This paper describes the criterion for the protection of building envelopes against the growth of mould. As a criterion for assessing the risk to envelopes, the fRsi temperature factor is adopted. The paper provides the resultant temperature factor fRsi,max for the critical month in 61 areas in Poland for which typical year-long meteorological data is available on the website of the Ministry of Infrastructure and Development. While calculating the temperature factor, various room humidity classes were taken into account. The results of calculations of the temperature factor fRsi,max have been illustrated with isolines drawn for the whole area of Poland.