The use of thermal mass needs to be considered carefully
Thermal mass has amazing benefits if used correctly. However, if it is not used correctly, it can increase your winter heating requirements.
In some cases thermal mass can create a negative winter effects with increased winter energy requirements. Thermal mass requires adequate solar gain to be effective. Thus you will need adequate north facing windows which are not affected by overshadowing in winter.
If thermal mass isn’t adequately heated by the solar gain it will need to be heated by supplementary heating. You will not only need to heat the air, you will also need to heat the thermal mass. An adequate area of north-facing glass can help offset this effect.