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Low-energy and passive house construction practices are characterised by increased insulation, high air tightness of the building shell and controlled mechanical ventilation with heat recovery. As a result of the interaction of mechanical ventilation and high air tightness, the pressure difference in a building can be markedly enhanced. This may lead to elevated indoor radon levels. Minor leakages in the foundation can affect the radon concentration, even in the case where such leaks do not markedly reduce the total air tightness. The potential for high pressures to affect indoor radon concentrations markedly increases when the air tightness ACH 50 , i.e. the air change per hour induced by a pressure difference of 50 Pa, is &lt;1.0 h −1 . Pressure differences in Finnish low-rise residential houses having mechanical supply and exhaust ventilation with heat recovery (MSEV) are typically 2–3 Pa, clearly lower than the values of 5–9 Pa in houses with only mechanical exhaust ventilation (MEV). In MSEV houses, radon concentrations are typically 30 % lower than in MEV houses. In new MSEV houses with an ACH 50  of 0.6 h −1 , the limit for passive construction, the analytical estimates predict an increase of 100 % in the radon concentration compared with older houses with an ACH 50  of 4.0 h −1 . This poses a challenge for efficient radon prevention in new construction. Radon concentrations are typically 30 % lower in houses with two storeys compared with only one storey. The introduction of an MSEV ventilation strategy in typically very airtight apartments has markedly reduced pressure differences and radon concentrations.

Review of low-energy construction, air tightness, ventilation strategies and indoor radon: results from Finnish houses and apartments