Radon Test House: What Homeowners Need to Know
“Radon test house” is the search phrase homeowners type when they want to test their entire home for radon, not just a single room. The honest answer is that EPA Protocol calls for a single test at the lowest occupied level for the main action decision, with optional supplemental tests on other levels for context. This guide explains the whole-house testing strategy, why one well-placed test is usually enough, when to add additional tests, and how Colorado Front Range homes differ from national averages. This guide summarizes EPA Indoor Air Quality and Colorado Department of Public Health and Environment guidance current as of 2026.
What testing the whole house actually means
The most common misconception about radon testing is that every room needs a separate test. EPA Protocol does not require this. The protocol places one test at the lowest occupied level because that level represents the worst-case exposure and the basis for mitigation decisions. Radon enters from soil gas through the foundation slab, basement walls, sumps, and any other openings between soil and the conditioned air envelope. Concentration is almost always highest at the lowest level and decreases with vertical distance from the source.
For a single-family home with a basement, the right starting strategy is one test in the basement (or the lowest occupied level) for at least 48 hours under closed-house conditions. That single result determines whether mitigation is recommended. Additional tests on the main floor and upper floors are optional context and useful for understanding exposure in primary living and sleeping areas, but they do not override the basement result for mitigation decisions. The Front Range radon testing guide covers regional considerations.
When to add additional tests beyond the lowest level
Additional tests above the lowest occupied level are worth doing in several scenarios. A finished basement that is used regularly as living or sleeping space benefits from a long-term test (90-day or longer alpha-track) that captures actual exposure rather than worst-case short-term. The bedroom level matters because people spend the most continuous time there. If the bedrooms are on the second floor of a two-story home, a supplemental upper-level test reveals how much radon actually reaches the sleeping environment.
Multi-unit homes (duplex, mother-in-law apartment, basement rental) need separate tests in each independently conditioned unit. Each unit has its own pressure dynamics and HVAC, and radon exposure can differ significantly. Homes with a slab-on-grade addition off the original basement need a test in each foundation type because the soil-air interface differs.
Why the basement reading dominates the mitigation decision
Mitigation systems (sub-slab depressurization, the standard active mitigation approach) work by creating negative pressure under the foundation slab and venting soil gas above the roofline. The system reduces radon throughout the home in proportion to how thoroughly it intercepts the soil-air interface. Because the basement is closest to the source, the basement reading is the most sensitive indicator of soil-gas entry. A basement reading at or above 4.0 pCi/L means soil gas is entering at a rate that requires intervention.
Once mitigation is installed, follow-up testing confirms the system is working. Standard practice is a post-mitigation short-term test in the basement, ideally with the mitigation fan running and closed-house conditions maintained. The target is a basement reading below 4.0 pCi/L, often below 2.0 pCi/L. EPA recommends re-testing every two to five years after mitigation to verify continued performance.
Front Range geography: why every house in this region should test
Colorado sits over uranium-rich soils that produce radon as uranium decays. The entire Colorado Front Range is in EPA Zone 1 (highest priority), meaning the predicted average indoor radon concentration is greater than 4.0 pCi/L. CDPHE survey data has documented elevated readings across Denver, Boulder, Jefferson, Douglas, Adams, Arapahoe, Broomfield, and El Paso counties for decades.
What this means practically: the prior probability that a Front Range home has elevated radon is high, even before testing. Approximately half of Colorado homes test above 4.0 pCi/L based on CDPHE survey data. New construction is not exempt; even homes built with passive radon-resistant features show elevated readings often enough that testing post-construction is the rule rather than the exception. Single-family, townhome, and condominium units all benefit from testing.
Local variation within the same neighborhood
Radon concentration varies dramatically house to house even in the same subdivision because of differences in foundation construction, soil characteristics under each lot, age of foundation, presence of cracks or pipe penetrations, and HVAC pressure dynamics. A house testing at 1.8 pCi/L can share a property line with a house testing at 12 pCi/L. Do not assume your house is fine because a neighbor’s test was negative. Test your own house. The how to test your home for radon guide covers homeowner-level testing in depth.
