Radon mitigation in Fort Collins basements: what to do before you finish

Radon is a serious question in Fort Collins basements. About half of the homes tested in the city come in at or above 4 pCi/L, the level where the EPA recommends a fix.¹ So test before you frame anything. If the number is high, a certified mitigator seals the basement and installs a vent system while the space is still open. Your finish then goes up around the pipe and fan.

TL;DR

• High radon is common here. About half of tested Fort Collins homes reach the EPA's 4 pCi/L action level, and levels vary from house to house.¹
• Test first. The city sells a short-term kit for $6.¹
• Mitigation means sealing entry points and venting soil gas through a pipe to the roof, usually with a fan. The city's target for a working system is 2 pCi/L.¹
• Install the system before framing. Its route can cross slabs, framing, electrical work, firestopping, attics and the roof, so plan it with your finishing contractor while walls are open.²
• One Fort Collins directory page puts a typical system at $800 to $1,500.³ Against a typical $50,000 to $110,000 Fort Collins full finish, that works out to about 0.7% to 3%.

How serious is radon in Fort Collins basements?

Serious enough that testing belongs at the top of your finishing plan. The City of Fort Collins says about half of the homes tested have radon at or above 4 pCi/L.¹ The EPA recommends fixing radon at that level. Levels below 4 still pose a risk, and the city sets 2 pCi/L as the target for radon systems.¹

Larimer County's health department gives the same EPA line. Mitigate if the level is 4 pCi/L or more.⁴

Radon forms when uranium in soil breaks down. It gets into homes through cracks and openings in basements, crawl spaces and slabs.¹ A basement sits in that soil, so it's the first room the gas reaches.

State health officials put numbers on the risk. CDPHE calls radon the leading cause of lung cancer among non-smokers in the United States. It links radon to about 500 lung cancer deaths a year in Colorado.⁵ The city notes that your risk depends on how much radon is in the home and how long you're exposed.¹

Exposure time is why finishing changes the picture. A bare basement gets a few minutes of use a day. A finished bedroom, office or family room gets hours.

Your neighbor's result won't answer the question for your house. The city says levels vary from house to house and depend on the age, quality and upkeep of the home.¹ Two houses on one street can test differently. Only your own test counts.

Does Fort Collins require radon testing or mitigation?

The city doesn't require radon testing, according to a Fort Collins real estate radon brochure. It does require radon-resistant construction in new homes.⁶ Colorado law adds disclosure duties when you sell. The brochure is undated, so confirm the current rules with the city before you plan around them.

If you sell later

Colorado law requires sellers to tell buyers about radon levels they know of. Sellers use the Sellers Property Disclosure form for this.¹ Colorado also requires sellers to give buyers the CDPHE brochure on radon in real estate and rental transactions.¹

In practice, the result from your pre-finishing test becomes a number you disclose. A buyer gets a better answer from a documented system with a follow-up result than from a high number with no fix.

If you plan to rent the finished space

A basement suite for a tenant brings its own rules. The landlord must give the tenant the CDPHE brochure before the lease is signed.¹ Landlords aren't legally required to install a system. A tenant can still break the lease if the landlord fails to disclose known radon levels. The same applies if the landlord makes no reasonable effort to fix high levels within 180 days of a radon professional's finding. From January 1, 2026, those remedies don't apply to leases of one year or less.¹

The finishing permit is a separate step

Radon rules and the finishing permit run on two different tracks. For the basement build, your contractor pulls the permit and books the inspections. Our Fort Collins basement remodeling hub covers the local picture for the rest of the project.

How do you test a basement in Fort Collins before finishing?

Start with a short-term kit in the basement before framing starts. The city sells short-term kits for $6 and long-term kits for $20 at the Fort Collins Senior Center, 1200 Raintree Drive.¹ A short-term test runs three to seven days with the house closed up. It gives a quick snapshot and shows whether you need more testing.¹

The city describes three kinds of test.¹

If you live outside city limits, the city points you to CDPHE for a kit.¹ Check which side of the line your address sits on before you buy.

Short-term kits are time sensitive. Mail them right after sealing and follow every instruction, or the lab may reject the result.¹

Time the test so the result arrives before you sign a finishing contract. A high number changes the layout conversation. It decides where a pipe can run and which closet or corner the finishing plan gives up.

What does radon mitigation involve?

Two jobs: sealing and venting. The city describes the fix as sealing the basement or crawl space, then installing a system. A passive system runs a pipe from the soil to the roof and works like a chimney. An active system adds a fan to pull radon from the soil. The goal is 2 pCi/L.¹

For a deeper look at the parts, see how radon mitigation systems work.

