When positioned correctly inside the house, thermal mass can store heat during the day in winter, and re-radiate it at night time. During summer the thermal mass is protected from direct solar radiation and can act as a heat sink in hot weather to give cooler indoor temperatures. Insulation is vital to maintain even temperatures in a passive solar home. Insulation keeps warmth inside the house in cold weather, and prevents heat from entering when it is hot. There are many types of insulation available today to suit a wide range of needs.
Windows are also an important consideration.
If a house is well sealed with an insulated ceiling and walls, then the windows will be the weak point for heat transfer in and out of the house. In climates that experience cold weather, large north facing windows are required to allow the winter sun in during cold weather, and naturally their value will be diminished if they then allow that day's heat to escape. Close fitting heavy curtains or blinds with a pelmet will reduce heat transfer. Double glazing will reduce heat transfer by a similar amount to effective curtains, but will be more expensive. There are now readily available double glazing films that are cheap and easy to apply to most existing windows.