Crawl space and mixed-foundation radon
Plenty of Helena houses sit on more than one kind of foundation. Each part that touches the soil may need its own piece of the system.
Helena's housing runs from 1870s and 1880s brick and stone buildings to new construction in the valley. A single house might have a basement under the original structure, a crawl space under a later addition and a slab-on-grade family room. Radon can enter through any of those, so a radon company looks at each section separately.
How is a crawl space handled?
The standard approach is sub-membrane depressurization. A heavy plastic or rubber sheet is laid over the dirt, overlapped and sealed at the seams, and sealed to the foundation walls and piers. A pipe passes through the membrane, and a fan draws soil gas from underneath it and vents it above the roof. It works like a sub-slab system, with the membrane standing in for the concrete.
What about slab additions and garages turned into rooms?
A slab section gets a suction point through the concrete, often with the pipe running up a garage wall or a closet. If the soil under the addition doesn't connect with the soil under the main basement, it may need its own point, sometimes tied into the same fan.
Common combinations
- Basement plus crawl space: a sub-slab point in the basement and a membrane in the crawl, often sharing one fan.
- Old stone or rubble foundation: extra sealing work, and often a membrane over any exposed soil in a cellar.
- Manufactured home: a sealed membrane over the ground under the home, routed with the skirting and access in mind.
Can I just open the crawl space vents?
More outside air sometimes lowers radon a little, but results are inconsistent, and with Helena's subzero January nights, open vents can freeze plumbing in the crawl. A sealed membrane with a fan is the dependable option.
Anything else to think about?
A sealed membrane also cuts ground moisture coming into the crawl. If you store things down there or need to reach plumbing and ductwork, ask how the installer will protect the membrane where people crawl, and whether a heavier material makes sense in those paths. If the house is on a private well, mention it; DEQ notes that radon in well water can add to indoor air levels.
We don't publish or quote prices. The radon company prices the job after it understands your foundation, and usually after someone has walked through the house, so treat any figure given over the phone as a starting point, not a quote.
Where we connect you with help
Common questions
Does a house without a basement need a radon test?
Yes. Any house in contact with the soil can collect radon, whether it sits on a slab or a crawl space.
Can a membrane go in a very low crawl space?
Often, though tight access affects how the work is done. Tell the radon company the clearance when you call so they can plan.
Related services
More background: Montana radon guide.
Radon result came back over 4 pCi/L?
Call and get connected with a Helena-area radon company that can look at your foundation and design a system.