The risk and the reference level
Radon comes from the natural decay of uranium present in certain ground, with granitic substrates as the characteristic case, which explains the Spanish geography of the problem: Galicia, wide areas of the peninsular west, Extremadura and the central mountain ranges concentrate the most affected municipalities. The gas rises from the ground, enters buildings through the elements in contact with the soil, cracks, joints, service penetrations, and accumulates in enclosed rooms, especially in habitable ground floors and basements.
The regulatory requirement is articulated around a reference level: the annual average radon concentration in habitable rooms must not exceed 300 becquerels per cubic metre, the value aligned with the European radiological protection framework. That number is the objective; the section's construction solutions are the means.
The geography of the requirement: the appendix of municipalities
The section applies in the municipalities listed in its appendix of municipalities where a significant radon potential has been identified, classified into two categories: zone I, of appreciable risk, and zone II, of high risk. Outside those municipalities, the section imposes no action; inside, it imposes graded action according to the zone.
The operational consequence is a design routine already anticipated in the document's general article and formulated here as a rule: consulting the appendix is a minutes-long check to be done when the file is opened, because it switches an entire package of solutions on or off. The detail of how to consult the classification and its practical implications is developed in the article on municipalities in radon zones.
The solutions required by zone
The section articulates protection with a repertoire of three families of solutions, combined according to the zone and the building's configuration. All three can be combined, and the choice depends on the zone and on how the building meets the ground.
The protection barrier is the base element: a continuous, radon-tight layer, with its conditions of material, continuity and sealing of junctions and penetrations, interposed between the ground and the habitable rooms. Its working plane coincides with that of the waterproofing of section HS 1, and that coincidence is a direct invitation to resolve both requirements with a joint design of the building's base.
The ventilated air cavity interposes between the ground and the rooms a space, a suspended sanitary floor in ordinary practice, whose ventilation dilutes and removes the gas before it reaches the habitable spaces, with defined opening and communication conditions. Its effectiveness depends on the openings staying clear for the building's whole life.
Ground depressurisation systems act at the source: collection pits or networks under the building connected to extraction ducts, creating a depression that draws the gas from the substrate and expels it above the roof, with natural or mechanical extraction as the case requires. Their design is coordinated with the foundations and with the drainage network.
The required combination grows with the risk. In general terms, zone I is satisfied with the barrier or the ventilated cavity in the usual configurations, while zone II requires the barrier accompanied by a second line, ventilated cavity or depressurisation, with the application particularities the section details according to the existence of basements and the use of the floors in contact with the ground.
| Zone | Typical solution | Nature of the cost |
|---|---|---|
| I | Barrier or ventilated cavity | Moderate item integrated into the building's base |
| II | Barrier plus cavity or depressurisation | Double line, higher cost and trade coordination |
What protecting costs, what repairing costs
In planned new construction, radon protection is a contained item: the barrier is one more membrane in the base package, with its careful sealing as the real cost, the cavity leverages suspended floor solutions many projects already adopt for other reasons, and depressurisation, the most expensive route, is reserved for the highest-risk configurations. As an indicative order of magnitude, the package moves in fractions of a percentage point of the PEM (presupuesto de ejecución material, the direct construction cost) in ordinary zone I cases, scaling up in zone II and in complex configurations.
The arithmetic changes completely when acting on the already built: measuring, diagnosing the entry paths and retrofitting a building in use, sealing, added extraction systems, is expensive intervention with less guaranteed results. The asymmetry repeats the pattern of all health requirements: designed protection costs little, retrofitted protection costs a lot. In renovation, the section applies to interventions under the code's general terms for existing buildings, and works touching the building's base in an affected municipality are the natural, and economically rational, moment to incorporate the protection.
Frequent errors
The first is not consulting the appendix at kick-off, the package's foundational error, with the requirement discovered at permit stage and the ensuing change order. The check belongs to the appointment and takes minutes.
The second is treating the barrier as a membrane to be rolled out rather than a tightness to be built: radon enters through the missing seals, service penetrations, joints, junctions, exactly where careless execution leaves gaps. The seals are detailed one by one and controlled on site.
The third is designing the radon barrier and the HS 1 waterproofing separately, duplicating layers or, worse, making them incompatible. Both share a working plane and are resolved in the same detail.
The fourth is forgetting the operation of active systems: a mechanical depressurisation is a fan that must run and be maintained, and its silent stop cancels the protection without anyone noticing. The maintenance is recorded in the building's log book.
The fifth is ignoring radon in renovation appraisals and property purchases in affected municipalities, where prior measurement is a cheap diligence that avoids inheriting an expensive problem. Prior measurement costs little and changes the purchase price.
Note: the reference level, zones and solutions cited correspond to section HS 6 incorporated by Royal Decree 732/2019; each municipality's classification and the applicable conditions are those of the appendix and text of the consolidated DB-HS in force.