This paper is devoted to analysis of influence of external wall thermal mass on indoor air parameters stability and energy performance of buildings. The stability of indoor air parameters, i.e. minimal fluctuation of temperature and relative humidity, is of particular importance for such buildings as museums, schools and hospitals. The paper presents analysis of thermal mass impact on buildings energy performance and indoor air parameters stability in cold and humid climate of Baltic region. In winter time the average outdoor air temperatures fluctuation is ±10. In some cases the outdoor temperature can rapidly drop below heating system design temperature. In that case it is difficult to insure stability of indoor air parameters. In the scope of this paper the dependency between heat boiler capacity and level of thermal mass of building’s envelope was evaluated. The paper also presents simplified approach for practical use of thermal mass in design of heating systems’ maximal capacity. The existing criteria of building thermal mass – heat inertia – could be used for evaluation of heating systems’ maximal capacity for winter time. The average temperature of coldest 5 days with probability of 0,92 cannot be used as heating design temperature for timber-frame building envelopes in Latvian climatic conditions.