The choice of foundation type affects the radon concentration. Furthermore, the foundation type affects the number of radon-technical solutions needed. In areas with very high measured radon levels, the choice of foundation is essentially important in terms of successful radon control. The critical point is how efficiently the foundation solution prevents radon-bearing air in the soil from entering the inside of the building.
Radon-safe foundation types include
- Crawl-space construction
- Monolithic slab foundation
- Ground slab foundation, with the slab being cast separately inside the foundation wall – providing that the tightness of the joint between the slab and the footing is ensured.
In houses with a crawl space construction or a monolithic slab, it is considerably less common to find radon concentrations above the maximum value compared with houses that have a ground slab separately cast inside the foundation wall.
A high-quality floor slab will not let high quantities of radon through, unless it has openings or cracks.
Radon-bearing air from the soil is diluted in the crawl-space, providing that it is ventilated. Sufficient ventilation is achieved by following the regulations concerning the size of vents. The radon-technical efficiency of a crawl-space construction is affected by the sufficiency of ventilation and the leakage tightness of the base floor. Particular attention must be paid to the leakage tightness of the base floor construction and the included joints and penetrations. At best, radon levels in wooden houses with a crawl space foundation are very low, below 20 Bq/m3.
Even in new houses with a crawl space foundation, radon concentrations above the maximum level of 200 Bq/m3 have been measured. This has resulted from shortcomings such as the following:
- Pipe penetrations have not been sealed
- Ventilation in a low crawl space is insufficient and the leakage tightness of the floor construction is lacking.
- Ventilation is prevented due to the use of mechanical drying of the crawl space.
Monolithic slab foundation
The best solution for ground-supported base floor is a base floor and foundation structure that is as joint-free and leakage-tight as possible, without much need for sealing. A monolithic slab is an example of such a construction. Penetrations may constitute a source of radon, as well as the concrete material of the slab, but its effect is normally only approximately 20 Bq/m3.
Thus, the sealing of penetrations is important also in this solution. A high-quality monolithic slab does not have cracks that would facilitate air leakage from the soil.
Ground-supported walls built on a monolithic slab may increase radon leakage. The recommendations of the RT reference card must be followed in order to prevent this.
Foundation wall and ground-supported base floor
In recent decades, the most common floor construction in low-rise residential buildings has been a ground-supported floor slab that is separately cast inside the foundation wall. Flow of radon-bearing air through joints in such a base floor is the most significant source of radon in dwellings. The RT reference card focuses mainly on the radon-safe implementation of this solution. The instructions include two key measures:
- The joint between the ground slab and footing is sealed by installing a strip of bitumen felt on top of the footing and at the bottom of the slab edge.
- In addition, radon piping is installed under the slab. If the control measurement in the completed house shows that the radon concentration exceeds the maximum value, the piping is activated by installing suction to the piping.
The sealing of penetrations is important also in this solution.