Heat travels in and out of buildings in three ways: radiation, convection, and conduction, all of which must be addressed to reduce unwanted heat transfer effectively.

Radiation

Radiation is the transfer of heat from a warmer body to a cooler one (regardless of position). The way to stop radiation heat transfer is by using reflective surfaces.

A reflective roof, for instance, can reduce solar heat gain through the roof by up to 40%. Radiant barriers in attics or crawlspaces can also be used to reflect heat away from or back into occupied spaces of a building. Using light pavement surfaces (or better yet, reducing pavement as much as possible) will lower ambient air temperature around a building, thus reducing the building’s cooling load.

High-performance window glazing often includes a thin film or films to reflect infrared light (heat) either out of a building (in a hot climate) or back into a building (in a cold climate).

Passive solar design in cold climates usually involves allowing the sun’s radiation to enter a building and be absorbed into thermal mass for re-release later.

Convection

Convection is the transfer of heat in a fluid or gas, such as in air. Green buildings achieve natural ventilation by using convective forces, such as wind, and differences in humidity and temperature.

Typically, we experience convection as unwanted heat loss. It is what we experience when we feel a cold draft next to a leaky window or when a door is opened and cold air rushes in. Methods of preventing convective heat transfer include:

• providing an air barrier; sealing gaps around windows, doors, electrical outlets, and other openings in the building envelopeM
• providing air-lock entrances
• using heat recovery ventilators, which transfer 50%–80% of the heat from exhaust air to intake air in cold climates, and vice versa in hot ones. They are an excellent way to ensure adequate ventilation in a tightly sealed house, while maintaining high energy efficiency.

Conduction

Conduction is the transfer of heat across a solid substance.

Every material has a specific conductivity (U-value) and resistance (the inverse of the U-value, called the R-value). Insulation is made of materials with particularly high resistance to conductive heat transfer (high R-values).

In climates with significant indoor/outdoor temperature differentials, it is important to insulate the entire building envelope—roof, walls, and foundation. Although heated or air conditioned buildings in any climate benefit from insulation, the greater the indoor/outdoor temperature differential, the more insulation is needed.

Related Articles

• Reducing Energy Use in Buildings
• Limiting Heat Loss Through Windows
• Efficient Cooling Methods for Green Buildings

This article is an excerpt from the book Green Building: Project Planning & Estimating which can be purchased through the RSMeans Bookstore.

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