Guide

Crawlspace Radon and Sub-Membrane Systems

Why a crawlspace is the difficult case

Under a basement slab, the concrete is already most of a barrier. The installer's job is to draw from the small volume beneath it and let the slab do the rest. A dirt-floor crawlspace has no barrier at all. Every square foot of exposed soil is releasing gas directly into a space that is connected to the house.

And it is connected, more than people expect. Air moves upward through a building as warm air rises and escapes at the top, drawing replacement air in from below. In a house over a crawlspace, a meaningful share of the air on the ground floor came through that crawlspace first. Whatever is down there is in the house.

How sub-membrane depressurization works

The membrane

Heavy polyethylene sheeting, typically 6 mil or thicker for a radon application, is laid across the entire soil floor. Seams are overlapped generously and sealed. The sheeting runs up the foundation walls and is mechanically fastened and sealed there, and it is sealed around every pier, post and penetration. Done properly, it is a continuous barrier with no free edges.

The suction

A perforated pipe or a gravel-filled pit goes beneath the membrane, connected to a riser and an inline fan exactly as in a slab system, venting above the roofline. The fan holds the space under the membrane at lower pressure than the crawlspace above it, so soil gas goes to the pipe.

Why the seal quality is the whole job

In a slab system, a small unsealed crack is an inefficiency. In a membrane system, the membrane is the barrier, so a torn section, an unsealed seam or a corner pulled up to get at storage is a direct hole in the system. This is the single most common reason a crawlspace system underperforms, and it is usually visible to anyone willing to go down there and look.

The vent question

Older crawlspaces were built with vents to the outside to let moisture escape. In humid Mid-Atlantic summers this frequently does the opposite: warm moist outside air enters, meets cooler surfaces under the floor, and condenses. Current building science generally favours sealing and conditioning over venting for that reason.

For radon specifically, it cuts both ways. Ventilation dilutes radon, so sealing vents without addressing the soil can raise concentrations. The right order is membrane and depressurization first, vent strategy second, not the other way round.

Doing it alongside encapsulation

Encapsulation, done for moisture, and sub-membrane depressurization, done for radon, are close to the same physical work: a sealed membrane across the soil and up the walls. If a crawlspace needs either, it is nearly always worth doing both at once.

Sequence matters:

1. Fix bulk water first: grading, gutters, downspouts, and a sump if groundwater genuinely requires one
2. Clear out debris, old insulation and anything sitting on the soil
3. Deal with any existing mold or rot on framing before it gets sealed in
4. Lay the suction field, pipe or gravel pit, before the membrane goes down
5. Lay and seal the membrane across soil, up walls, around every pier
6. Install riser, fan, manometer and roofline termination
7. Post-mitigation test to confirm the number
8. Address vents and add dehumidification if the moisture design calls for it

Sealing a crawlspace before step four means opening it again afterwards, which is how this job gets paid for twice.

What to look for before you call anyone

• Exposed bare soil with no sheeting, or sheeting that is torn, bunched or only partly covering the floor
• Standing water, damp soil, or a visible water line on a pier
• Insulation fallen away from the subfloor, which means it has absorbed moisture
• A crawlspace hatch inside the house that does not seal
• Ductwork or a furnace in the crawlspace, which moves that air around the house actively
• A section of the house on a different foundation type, which changes the design

The other foundation variants are on system types, the mechanism is the same one described on sub-slab depressurization, and cost drivers are on what it costs.

Common questions

Should I just seal the crawlspace vents?

Sealing vents is part of modern crawlspace practice for moisture reasons, but on its own it can make radon worse by removing the dilution that ventilation provided. Vent sealing belongs with a membrane and a depressurization system, not instead of them.

Does encapsulation fix radon?

Not by itself. Encapsulation is a sealed membrane for moisture control, and it does reduce soil gas entry, but without active suction beneath it, gas still finds seams, edges and penetrations. Encapsulation plus depressurization is the combination that works, and doing them together is cheaper than doing them twice.

My crawlspace is only part of the house. Does that matter?

Yes, a lot. A house with a basement under the original structure and a crawlspace under an addition has two separate soil-gas environments, and they usually need separate suction points.