It is a common experience to find indoor temperatures surprisingly higher than the outdoor air, even on a warm day. This temperature difference stems from fundamental principles of physics and the intricate ways buildings interact with heat energy. Understanding how heat moves and accumulates within structures explains why an indoor environment can become warmer than its surroundings.
The Basic Science of Heat Movement
Heat energy naturally transfers from warmer areas to cooler areas. This transfer occurs through three primary mechanisms. Conduction involves heat moving directly through materials by physical contact, such as heat transferring through a wall.
Convection is the transfer of heat through the movement of fluids. This occurs as warmer, less dense fluid rises, and cooler, denser fluid sinks, creating a circulation. Warm air rising in a room is an example.
Radiation is the transfer of heat through electromagnetic waves, which does not require a medium. The warmth felt from the sun or a fire is due to thermal radiation.
How Heat Enters Indoor Spaces
External heat primarily enters buildings through solar radiation, material conduction, and air infiltration. Sunlight directly warms a building’s surfaces like roofs, walls, and especially windows. When solar radiation strikes glass, some is transmitted through, and some is absorbed, increasing the temperature of the glass itself.
Heat also moves into indoor spaces through conduction via building materials. Walls, roofs, and floors can transfer heat from the warmer outdoor environment to the cooler interior. Different building materials conduct heat at varying rates; for instance, metals have high conductivity, while materials like fiberglass and foam have low conductivity.
Air infiltration further contributes to heat gain, occurring when warmer outdoor air enters a conditioned space through unintentional openings. This can happen through cracks around windows and doors, or other gaps in the building envelope, driven by wind pressure and temperature differences. This bulk movement of warm air directly increases the indoor temperature.
Why Heat Gets Trapped Inside
Once heat enters a building, several factors contribute to its retention. Insulation, while designed to slow heat transfer, can also trap heat inside if the exterior cools down. Insulation materials work by resisting conductive and convective heat flow, often by trapping air within their structure.
Lack of adequate ventilation is another factor, as it prevents hot air from escaping. Without a balanced system of intake and exhaust vents, such as soffit and ridge vents, hot air can accumulate in spaces like attics, leading to high temperatures. This lack of airflow means that heat cannot easily move out of the building through convection.
The “greenhouse effect” within buildings plays a substantial role, particularly through windows. Glass allows shortwave solar radiation from the sun to enter, which is then absorbed by interior surfaces and re-radiated as longwave infrared radiation. Glass is largely opaque to this longer wavelength radiation, trapping the heat inside and causing temperatures to rise. Furthermore, heat generated from within the building, such as from occupants, lighting, and appliances, adds to the total heat load, compounding the effect of trapped external heat.
Everyday Examples of Indoor Heat Buildup
The principles of heat entry and trapping are evident in common scenarios. A parked car on a sunny day provides a clear illustration. Sunlight penetrates the car’s windows, and the interior surfaces, like seats and the dashboard, absorb this solar radiation. The absorbed energy is then re-radiated as longwave infrared, which the car’s windows largely trap, leading to a rapid temperature increase inside. On an 80-degree Fahrenheit day, a car’s interior can reach 109 degrees Fahrenheit in just 20 minutes.
Unventilated attics also demonstrate heat buildup. Solar radiation heats the roof, and this heat conducts into the attic space. Since hot air rises, it accumulates within the attic, and without proper ventilation to allow this hot air to escape, temperatures can exceed 130 degrees Fahrenheit, sometimes even reaching 150°F on hot summer days.
A room with large, uncovered windows facing the sun experiences increased temperatures due to direct solar gain. The sunlight passes through the glass, heats the interior elements, and the resulting longwave radiation becomes trapped, creating a localized greenhouse effect. This phenomenon causes the room to warm considerably, even if the air outside is cooler, highlighting how building design and materials influence indoor thermal conditions.