The temperature inside a building is never static, but results from a constant thermal exchange with the exterior environment. Buildings continually work to maintain a controlled thermal balance against outside weather conditions. Understanding the physical principles governing how heat moves in and out is fundamental to improving occupant comfort and energy efficiency.
The Three Mechanisms of Heat Transfer
Heat energy always moves from warmer areas to cooler areas through three distinct physical mechanisms that work simultaneously within a structure. The first mechanism is conduction, which is the movement of heat through direct physical contact between solid materials. Conduction occurs as vibrating molecules in a warmer object transfer kinetic energy to adjacent molecules in a cooler object, such as when heat passes through a wall or window pane.
The second mechanism is convection, which involves the transfer of heat through the movement of fluids, specifically air or water. In a building, convection occurs when warm air rises and cool air sinks, creating circulation patterns. It also happens when air leaks through openings in the building envelope, which significantly impacts indoor temperature stability.
The third mechanism is radiation, which transfers heat as electromagnetic waves and does not require a medium for movement. Within a building, warm surfaces, such as a sun-heated roof, emit infrared radiation. This radiation is then absorbed by cooler interior surfaces or objects, contributing to the overall thermal flow between the inside and outside.
Primary Pathways for Temperature Exchange
The physical structure of a building, known as the envelope, dictates the primary pathways for heat exchange. Walls and roofs are major surfaces for heat transfer, with conduction moving heat through solid materials. Radiation contributes significantly to solar heat gain, especially through the roof in warmer months, and the attic space can be a large source of heat gain or loss.
Uncontrolled air leakage, known as air infiltration, is often the most substantial source of heat transfer via convection. This occurs through small cracks, gaps, and poorly sealed areas around utility penetrations, window frames, and door sills. Windows are also a major pathway for thermal exchange, contributing to heat loss through conduction and heat gain through solar radiation. The cumulative effect of air leakage can account for a considerable portion of the total energy required for heating and cooling.
Key Factors Resisting Temperature Transfer
To resist heat transfer, materials with specific thermal properties provide a barrier between the indoor and outdoor environments. Insulation is designed primarily to resist conductive heat flow, and its effectiveness is quantified by its R-value, which stands for thermal resistance. A higher R-value indicates a greater capacity to slow the movement of heat, helping maintain a consistent indoor temperature.
Addressing thermal bridging is necessary, which is the unintended movement of heat through highly conductive materials like wood studs or metal framing that bypass the insulation layer. A thermal break, a low-conductivity material placed between conductive components, interrupts this flow and restores the assembly’s intended thermal performance. Air sealing techniques, such as caulking and weatherstripping, are applied to the building envelope to mitigate convective heat loss by controlling air leakage. For windows, specialized low-emissivity (low-e) coatings reflect radiant heat, blocking solar gain in the summer while retaining interior heat in the winter.
Thermal Lag and Building Inertia
The relationship between outside and inside temperature is complicated by thermal lag, which is the time delay between a change in outdoor temperature and the resulting change felt indoors. This delay results from the building’s thermal inertia, which is the capacity of the structure’s mass to absorb and store heat energy. Materials with high thermal mass, such as concrete or brick, absorb heat slowly throughout the day, preventing it from immediately passing into the interior space.
This heat absorption dampens and delays peak temperature effects, shifting the time when the greatest heat load enters the building to several hours later. For instance, a hot afternoon may not cause the indoor temperature to peak until late in the evening. This heat storage capacity stabilizes the indoor climate, reducing temperature variation and providing a gradual transition in response to external fluctuations.