Basics of heat transfer

Heat transfer in buildings refers to the movement of heat from one location to another within a building. Heat transfer may occur through the walls, roofs, windows, etc. transferring heat between the exterior of the building and the interior. Heat may also be transferred from internal gains to the building spaces. Heat transfer processes can have a significant impact on the overall comfort and energy efficiency of a building. This page provides an overview of the three main mechanisms of heat transfer in buildings: conduction, convection, and radiation.

Conduction

Conduction is the transfer of heat through direct contact between two objects, without any movement of the objects themselves. In buildings, conduction occurs through various building materials, such as walls, floors, and roofs. For example, heat is transferred between the layers of a wall assembly. Heat can also transfer from a warm wall to a cold window through direct contact, causing the wall to lose heat to the window (and out of the building). The rate of heat transfer by conduction depends on the conductivity of the building material and the temperature difference between the two surfaces.

Building materials are often specified in terms of their resistance to conduction, represented by a value called the R-value. An overall resistance can be calculated for multiple layers of materials (used to construct walls, roofs, etc.). This can be represented by an overall R-value, or more often by the inverse of the R-value (1/R) which is called the construction's U-value.

Convection

Convection is the transfer of heat by the movement of a fluid, such as air. In buildings, convection is the most common way heat is transferred from one object to another or one location to another. For example, heat sources in a room such as the lighting and equipment can heat the air around them via convection. Another example is when warm air rises and cooler air falls, this can cause heat to be transferred from a warm room to a plenum (above the ceiling) or upper floors if the air rises through stairwells, elevator shafts, and other openings between floors.

Convection is also the mechanism used by many heating and cooling systems, where warm air is circulated through a building to provide heating, and cool air is circulated to provide cooling. The rate of heat transfer by convection depends on the velocity of the fluid, the temperature difference between the two surfaces, and the heat capacity of the fluid.

Radiation

Radiation is the transfer of heat through electromagnetic waves, which do not require a medium for transfer. In buildings, radiation occurs when heat is transferred from a warm object, such as a radiator, to a cooler object, such as a wall or a person. Other objects inside the building that produce radiation gains in a space include lighting and plug load equipment. Radiation can also occur through windows, where heat is transferred from the sun to the interior of a building. The rate of heat transfer by radiation depends on the surface area of the emitting and receiving surfaces, the distance between the surfaces, and the temperature difference between the surfaces.

Sensible and latent heat gains

Sensible heat gain refers to the heat that is transferred through conduction, convection, or radiation, and can be felt or sensed by human beings. Sensible heat gain typically comes from sources such as solar radiation, people, lights, and equipment.

Latent heat gain, on the other hand, refers to the heat that is transferred as moisture or humidity. Latent heat gain typically comes from sources such as respiration, perspiration, evaporation from surfaces, and infiltration of humid outdoor air into the building.

The main difference between sensible and latent heat gain is the way in which heat is transferred. Sensible heat is transferred through a temperature difference, while latent heat is transferred through a moisture difference. As a result, the two types of heat gain require different strategies for controlling indoor temperatures and humidity levels.

In buildings, it's important to manage both sensible and latent heat gain to maintain a comfortable and healthy indoor environment. HVAC systems can be designed to address both types of heat gain, by providing heating, cooling, and dehumidification as needed.

Heat transfer in buildings

The sections above provide some examples of how each mode of heat transfer is often present in a building, but it's important to understand that multiple modes of heat transfer are often occurring simultaneously. Lighting and equipment will produce radiation and convective gains. Walls will conduct heat through their layers, but also warm the air by convection. These concepts are explored in more detail in the Building Science section of this wiki.

Energy efficient design is highly focused on trying to reduce heat transfer, thereby minimizing energy from HVAC systems to offset the building's heat gains or losses. Proper insulation and air sealing can help reduce conductive and convective heat transfer to the building exterior; design features such as double-paned windows and shading devices can help reduce solar heat gain and reduce the amount of heat transferred into a building. Minimizing the amount of lighting and equipment power will reduce internal gain heat sources.