 range as other residential systems, but crawl space work usually lands at the upper end because covering and sealing soil by hand in a low, awkward space takes far more labor than coring a hole in a basement floor.
The short version: plastic over the dirt does almost nothing by itself. Plastic over the dirt with a fan pulling underneath it is one of the most effective radon systems there is. The membrane is not the fix — the vacuum under the membrane is. Call (218) 520-9679 to have your crawl space looked at.
A heavy polyethylene membrane is laid over every square foot of exposed soil in the crawl space. Sheets are overlapped generously at the seams and sealed together, the edges are run up and sealed to the foundation wall, and the membrane is cut and sealed tight around every pier, post, and column it meets. A perforated pipe or suction pit goes in underneath the membrane, connected to solid PVC that runs out of the crawl space to an inline fan and up to a discharge point above the roofline.
The important mental shift is this: the membrane is not really a barrier. It is a plenum. Because it is sealed at every edge, the running fan turns the entire space between the soil and the plastic into a low-pressure zone. Soil gas rising anywhere across that whole floor is captured and carried up the pipe rather than pushed into the crawl space. The system does not have to be perfectly airtight to work, but the better the seal, the wider the vacuum spreads and the less the fan has to fight.
That is the same pressure-reversal principle behind the sub-slab systems on our radon mitigation page. The only real difference is that a slab is already the membrane; in a crawl space, we have to build one.
Plenty of Duluth crawl spaces already have a sheet of plastic thrown over the dirt, often loose, often torn, rarely sealed at the walls. That is a passive vapor barrier. It slows moisture and it slows radon a little, but it does not stop soil gas, and it cannot. Without a fan there is no pressure difference, so gas simply accumulates under the plastic until it finds a way out. Every tear, every unsealed seam, every gap at a pier becomes a concentrated exit point — sometimes worse than no barrier at all, because the gas is now funneled through a few openings instead of diffusing evenly.
Passive sealing has the same limitation everywhere in radon work. Sealing cracks helps a system perform better. Sealing by itself is not a system.
| Approach | Radon reduction | Moisture control | Relative cost |
|---|---|---|---|
| Bare dirt or gravel crawl space | None — full soil exposure | None; damp soil evaporates into the house | $0 |
| Loose or partially sealed vapor barrier | Minimal; gas escapes at tears and edges | Moderate if fully covered | Low |
| Sealed membrane + suction point + fan (SMD) | Large and verifiable; usually well under 2 pCi/L | Strong; ground moisture is captured and vented | Upper end of $1,500–$3,000 |
A large share of older Duluth homes do not have one foundation, they have two or three. A poured or block basement under the original structure, a crawl space under a rear addition or a kitchen wing, sometimes a third slab under a later porch enclosure. This is extremely common in the pre-1950 housing across Central Hillside, Lincoln Park, West Duluth, and the older parts of Lakeside.
Mixed foundations need a combined system. That usually means a suction point under the basement slab and a sub-membrane suction point in the crawl space, with both runs of pipe tied into a single fan and a single vent stack. Treating only the basement is one of the most common mistakes we see. The crawl space keeps feeding soil gas into the house through the shared floor and wall assemblies, the post-mitigation number never drops the way it should, and the homeowner is told the house is just "difficult."
Diagnostic pressure testing before the design is settled tells you whether one fan can serve both areas or whether the crawl space needs its own. Skipping that step is how a system ends up undersized.
This city gives crawl space work its own set of complications. Pre-1950 housing stock means fieldstone and hollow block foundation walls that are irregular, porous, and hard to seal a membrane against. Hillside homes frequently have crawl spaces cut directly into rock, so the "soil" floor is a mix of fractured gabbro, rubble, and fill — and fractured bedrock near the surface is an efficient pathway for soil gas to travel from a long distance away.
Headroom is often 24 to 36 inches, sometimes with a hatch access the size of a kitchen cabinet door. Every roll of membrane, every length of pipe, and every tool goes in through that opening and gets worked by hand while lying down. And because a Duluth crawl space is usually unheated and only partly insulated, it is often already a moisture problem and a freezing-pipe problem before radon ever enters the conversation.
A properly installed SMD membrane does double duty. Sealing the soil and venting the space beneath the plastic cuts ground moisture evaporation substantially, which usually means less mustiness in the house above, less humidity feeding wood rot and mold in the floor joists, and warmer floors in the rooms over the crawl space.
