Radon Test for House: Methodology and Placement Guide 2026
A radon test for house is the only reliable way to know whether your home has elevated radon — the gas is odorless, colorless, and tasteless, and no symptoms or visual signs indicate its presence. Three testing methodologies are available to homeowners: short-term test kits (2 to 7 days, $15-30 retail), long-term test kits (90 days to 12 months, $30-50 retail), and professional continuous monitoring (48 hours minimum, $150-200 professional). Each has trade-offs in cost, accuracy, and use case. This guide walks through which test to use when, where to place it in the home, the closed-house protocol that makes results valid, and how to interpret results against EPA’s 4.0 pCi/L action level. This guide summarizes EPA and CDPHE guidance current as of 2026 — consult a certified radon professional for testing decisions specific to your home.
The Three Test Methodologies
Short-Term Test Kit ($15 to $30 Retail)
The most common first-screen test. Two main technologies:
- Charcoal canister — 2 to 7 days of exposure. Activated charcoal absorbs radon and its decay products. Mailed to lab for gamma-spectroscopy analysis.
- Activated charcoal liquid scintillation — similar exposure window, different analysis method, often slightly more sensitive.
Available at hardware stores, online, and through state radon programs (many Colorado counties offer free kits through CDPHE’s Radon Program). Results in 5 to 10 business days after lab receives the sample. Best use: initial screening of an unknown home, especially before purchase.
Long-Term Test Kit ($30 to $50 Retail)
Alpha-track detectors are small plastic discs that record alpha-particle hits from radon decay over a 90-day to 12-month window. The integrated reading averages over seasonal variation (radon levels are typically higher in winter when homes are closed up). Best use: follow-up after a marginal short-term result, or year-long characterization of a home with mixed climate.
Professional Continuous Radon Monitor ($150 to $200 Professional)
Electronic instrument with hourly reading capability over 48 hours minimum. Used by certified home inspectors and ancillary radon services during real-estate transactions. The hourly data shows whether closed-house protocol was followed and reveals diurnal patterns. Best use: real-estate transaction radon testing, post-mitigation verification, due-diligence before purchase. Our radon test kit buyer’s guide compares consumer-purchase options against professional rentals.
Where to Place the Radon Test in the House
Placement matters more than people realize. Wrong placement can produce results 30% to 50% off from actual.
Lowest Livable Level
Place the test on the lowest level of the home that is regularly occupied. If the basement is finished and used as living space (bedroom, family room, home office), test there. If the basement is unfinished and only used for laundry/storage, test the first floor instead. Crawlspaces themselves are not tested — they’re sealed off and the testing happens in the lowest occupied level above. For broader pillar context, see our radon testing pillar guide for Front Range homeowners.
Height Above Floor
20 to 72 inches off the floor. Most homeowners place the kit at 20 to 30 inches — about table height. This represents the breathing zone of seated occupants.
Distance from Windows, Doors, Exterior Walls
At least 4 feet from any exterior wall, window, or exterior door. The kit should sample interior breathing-zone air, not air near pressure-influence points where indoor and outdoor exchange happens.
Distance from Drafts and HVAC
Avoid placement near HVAC supply registers, ceiling fans, fireplaces, or other strong air-movement sources. Drafts can dilute or concentrate radon at the test location in non-representative ways.
Central Room Position
A central position in the room, on a stable surface (table, shelf, mantle), is ideal. Don’t place on the floor (humidity), don’t place hanging from the ceiling (uncommon and untested protocols).
The Closed-House Protocol
Short-term tests (2 to 7 days) require closed-house conditions to produce comparable results. Per EPA’s testing protocol:
- 12 hours before test start, close all windows and exterior doors
- Keep windows and exterior doors closed throughout the test (except for normal entry and exit)
- Keep HVAC fan in normal operation mode (don’t run continuously or shut off)
- Don’t run window fans, ceiling fans pulling outdoor air, or whole-house fans
- Don’t operate fireplaces or wood stoves unless that’s normal household practice
The closed-house protocol is what makes a 48-hour test result comparable to the EPA 4.0 pCi/L action level. Long-term tests (90+ days) are less sensitive to closed-house compliance because they average over multiple weeks of household use.
When to Test
Anytime — but some seasons are better for representative results:
- Winter: Front Range Colorado has highest indoor radon in winter when homes are closed up and HVAC is running. Testing in winter gives a worst-case characterization. This is when most Front Range testing happens.
- Summer: Some homes open windows daily, which dilutes indoor radon and may produce falsely low results. Closed-house protocol becomes critical.
- Real-estate transactions: Whenever the transaction happens — most use 48-hour continuous monitors with documented closed-house compliance.
- After renovations: Foundation work, basement finishing, HVAC changes, or addition construction can shift radon entry patterns. Retest.
EPA recommends testing every 2 years even after a low initial result. Soil-gas conditions change; new cracks develop; HVAC patterns shift.
Interpreting Results
- Below 2.0 pCi/L: Low. EPA recommends retesting every 2 years. Continue normal home use.
- 2.0 to 4.0 pCi/L: Consider mitigation. EPA recommends confirming with a long-term alpha-track test or a second short-term test. Many homeowners proactively mitigate in this range.
- 4.0 to 10.0 pCi/L: Above action level. Mitigation recommended. Get three quotes for ASD installation.
- Above 10.0 pCi/L: Significantly elevated. Mitigate as soon as practical. Some homeowners limit basement occupancy until mitigation is installed.
Front Range Colorado median results often cluster in the 4 to 8 pCi/L range. Mountain-corridor and granite-bedrock homes can read 15 to 25+ pCi/L in extreme cases. CDPHE’s county-by-county distribution data shows Boulder, Jefferson, Larimer, and Douglas counties with 50%+ of tested homes at or above 4.0 pCi/L.
