Crawlspace radon mitigation

The standard method: sub-membrane depressurisation

The approach is conceptually the same as a slab system, with the membrane playing the role of the concrete. A durable polyethylene sheet is laid over the exposed soil, sealed at the seams, sealed up the foundation walls and around piers and posts, and a suction pipe is placed beneath it. A fan draws air from under the membrane and vents it above the roofline.

The quality of that sealing is the whole job. A membrane loosely laid over the dirt with gaps at the walls performs poorly regardless of the fan on the other end. Seams properly lapped and sealed, edges mechanically fastened and sealed to the foundation, penetrations addressed — that is what separates a system from a tarp.

Where a crawl space has a rough or rubble floor, or standing moisture, preparation work comes first. That is legitimate scope and it belongs on the quote in writing.

• Durable membrane over all exposed soil, not just the open middle
• Seams lapped and sealed rather than merely overlapped
• Edges sealed to foundation walls, piers, and posts
• Suction point beneath the membrane, sized to the area
• Vent piping to above the roofline, away from windows
• Manometer and dedicated electrical, as with any system

Why crawl spaces raise the price

Access is the first driver. A crawl with eighteen inches of headroom, a small hatch, and ductwork in the way is slow, uncomfortable work, and time is what you are paying for.

Area is the second. Membrane cost scales with square footage, and a large crawl uses a lot of material and a lot of sealing.

Condition is the third. Debris removal, grading a rough floor, dealing with standing water, or working around old vapour barrier that has to come out all add hours before the actual mitigation begins.

None of that is padding. It does mean a crawl-space quote should itemise those elements, so that you can see what you are being charged for rather than comparing two round numbers.

Houses with both a basement and a crawl

This is extremely common — a basement under the original house, a crawl under an addition or a porch, sometimes a slab section too. The correct approach treats the house as its zones: a suction point for the basement slab, sub-membrane suction for the crawl, and whatever the addition needs, frequently all tied into one fan.

The thing to watch for in quotes is a bid that has priced only the basement. A system that leaves an untreated crawl open to the house is drawing against a large uncontrolled opening, and it commonly explains a verification test that does not come down.

Ask every bidder explicitly which zones their price covers. When two quotes differ sharply on a mixed-foundation house, this is usually why.

Vented crawl spaces and the ventilation question

Some crawl spaces are vented to the outdoors by design. Increasing that ventilation can dilute radon in the crawl, and in mild climates it is occasionally offered as an approach. It comes with trade-offs: energy loss, moisture and freezing risk depending on climate, and a result that varies with the weather rather than being controlled.

Sub-membrane depressurisation is the more reliable method because it removes soil gas before it enters rather than diluting it afterward, and its performance can be verified. Where ventilation is proposed, ask what post-mitigation number the contractor expects and how it will be measured across seasons.

The moisture side-benefit, stated honestly

A sealed membrane over crawl-space soil substantially reduces the moisture entering the space from the ground, and the fan continuously removes humid soil gas. Homeowners frequently report a drier crawl, less musty odour, and better conditions for anything stored or ducted there.

That is a genuine side-benefit, and it is worth mentioning to a contractor because it may influence membrane choice. What it is not is a substitute for grading, gutters, drainage, or a dehumidifier where those are the real problem. Treat the improvement as a bonus rather than as part of what you are buying.

What to check when the work is done

Go and look, or ask for photographs if the access genuinely does not permit it. The membrane should cover all exposed soil, be sealed at walls and piers, and be intact — not torn by the installer working across it. The suction pipe should be beneath it, the manometer should show an offset, and the discharge should be above the roofline.

Then verify with a post-mitigation test in the same location as the original, under closed-building conditions, after the system has run at least twenty-four hours. On a mixed-foundation house it is reasonable to ask for measurement in the ground-contact spaces as well.