By Grizzly Insulation CO

Spray foam is one of the most effective choices available for a Denver basement. It helps seal gaps that traditional insulation simply cannot reach and turns a cold, drafty space into a warmer, drier one. But that same sealing effect changes how air moves through the basement, and if radon gas is already rising from the soil beneath your home without your knowledge, sealing the space can trap that gas inside instead of allowing it to escape. This is a genuinely common scenario in the Denver area, where roughly half of all homes register radon levels above the safe threshold. Statistically, there is a real possibility your home is one of them.

So, yes, you should test for radon before starting spray foam insulation work in your Denver basement, not after the job is finished.

Why Radon Is a Bigger Problem in Denver Than in Most Cities

Radon is a naturally occurring gas released as uranium decays underground, and Colorado’s soil, particularly along the Front Range where Denver sits, contains an unusually high concentration of it. The gas is invisible, odorless, and tasteless, which means testing is the only reliable way to know whether it is present in your home. The average indoor radon level nationally is about 1.3 picocuries per liter, a unit written as pCi/L. The Environmental Protection Agency recommends taking action once indoor levels reach 4 pCi/L or higher.

Denver falls within EPA Zone 1, the agency’s highest radon risk classification, alongside Boulder, Jefferson County, and Douglas County. Colorado State University Extension reports that roughly half of all homes statewide test above the 4 pCi/L threshold, a pattern that holds true across the state rather than being isolated to a handful of counties.

For a Denver homeowner, that translates to nearly a coin-flip chance that your basement already carries radon levels above the safe limit, and the only way to know is to test.

What Spray Foam Actually Does to a Denver Basement

Spray foam functions as both insulation and an air seal. When a contractor applies it to your basement walls and rim joist, it fills the small cracks and gaps that previously allowed air to move in and out of the space. The complication is that a basement sits directly on soil, and radon originates from that soil, not from the walls above it. Most Denver basements have a concrete floor with small cracks, a seam where the floor meets the foundation wall, and openings around the sump pit and floor drains, all of which serve as entry points for radon gas.

A standard spray foam job addresses the walls and rim joist. It does not seal the floor, the sump pit, or the seam where the floor meets the wall, since those require different materials and a separate scope of work. Once spray foam seals the walls, the basement becomes considerably tighter overall, but the openings where radon actually enters, down at the floor level, remain exactly as open as they were before the project started.

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What Happens in a Denver Basement If You Skip the Test and Spray Foam Anyway

Before a basement is sealed, a small amount of air naturally moves in and out through gaps in the walls. That airflow works against energy efficiency, but it also performs one unintentional service: it allows some of the radon rising through the floor to escape outside rather than accumulating indoors.

Once spray foam seals the walls, that natural airflow largely stops. Radon continues rising through the unsealed floor, but with nowhere left to go, it now concentrates inside a smaller, tighter space. A second effect compounds the first. Sealing a basement can slightly lower the air pressure inside relative to the pressure in the soil beneath it, creating a gentle vacuum effect that pulls additional radon gas up through the floor cracks rather than reducing it.

In practice, this means two things happen simultaneously:

  • More radon enters through the floor, since the pressure change actively draws it upward.
  • Less radon escapes through the walls, since the sealing has closed off its previous exit points.

Both effects push the basement’s radon level higher rather than lower, and none of it is detectable without testing. Radon has no smell, no color, and no warning sign. The basement will simply feel warmer and more comfortable, which reads as a successful project. Weeks or months later, often during a home sale, a test can reveal a level considerably higher than it would have been before the spray foam work.

By that stage, correcting the problem is more involved. A radon fix typically requires drilling through the concrete floor and running a pipe up through the house to a fan, and if the basement has already been finished with new foam, drywall, or flooring, the mitigation contractor has to work around all of it. What could have been a single straightforward project becomes two separate, more expensive ones.

Spray foam does not create radon; the uranium in the ground does. But applied without addressing the floor, spray foam can change where the radon accumulates and how much of it ends up in the air you breathe.

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Should You Retest for Radon After Spray Foam Work in a Denver Basement?

Yes, a test conducted before the project establishes a baseline for your basement’s air quality prior to any changes. A test conducted afterward shows what conditions look like once the walls have been sealed. These two numbers can differ, even though the geology beneath your house hasn’t changed at all.

If your initial test comes back close to but under 4 pCi/L, at 3.2 pCi/L for example, that number can rise once the basement is sealed, since the seal removes the airflow that had been keeping the level lower. A result that was safe before the project might not remain safe afterward.

A second, post-installation test resolves this uncertainty one of two ways: either the level held steady or decreased, which is the desired outcome, or it increased, which signals it’s time to consult a radon specialist. 

The comparison to a familiar medical practice is useful here: checking blood pressure before starting a new medication, then checking again a few weeks later. The first reading establishes where you started. The second confirms whether the intervention actually worked.

Five Ways to Test for Radon Before You Spray-Foam a Denver Basement

Radon testing is inexpensive and straightforward. Denver homeowners have five practical options.

  1. A short-term test kit – A small charcoal container, available at most hardware stores or online, placed in the basement for two to seven days with windows and doors kept closed as much as possible. The kit is then mailed to a lab for results. This is the fastest option for homeowners with a spray foam project already scheduled.
  1. A long-term test kit – This version remains in the basement for 90 days or more, producing a more accurate seasonal average, since radon levels fluctuate throughout the year. It suits homeowners with more lead time before their project begins.
  1. A digital radon monitor – A plug-in device that displays a live reading on screen. It costs more than a mail-in kit but allows day-to-day tracking, and the same device can be reused to check the basement again once the spray foam work is complete.
  1. A certified radon tester – A professional who visits the home with specialized equipment and provides an official, lab-verified result. This option carries more weight for homeowners planning to sell, since Colorado law requires sellers to disclose known radon results to buyers. 
  1. Coordinating through your insulation contractor – Ask your spray foam contractor to build a radon test into the project schedule before work begins. A contractor experienced with basement sealing should be able to pause the timeline for a short test or connect you directly with a radon tester, rather than leaving the coordination entirely up to you.

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What to Do If Your Denver Radon Test Comes Back High

A result at or above 4 pCi/L does not mean canceling the spray foam project. It means adjusting the sequence of work. Engage a licensed radon mitigation contractor first, or in parallel with your insulation contractor. A typical mitigation system uses a sealed pipe running from beneath the concrete floor, up through the house, and out above the roofline, with a small fan drawing radon gas through the pipe before it can accumulate in the basement. Once that system is in place, sealing the basement walls with spray foam accomplishes exactly what it should: a warmer, more comfortable basement without the added risk.

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At Grizzly Insulation Co., we ask homeowners about radon testing before providing a quote for basement spray foam work anywhere in the Denver area. It’s a brief question that can prevent a considerably larger problem down the line.

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The Bottom Line for Denver Homeowners

Test your Denver basement for radon first. Apply spray foam second. If the initial test comes back high, bring in a radon specialist and coordinate both projects together rather than sequentially. Test again once the spray foam work is complete, particularly if the original reading was close to the 4 pCi/L threshold. These steps simply need to be followed in the correct order in a city where the ground beneath most basements presents a genuine risk of radon gas rising into the home above it.

The team at Grizzly Insulation CO is known for providing efficient spray foam insulation services across the Denver metro area.