Radon Survey

What is a radon survey?

A radon survey is a soil gas measurement conducted at the outset of site assessment to evaluate the concentration of radon—a naturally occurring radioactive gas—in the ground beneath a proposed building. The survey determines the radon risk category of the site (low, medium, or high) and directly informs the design of the foundation radon barrier and protection systems. This assessment is a mandatory step under Slovak law before any residential or commercial building can proceed to schematic design.

Why is radon testing required in Slovakia?

Slovakia lies in a region of elevated natural radon emission. Act No. 87/2018 on radiation protection establishes that indoor radon must not exceed 300 Bq/m³ (becquerels per cubic meter) annually. This reference level exists because radon is the second-leading cause of lung cancer after smoking, responsible for an estimated 3–14% of all lung cancers depending on national exposure levels. The country has identified 99 municipalities classified as radon-prone areas where soil radon concentrations regularly exceed safe thresholds. As a result, all building developments in Slovakia must undergo radon risk assessment before foundation design can proceed, and proof of measurement must be submitted with building permit applications and verified again before occupancy approval.

How is a radon survey conducted?

A certified specialist measures soil gas radon concentration using a probe inserted to a depth of approximately 0.8 meters—deep enough to avoid surface air dilution while representing conditions beneath the future building. The measurement is typically performed using a RAD7 or equivalent alpha-particle detector, which counts the decay of radon progeny (polonium-218 and polonium-214) to determine activity concentration in Bq/m³. Simultaneously, soil gas permeability is measured at the same location using a specialized permeameter, which indicates the ease with which radon can travel through soil toward the building. Measurements are conducted during dry periods, as soil moisture content significantly affects both radon mobility and detector accuracy. The complete survey covers multiple test points across the building footprint to account for spatial variation in soil properties. Results are documented in a protocol that records radon concentration, soil permeability classification, and assigns a final radon index (low, medium, or high), which becomes the basis for foundation protection design decisions.

What are the radon risk categories and their mitigation approaches?

How does radon survey determine the foundation strategy?

The radon survey output—the soil radon concentration and permeability classification—becomes a controlling parameter in foundation design. For passive-house and energy-efficient homes common in Slovakia, the foundation must serve dual purposes: thermal performance and radon exclusion. Low-risk sites require no special radon measures beyond standard construction practice. Medium-risk sites mandate installation of a plastic radon barrier (typically 0.15 mm polyethylene) beneath the entire foundation slab, positioned to block radon ingress while managing groundwater and capillary rise. High-risk sites require active mitigation—usually a sub-slab depressurization (passive or active) system that maintains slightly negative air pressure in the soil beneath the building, forcing radon to vent to the exterior rather than enter the structure. The barrier and ventilation measures must be installed and tested before occupancy, with post-construction radon measurements submitted to authorities for verification.

How does radon survey differ from a geotechnical survey?

While both surveys occur during the site analysis phase, they answer different questions: radon survey addresses chemical-radiological risk, while geotechnical survey addresses mechanical soil behavior. Both inform foundation strategy—radon risk determines the barrier type and ventilation system, while soil properties determine depth, bearing capacity, and settlement control.

What are common misconceptions about radon surveys?

A frequent misunderstanding is that radon survey results are optional or can be waived for well-ventilated buildings. In Slovakia, the survey is legally mandatory regardless of future building performance; the assessment determines what passive or active radon controls must be designed in. Another misconception is that high radon risk makes a site unsuitable for building—in reality, high-risk sites require engineered protection (radon barriers and ventilation) that, when properly installed and verified, reliably prevents radon entry. Lastly, some assume radon is confined to particular geological zones; while certain municipalities are designated radon-prone, radon can occur sporadically even in lower-risk areas, making site-specific measurement the only reliable assessment method.

How does post-construction radon verification work?

After the building envelope is sealed, a post-construction radon measurement is required to verify that mitigation measures have been effective. This measurement is typically conducted several weeks after occupancy to allow radon equilibration within the structure. If measured indoor radon levels exceed the legal reference level of 300 Bq/m³, additional corrective measures must be implemented. These verification protocols are submitted to the relevant public health authority as proof of compliance with Act No. 87/2018, and the documentation becomes part of the building's permanent record.