Single-family detached home testing strategy
For a typical single-family detached home with a basement: place one short-term test (charcoal canister or electret) in the basement on the lowest occupied level. Maintain closed-house conditions throughout. Mail in the kit promptly. Wait for results.
If results are at or above 4.0 pCi/L: confirm with a second short-term test or a 90-day alpha-track in the same location. If both short-term results average at or above 4.0 pCi/L, install mitigation.
If results are between 2.0 and 4.0 pCi/L: consider mitigation, especially if the basement is occupied or used as sleeping space. EPA recommends consideration of mitigation in this range.
If results are below 2.0 pCi/L: no mitigation needed. Re-test every two to five years. Re-test after any significant home modification (new windows, new HVAC, radon mitigation system installation in a nearby unit, foundation repair).
Multi-level home strategy
For a two-story home with a basement: start with a basement test. If basement results are elevated and the home has a slab-on-grade attached addition, add a separate test in the addition. If the family spends extensive time on the upper floors and you want to understand actual exposure there, add an alpha-track on the main bedroom level for 90 days.
The mitigation decision is still made on the basement reading. Upper-floor results are context, not a substitute. A basement at 6.0 pCi/L with upper-floor readings at 2.5 pCi/L still warrants mitigation; the post-mitigation readings should drop on all levels.
Crawlspace homes and slab-on-grade
Homes without basements have different testing dynamics. A crawlspace home with a vented or unconditioned crawlspace under the main floor: test the lowest occupied level (main floor) with one short-term kit placed in a regularly occupied room (living room or bedroom, not a kitchen or bathroom). Crawlspace pressure dynamics are complex; sealed and conditioned crawlspaces can produce different radon entry rates than vented crawlspaces.
Slab-on-grade homes (concrete slab directly on soil, no basement, no crawlspace): test the main floor at the lowest occupied level. Radon enters through cracks and pipe penetrations in the slab. Mitigation in slab-on-grade homes works similarly to basement mitigation but the technical details differ.
Condos and townhomes
Lower-level condominium units (especially garden-level units below grade) need testing because they share the soil-air interface with the building’s foundation. Upper-floor condo units further from grade typically have lower radon concentrations but should still be tested if the building was constructed before radon-resistant practices became standard. Test the unit as a single zone: place the kit in a regularly occupied area, follow closed-house protocols, and use the result to make the mitigation decision for that unit only.
Townhomes have individual foundations and behave like single-family homes for testing purposes. Each townhome unit tests independently. Side-by-side townhomes can have very different radon results.
New construction with passive radon features
Many new Colorado homes are built with passive radon-resistant features per the International Residential Code, including a sub-slab gas-permeable layer, a slab vapor retarder, a vertical riser pipe from the sub-slab to above the roofline, and electrical provision for a future fan. The passive system reduces radon entry but does not always reduce concentration below 4.0 pCi/L. Testing the home after construction is essential to verify whether the passive system is sufficient or whether the riser needs to be activated with a fan.
Builders sometimes test as part of the closing process; buyers should ask for the test results and consider repeating the test after move-in to confirm. The test radon in home action guide walks through homeowner steps.
How often to re-test after a clean initial result
EPA recommends re-testing every two to five years even after a clean initial result. Re-testing is also recommended after: a significant home renovation that disturbs the foundation, new HVAC installation, replacement of windows or doors (which changes the home’s pressure dynamics), installation of a wood stove or other combustion appliance, and any major change in how the basement is used (finishing a previously unfinished basement, for example).
Long-term alpha-track measurement every few years is a low-cost insurance policy for Colorado homeowners. A 90-day or 365-day alpha-track in a frequently occupied lower-level room captures annual-average exposure and catches slow drift that short-term tests might miss.
References
- EPA radon homepage — U.S. Environmental Protection Agency
- Colorado radon program and survey data — Colorado Department of Public Health and Environment
- EPA radon zones map — U.S. Environmental Protection Agency
- CDC radon health information — Centers for Disease Control and Prevention
Front Range homeowners who want a professional whole-house radon assessment or mitigation referral can get in touch here.