Common Test Errors and How to Avoid Them
- Placement near a window or exterior door: Falsely low results from outdoor-air dilution
- Whole-house fan running during test: Falsely low results from outdoor-air dilution
- Test placed on floor: Floor-level humidity and condensation can interfere with charcoal media
- Test placed in laundry room with dryer venting: Outdoor-air infiltration via dryer vent skews results
- Mailing delays: Charcoal tests degrade past their analysis window — mail same day or next-day after retrieval
- Mid-renovation testing: Construction activity disturbs soil-gas patterns; wait 2 weeks after major work before testing
Where to Get a Radon Test Kit
Multiple sources:
- Hardware stores and big-box retail: $15 to $30 short-term kits with lab analysis included in price
- State radon programs: CDPHE offers free kits for Colorado residents in some counties — check the county map
- Online retailers: Same kits typically slightly cheaper online; same lab analysis
- Certified home inspectors: Continuous-monitor test bundled with inspection or as standalone $150-200
- Real-estate transaction radon services: Continuous-monitor test bundled with home inspection or as ancillary
Our where can I get a radon test kit guide covers each source in depth with current pricing and turnaround times.
Seasonal Variation and Why It Matters
Indoor radon levels are typically highest in winter when homes are closed up and the temperature differential between warm interior and cold exterior creates a stack effect — warm indoor air rises and exits through upper-floor windows and roof penetrations, drawing replacement air (and soil gas containing radon) up from below the foundation. In summer, with windows open and reduced stack effect, indoor radon levels drop substantially in many homes.
This means a short-term test in February can show 8 pCi/L while a short-term test in July of the same home can show 2 pCi/L — both correct, both representing different exposure scenarios. Long-term alpha-track tests (90 days or longer) average across seasonal variation and give a better picture of annual cumulative exposure.
For Front Range Colorado homes, winter testing typically produces the most actionable results because it represents worst-case household exposure during the heating season when homes are most closed up. Real-estate transaction tests happen year-round regardless of season; the documented closed-house protocol normalizes the result. CDC radon guidance acknowledges seasonal variation and recommends long-term testing follow-up when short-term results are marginal.
Multi-Zone Homes and Where to Place Multiple Tests
For homes with multiple living levels — basement plus first floor plus second floor — EPA recommends one test in the lowest livable level for screening purposes. If the screening result is elevated and you want a more complete picture, place a second test on the first floor for comparison.
Multi-zone HVAC homes can have different radon levels in different zones because the air-handling differs. If zones are physically separated (a basement zone with its own thermostat and ducting, separate from the main-floor zone), test each zone separately. If zones share ducting, one test in the lowest occupied level suffices.
Open-concept homes with vaulted ceilings and minimal floor separation should be tested in the lowest level — single test usually suffices. Loft-conversion situations with one ground-level and one mezzanine level: test the ground level.
Continuous Monitor Specifics for Real-Estate Transactions
Real-estate transaction testing typically uses a continuous radon monitor (CRM) rather than a charcoal canister. The CRM records hourly readings over 48 hours minimum, producing a graph that shows pattern variability and confirms closed-house protocol compliance. Common CRM models in 2026: Sun Nuclear 1027 and 1028, Femto-Tech CRM-510, AirThings Corentium Pro, Radon Eye Plus. Per EPA testing protocols, CRMs require annual calibration certificates from manufacturers — ask any inspector deploying a CRM to confirm their instrument’s calibration is current.
The hourly log on a CRM result is the documentation that protects both buyer and seller. If the seller opened windows mid-test, the hourly graph reveals it; if the buyer challenges the methodology, the calibration certificate and log provide the audit trail. This is why real-estate transactions favor CRMs over consumer charcoal kits despite the higher cost.
What to Do With the Results
Whatever the result, document it in writing. Keep the test report with home records. If selling, disclose to the buyer. If buying, request the seller’s prior test results (if any) and consider an independent test as part of the inspection.
If results are above the action level, get three quotes for ASD mitigation. Front Range pricing typically runs $1,000 to $2,500. CDC’s radon overview and American Lung Association’s radon program both align with EPA recommendations on testing-to-mitigation sequencing.
Testing Older Homes vs Newer Homes
Both older and newer homes can have elevated radon — the assumption that newer construction is automatically safer doesn’t hold across the board. Newer homes built without RRNC features (radon-resistant new construction passive vent stack, sub-slab vapor barrier, sealed slab penetrations) often test similarly to older homes in the same geology. Newer homes built with RRNC features tend to test lower because the passive stack provides natural depressurization even without an active fan.
For Front Range homes, the typical pattern: pre-1990 homes test elevated at higher rates because RRNC wasn’t standard construction practice. Post-2000 homes in Boulder County and other RRNC-required jurisdictions test elevated at lower rates because of the passive features built in. Even RRNC homes can test elevated and need active fan installation — the passive system reduces but doesn’t always eliminate the issue.
Newer-home buyers shouldn’t skip radon testing because the home is new. Test regardless of vintage. Per EPA’s testing recommendations, every home in EPA Zone 1 should be tested regardless of construction era.
References
- EPA — Radon testing protocols and action levels — U.S. Environmental Protection Agency
- CDPHE Radon Program — Colorado free-kit program and county data — Colorado Department of Public Health and Environment
- CDC — Radon health effects and testing recommendations — Centers for Disease Control and Prevention
- American Lung Association — radon testing context — Lung.org
Front Range homeowners ready to test or with results in hand can reach out via our contact page for connection to a certified radon professional.