Guide

Does Sealing Cracks Reduce Radon?

Why sealing alone underperforms

The instinct is reasonable: gas is coming in through cracks, so close the cracks. What defeats it is that a slab has far more openings than anyone can find, and the building is actively pulling.

Warm air rises and escapes through the upper part of a house, and replacement air is drawn in low, through the foundation. Exhaust fans, dryers and combustion appliances add to that. The result is a persistent pressure difference pulling soil gas upward. Reduce the number of openings and the same pressure difference simply pulls harder through the ones that remain.

And there are always openings that remain:

• The perimeter expansion joint, running the entire edge of the slab where it meets the wall
• Every plumbing and conduit penetration
• The gap around a sump pit, and the pit itself if it is open
• Floor drains
• Hollow block wall cavities, which can carry gas inside the wall and release it above floor level
• The joint where the slab meets a support column or a chimney base
• Hairline cracks that are invisible under paint or flooring
• Concrete itself, which is slightly permeable

What it does buy you

Sealing is genuinely valuable in the right role. Inside a mitigation system, every sealed opening is one the fan no longer wastes capacity on.

A fan is pulling a fixed amount of air. If a large fraction of that is conditioned indoor air sucked down through an open sump pit, the fan is doing less work on the soil under the slab and more work on your heating bill. Sealing redirects the fan's capacity where it belongs, which both improves the suction field across the slab and reduces the conditioned-air loss.

This is why a proper installation includes sealing and why it is listed in the scope rather than sold separately. See sub-slab depressurization.

The openings worth sealing, in order

1. An open sump pit. The single biggest one in most basements. A gasketed, airtight lid with sealed pass-throughs for the pump cable and discharge line, still removable for service
2. The perimeter expansion joint. Long, continuous, and usually hidden behind baseboard or flooring
3. Plumbing and conduit penetrations, with an appropriate flexible sealant
4. Floor drains, which need a trap that holds water or a proper one-way device, not a plug
5. Visible slab cracks, using a flexible polyurethane rather than a rigid filler that will crack again as the slab moves
6. The top course of hollow block walls, which is often open
7. Crawlspace to basement openings, and the crawlspace hatch

Note the material: flexible sealant, not rigid. Slabs move seasonally. A rigid filler cracks and the opening returns, which is why DIY crack repairs so often stop working within a year or two.

The one case where sealing alone is worth trying

If your result is only marginally above 4 pCi/L and you have one large, obvious opening, most often an uncovered sump pit, sealing it and retesting is a reasonable, cheap experiment. It occasionally works.

The condition is the retest. Sealing and assuming is how people spend two years believing a problem is solved. A short-term kit costs about fifteen dollars, so verifying is trivially cheap compared with being wrong. See short-term vs long-term tests.

What sealing definitely will not do

• Bring a high reading, say above 8 pCi/L, down to a safe level
• Address radon entering through hollow block wall cavities above the floor
• Do anything about a crawlspace with bare soil, which needs a membrane. See crawlspace radon
• Help with waterborne radon from a private well. See radon in well water
• Substitute for a system on a house being sold, where a buyer's agent wants a documented post-mitigation result

Waterproofing is not radon sealing

Worth separating these, because they get conflated. Interior waterproofing systems frequently install a drainage channel at the slab perimeter that is deliberately open to the sub-slab gravel, because it is designed to let water in so it can be routed to a sump. That is the opposite of a radon seal.

A house that had basement waterproofing done and then tested higher than before is not a coincidence. See radon after waterproofing.

Common questions

Can I just seal the cracks and skip the system?

It is worth trying only if your result is marginally above the action level and you have an obvious single opening such as an open sump pit. Even then, retest afterwards rather than assuming. In most houses the reduction is modest and inconsistent.

Why doesn't it work when the slab looks sealed?

Because the visible cracks are a fraction of the openings. The perimeter expansion joint runs the entire edge of the slab, there are gaps at every pipe penetration, and concrete itself is slightly permeable. The house is a pump and it will find what you missed.

Does sealing help once a system is installed?

Yes, meaningfully. An unsealed opening is somewhere the fan pulls conditioned air from inside the house instead of soil gas from under it. Sealing improves the suction field and reduces the heating and cooling penalty.