That said, be clear about scope. Full encapsulation — insulating the foundation walls, air-sealing the rim, and adding conditioned air or a dedicated dehumidifier — is a larger project than radon mitigation, with its own cost and its own set of decisions about how the crawl space is meant to perform. SMD is a radon system that helps with moisture. It is not a substitute for encapsulation, and anyone selling you the two as the same thing is blurring the line. Decide on radon first, based on a measurement.
Crawl space systems sit high in the Minnesota range for reasons that are all about labor, not materials. Tight headroom slows everything down. Standing water or saturated soil may require a sump or drainage work before a membrane can go down. Debris removal adds hours. A large crawl space footprint means more membrane and more seams. Irregular fieldstone piers and rough rock walls take longer to seal properly than clean poured concrete. A mixed-foundation home needs a second suction point and a longer pipe run.
A full-day install is normal for crawl space work, and larger or worse-access spaces run longer. Any quote you take seriously should specify membrane thickness, how many suction points, where the fan goes, and whether a confirmation test is included. Send us the details of your crawl space and we will tell you what to expect before anyone shows up.
Minnesota requires a license. Anyone performing radon testing or mitigation in a building they do not own or lease must be licensed by the Minnesota Department of Health. Industry certification alone is not sufficient in this state. Ask for the MDH license number before work begins.
Crawl space systems fall in the same $1,500 to $3,000 Minnesota range as other residential mitigation work, but they typically land at the upper end. The reason is labor, not materials. Covering and sealing soil by hand in a space with 24 to 36 inches of headroom takes far longer than coring a hole in a basement slab. Cost climbs further with a large crawl footprint, standing water, heavy debris, irregular fieldstone piers, or a mixed foundation that needs a second suction point in the basement.
No. A passive vapor barrier with no fan slows soil gas down but does not stop it. Gas accumulates under the plastic and escapes through any tear, unsealed seam, or gap at a pier or wall edge, which can concentrate it at a few points rather than reducing it. The membrane only works as radon control when a fan is pulling a vacuum beneath it, turning the sealed space between the soil and the plastic into a plenum that captures gas across the entire floor.
Six-mil polyethylene is the practical minimum. If anyone will ever crawl over the membrane to reach ductwork, plumbing, or storage, 10 to 20 mil reinforced material is the better choice. Thin sheeting punctures easily on rock and rubble, and in a Duluth crawl space cut into fractured bedrock it can be shredded within a few years. Every puncture is a leak in the plenum, so thickness directly affects how long the system keeps performing without repair.
Usually one system with two suction points. A sub-slab point under the basement and a sub-membrane point in the crawl space are tied into a single pipe run and a single fan. Mitigating only the basement is a common and expensive mistake, because the untreated crawl space keeps feeding soil gas into the house through shared floor and wall assemblies. Diagnostic pressure testing during the design determines whether one fan can serve both areas or whether the crawl space needs its own.
It helps considerably. Sealing the soil and venting the space beneath the membrane cuts ground moisture evaporation, which usually reduces mustiness in the house above, lowers the humidity that feeds wood rot in the floor framing, and makes the floors over the crawl space feel warmer. It is not the same as full encapsulation, though. Insulated foundation walls, air sealing, and conditioned or dehumidified air are a separate and larger project with their own cost, and that decision should be made on its own merits.
Plan on a full day. Basement sub-slab systems are often finished in four to six hours, but crawl space work involves clearing debris, excavating a suction pit by hand, laying and sealing membrane across the entire soil surface, and sealing around every pier before the pipe and fan go in. Low headroom, standing water, or a large footprint can stretch it beyond one day. After the fan is running, wait at least 24 hours before starting the confirmation test.
At least 24 hours after the fan starts running, so the crawl space and the house have time to reach a new steady state. Testing sooner captures a system that is still drawing down and gives you a number you cannot rely on. The test itself typically runs 48 hours under closed-house conditions, and the result should be provided in writing. A mitigation system that has never been verified with a measurement is just a fan on a pipe, and no reputable quote should leave that step out.
Send us the basics — headroom, whether the floor is dirt or gravel, and whether the house also has a basement — and we will tell you what a sub-membrane system would take.