The ceiling joins in. Above a sunlit car, a concrete slab absorbs energy through conduction and radiation, then releases it after direct light has moved, meaning the garage can keep loading the vehicle with heat when obvious glare is gone. The lag is real. Thermal capacitance makes the slab a slow heat battery, a metaphor grounded in sensible heat storage.
This delay matters most. Concrete's thermal diffusivity governs how quickly stored energy reaches the lower surface, while its specific heat capacity sets how much energy the material can hold for each degree of temperature change. Heat travels in layers. As that lower face warms, longwave infrared radiation reaches the car and convection warms the air around it, adding load to glass, metal, seats, and cabin surfaces.
Designers should stop guessing. A garage that maps slab thickness, orientation, airflow, and surface temperature can identify bays where delayed release turns shade into an illusion, then use ventilation, reflective finishes, or layout changes to reduce the heat burden. Sensors can close the loop. If building controls learn these thermal rhythms, a parking deck becomes an active climate machine, shifting comfort and energy demand before a driver opens the door.
