In most studies the effect of the high thermal capacity of the building envelope on interior temperatures is considered as reduced to a 24-hour period within which daily heat gains are balanced by night-time heat losses. The obtained effect is evaluated by the reduction of the maximum indoor temperature, the reduction of the diurnal variation of the indoor temperature and the increase of the time delay of the occurrence of the maximum daily temperature. The results of research conducted in the summer of 2018 in two identical detached residential buildings differing only by the thermal capacity of the walls (cellular concrete walls versus walls on a lightweight frame structure) allow us to believe that during the period when the outdoor temperature increases on consecutive days, the difference between the indoor temperature in low and high thermal mass building also increases systematically. The temperature difference in rooms with different thermal capacity is determined not only by the diurnal pattern of outdoor temperature and solar radiation, but also by their cumulative effects from the preceding days. The difference in peak temperatures in different rooms of the two buildings increased from 0 to as much as 3.5 K on consecutive days. On the other hand, when the outside temperature significantly and systematically decreased, the temperature difference in individual rooms of both buildings decreased in approximately the same range. The results of presented research indicate that in single-family detached buildings in moderate climate, the effect of positive influence of increasing wall thermal capacity on internal temperatures during hot weather increases systematically for consecutive days of increasing outdoor temperature, causing that in the building with higher wall thermal capacity both maximum daytime and minimum night-time temperatures decrease by 2 - 3.5 K.