Based on field studies of winter and summer indoor thermal conditions of school buildings with or without moderate thermal mass (MTM) in Auckland, New Zealand, this study identified difference of indoor thermal comfort between the school indoor spaces with and without MTM in their building envelopes, and evaluate the impacts of MTM on winter indoor health conditions of the school indoor spaces with and without MTM under a temperate climate with warm, dry summers and mild, wet winters. Although the thermal inertia effect can only increase or decrease the indoor temperature by a few degrees in winter or summer, under a local climate characterized by an average diurnal temperature range, this study revealed that, when the indoor temperature is within thermal comfort conditions, a school interior space with thermal inertia offers approximately 20% more comfort time in summer compared to a school interior space without thermal inertia; when the indoor temperature reaches 18°C, this space offers approximately 30% more comfort time in winter compared to a school interior space without thermal inertia; and when the indoor relative humidity is within the optimal range of 40% to 60%, this space offers 20% more comfort time in winter compared to a classroom without MTM.