Passive and active systems

A passive system relies on the pipe alone. An active system adds a fan to that same pipe.¹ Newer homes built to radon-resistant rules may already have some of this in place.⁶ One Fort Collins radon contractor lists "passive activation" among its services.² By the city's definition, activating a passive system means adding a fan. Check your builder's records or ask a mitigator whether your house has a passive pipe before you pay for a new route.

Who should do the work

The City of Fort Collins recommends certified radon professionals for any radon project. It points to the National Radon Proficiency Program's list of certified professionals in Colorado.¹ Credentials and rules change. Check current credentials in official licensing records and confirm them in the written proposal.²

Where the pipe and fan go

Systems run inside or outside the house. One Colorado mitigator shows an interior system with the fan in the attic. It also shows an exterior system on a multifamily building where no interior system was possible.⁷ The same firm says it weighs noise and aesthetics in its installs.⁷ Both matter once a finished room sits beside the pipe.

Proof the system works

Ask for a result after installation. One Fort Collins radon firm lists documented post-mitigation results as part of its work.² Hold framing until you have that number.

How do you finish a basement around a radon system?

Install the system first and build around it. One Fort Collins radon contractor notes that radon routes can cross slabs, framing, siding, electrical work, firestopping, attics and roof penetrations.² Your finishing contractor schedules those same trades. Planning the route while walls are open cuts rework later.²

Put the steps in order

1. Test the basement and get the result.
2. If the level is high, get a written proposal from a certified mitigator.
3. Seal the basement openings, then install the system.¹
4. Get the post-mitigation result.²
5. Frame, insulate and drywall around the finished system.

The sealing step drives this order. Radon comes in through cracks and openings in basements and slabs.¹ Flooring and framing cover those surfaces. Sealing them is simplest while the concrete is bare.

Agree the pipe route with both contractors

Decide where the vertical pipe runs before anyone frames. Options include a utility room, a closet, a framed chase or an exterior run. Each one costs floor space or wall area in the finished plan. Where the pipe passes through framing and floors, firestopping comes into play.² Settle in writing who handles it.

Keep the system reachable

One Fort Collins radon firm lists repairs among its services.² Systems need upkeep over time, so keep the pipe and any fan where a technician can reach them. An access panel costs little in a new wall. Cutting into finished drywall costs more.

If the basement is already finished

A system can still go in. The route choices shrink, and an exterior run may be the practical answer.⁷ Plan to open some finished surfaces if the pipe goes inside. If you're remodeling an already finished basement in Fort Collins, fold the radon work into that project.

For the wider build sequence, see how basement finishing works. Our Colorado basement finishing guide covers the state's other planning issues.

What does radon mitigation cost in Fort Collins?

Budget roughly $800 to $1,500 for a system. That range comes from one directory's Fort Collins page, which puts the average around $1,000.³ Your mitigator's written proposal sets the real number. Our radon mitigation cost guide covers pricing in more depth. Testing costs far less. City kits run $6 short-term and $20 long-term.¹

At least one Fort Collins firm prices by foundation type. Its planning ranges split into passive activation, slab, crawl-space, mixed-foundation, complex and service work.² Expect a slab-only basement and a house with a crawl space to price differently.

Set against the finish, the system is a small line. That works out to about 0.7% ($800 against $110,000) to 3% ($1,500 against $50,000) of a typical Fort Collins full finish. You can see what finishing costs in Fort Collins in more detail.

Help exists for some owners. CDPHE's Low Income Radon Mitigation Assistance program gives financial help for radon fixes. You must be a Colorado resident, live in the home as your main residence and qualify as low-income.¹

What to ask your radon mitigator and basement contractor

Ask questions that make both contractors plan the same route. The mitigator owns the radon result. The finishing contractor owns everything built around it. A good pair of quotes shows where the pipe runs, where the fan sits and who handles each opening through framing, floors and the roof.

Ask the radon mitigator:

• Are you on the National Radon Proficiency Program's Colorado list?¹
• Where will the pipe and fan go, inside or outside?⁷
• Which openings will you seal before installing the system?¹
• Will I get a written post-mitigation result?²
• Are you designing for the city's 2 pCi/L target?¹

Ask the basement contractor:

• Does the written, itemized quote show the chase or closet around the pipe?
• Who handles firestopping where the pipe passes through framing and floors?²
• Will framing wait until the system is in and tested?
• Where will the access panel go?

When your radon plan is settled, get a written quote for finishing your Fort Collins basement. Basement Remodeling Co. sends your request to one vetted, licensed basement contractor. That contractor books a free on-site estimate and gives you a written, itemized quote. Planning a basement somewhere else? You can find the contractor for your ZIP.