Radon in Colorado Homes: Testing, Mitigation, and What It Costs

If you are buying a home anywhere along the Front Range, radon is not an exotic risk you might run into. It is a normal line item. Colorado sits on granite, shale, and uranium-bearing soils that release radon gas as they break down, and the state health department has estimated for years that roughly half of Colorado homes test above the EPA's recommended action level. Every county in the Denver metro is rated as high-potential.

The good news is that radon is one of the most fixable problems a house can have. The bad news is that buyers regularly mishandle it in a transaction, either by skipping the test or by treating a high result like a structural defect. Here is how to think about it.

What Radon Is and Why It Matters

Radon is a colorless, odorless radioactive gas that seeps up from the ground and collects in the lowest enclosed spaces of a home — basements, crawlspaces, and slab-on-grade living areas. Long-term exposure is the leading cause of lung cancer among non-smokers in the United States. It is not an acute hazard; nobody gets sick from a week in a high-radon house. It is a cumulative one, which is exactly why it is worth fixing in a home you plan to live in for years.

The EPA's action level is 4.0 picocuries per liter (pCi/L). The EPA also recommends considering mitigation between 2.0 and 4.0, and many Colorado buyers now use that lower threshold, especially if the basement is finished and someone will be sleeping down there.

How Testing Works During a Purchase

In a standard Colorado contract, radon testing happens during the inspection period, before the Inspection Objection Deadline. Most inspectors in the metro offer it as an add-on for roughly $150 to $250. The test is almost always a continuous radon monitor placed in the lowest livable level of the house for a minimum of 48 hours.

Short-term tests depend on closed-house conditions: windows and exterior doors shut for 12 hours before and during the test, except for normal entry and exit. This is where tests get compromised. A seller who leaves basement windows open, or runs a whole-house fan, can drag the number down. Good continuous monitors log temperature, humidity, and movement, and a competent inspector will flag a test that looks tampered with or disturbed.

Plan the timing. A 48-hour test plus report turnaround means you want it placed in the first few days of your inspection window, not the last two. If your inspection period is ten days, ask your inspector to drop the monitor on day one or two.

Check the Seller's Disclosure First

Colorado's Seller's Property Disclosure asks specifically about radon — whether the seller has tested, the results, and whether a mitigation system is installed. Read that section before you write the offer. If there is already a system, that changes your job from 'find out if there is a problem' to 'confirm the system works.'

An existing system is a U-tube manometer on the vent pipe (the little gauge with colored liquid) and a fan, usually outside or in the attic. If the liquid levels are uneven, the fan is pulling. If they are even, the fan is off or dead. You still want a post-mitigation test during your inspection period, because systems fail and fans typically last about 5 to 10 years.

What Mitigation Actually Costs

The standard fix in Denver-area homes is sub-slab depressurization: a contractor cores a hole through the basement slab, runs PVC pipe up and out of the house, and installs an in-line fan that pulls soil gas from under the slab and vents it above the roofline. For a typical metro home with a basement, expect roughly $1,200 to $2,500 installed.

Costs go up with complications. Crawlspaces need a sealed vapor barrier across the dirt, which adds material and labor. Homes with both a basement and a crawlspace, or with multiple slab sections from additions, may need more than one suction point. Finished basements can require routing pipe through closets or the garage to avoid tearing into drywall. Budget closer to $3,000 to $4,000 for a complicated house.

Operating cost is small — the fan draws about as much as a light bulb, so figure $50 to $150 a year in electricity. And a properly installed system routinely brings levels below 2.0 pCi/L.

Use a certified mitigator. Colorado encourages certification through the National Radon Proficiency Program or the National Radon Safety Board, and a certified contractor will do a post-installation test and give you documentation you will want when you sell.

How to Negotiate It

A high radon result is a legitimate inspection objection, and in the current Denver market, sellers generally expect to address it. You have three reasonable options.

Ask the seller to install a system before closing. This works, but you are trusting the seller's choice of contractor. If you go this route, require a certified mitigator, a post-mitigation test showing a result under 4.0 (or under 2.0 if you negotiated for it), and the paperwork before closing.

Ask for a credit or price reduction. This is often the better choice for buyers, because you pick the contractor and control where the pipe runs. Base the credit on an actual written bid, not a guess. Keep in mind lenders cap how much a seller can contribute toward closing costs, so a credit may need to be structured as a price reduction instead.

Split it. In a slower fall market, a request to split mitigation cost is easy for most sellers to accept and keeps goodwill for the bigger items on your objection list.

What you should not do is walk away from an otherwise great house over radon alone. A $2,000 fix is not a reason to lose a home you love.

New Construction Is Not Automatically Safe

Many Colorado builders now install passive radon systems — a pipe from under the slab to the roof with no fan. Passive systems help, but they do not guarantee a low reading. Test new construction the same way you would a resale, and if the result is high, adding a fan to an existing passive pipe is one of the cheapest mitigations there is, often a few hundred dollars.

The Bottom Line

In Colorado, you test for radon on every purchase, you read the seller's disclosure before you offer, and you treat a high number as a routine, fixable negotiation item. Get the test placed early, use a certified mitigator, and document everything, because your future buyer is going to ask the same questions. If you want help working radon into your inspection strategy on a specific house, that is exactly the kind of detail we sweat for our clients.