Crawlspaces: Sub-Membrane Depressurization
A crawlspace with exposed dirt or gravel is essentially an open window to the soil — and the fix is to build the "slab" the house never had, out of a sealed membrane. It's called sub-membrane depressurization, and it's the crawlspace equivalent of the standard sub-slab system.
Heavy-duty plastic sheeting is laid across the entire crawlspace floor, sealed at every seam, and sealed to the foundation walls and support piers. Perforated pipe or a suction point goes underneath the membrane, connected to the same kind of inline fan used in a sub-slab system — mounted outside the living space, vented above the roofline, with a manometer you can read. The fan holds gentle vacuum under the membrane, so soil gas is captured at the dirt and exhausted before it ever reaches your crawlspace air, which matters because crawlspace air doesn't stay in the crawlspace — it gets pulled up into the house by the same stack effect that drives radon entry everywhere else.
Done thoroughly, this is full encapsulation, and the side benefits are real: less moisture, less musty smell, a cleaner space under your floor. It's also more labor than drilling a slab, which shows up in the price — crawlspace sub-membrane and encapsulation work runs $2,000–$5,000+, compared with roughly $1,200–$3,000 for a standard Colorado sub-slab system. The spread depends on crawlspace size, access, and how much prep the space needs before a membrane can seal. Details in the cost guide.
Walkout Basements and Hillside Homes
Walkouts are the valley's signature foundation — and a radon-design puzzle, because the soil pressure under the house isn't uniform. On a hillside lot, the uphill side of the basement may sit eight feet below grade while the downhill walkout wall sits at grade. Below-grade depth drives soil-gas pressure, so one end of the same slab experiences very different conditions than the other. A single suction point placed by habit — instead of by measurement — can depressurize the shallow end beautifully while the deep uphill corner keeps feeding radon into the house.
This is why we run a sub-slab communication test before quoting: applying vacuum at a candidate point and measuring how far it actually reaches through the sub-slab material. In the rocky, variable fill under mountain homes, communication distances can be short, and hillside slabs often have interior footings that split the sub-slab space into separate zones. The test tells us whether your walkout needs one suction point or three — before the drill comes out, not after a failed retest.
Mixed Foundations: Basement + Crawlspace + Slab in One House
If your house has a basement under the original structure, a crawlspace under the addition, and slab-on-grade under the garage or great room, that's not unusual here — it's the default. Decades of remodels and hillside-stepped construction leave many valley homes with two or three foundation types in a single footprint, each of which is a separate radon pathway needing its own treatment: sub-slab suction for the basement, sub-membrane for the crawlspace, its own suction point for the slab section. Treating only the basement is the classic mistake — the retest comes back barely improved, because the crawlspace next to it was left wide open to the soil.