Sub-slab suction sucks air from under the floor slab. The underpressure developed under the slab and the ventilation effect efficiently reduce the flow of radon into the house.
Sub-slab suction usually reduces the radon concentration by 60–90 per cent. Sub-slab suction can be implemented through the floor slab or footing. Sub-slab suction through the floor is implemented by making a 20–30 litre suction pit under the slab. An exhaust duct is led from the suction pit to the roof, where a 50–100 watt extractor fan is placed.
With one suction point, it is possible to repair even a dwelling unit of 120 m2, if the house has a monolith slab and it is not divided into sections by load-bearing partition walls. It is best to locate the suction point centrally in the part of the building that affects the radon concentration in the living areas. When necessary, several suction points must be built.
The advantage of sub-slab suction implemented through the footing is that no repair work is needed indoors. On the other hand, one suction point through the footing is not necessarily enough.
Rated values have been assigned to the air sucked from soil, limiting possible problems caused by cold air. The rated air flow is 0.2 m3/h per each floor and wall square metre in contact with soil.
Sub-slab suction costs EUR 2,000–3,000, but by doing some of the work yourself you can save money. Increasing the number of suction points increases costs.
According to a survey, the most common problem detected in connection with radon mitigation was that excessive ice accumulated in one in three sub-slab suction systems. Sometimes the accumulation of ice was detected on the basis of a disturbingly loud noise. If enough ice accumulates in the extractor to make it stop, the radon concentration in the house rises to the original high level. Therefore, the functioning of the sub-slab suction system should be monitored, particularly during long periods of very cold weather, or if a lot of snow accumulates on the roof. Low-cost electronic alarm systems are available for the monitoring of the suction system.