What is 'Thermal Lag'?

Thermal lag is the delay of heat transmitted through a wall. It's a measurement of the ability of walling material to slowly absorb and release heat energy. Only brick and other heavyweight materials offer this property. Exposing a brick wall to winter sunlight can be crucial to passive solar hearing design in regions with large swings of temperature from day to night. While insulation will stop heat, brick stores heat from sunlight during the day and releases it during winter evening hours. Because of the higher angle of the midday sun during summer months, a passive solar heating design will throw shade on the brick wall during summer months to block sunlight, keeping a home cooler.