What is the Definition of a Building Envelope?

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The roof, exterior walls and floors of a structure form the building envelope
The roof, exterior walls and floors of a structure form the building envelope (Image: building 2 image by jimcox40 from Fotolia.com)

The building envelope consists of the roof, exterior walls and floor of a structure. These elements form a barrier that separates the interior of the building from the outdoor environment. According to the website of the Pew Center on Climate Change, the building envelope determines how much energy will be needed to maintain a comfortable indoor environment relative to outdoor conditions. A properly designed building envelope can substantially reduce heating costs in the cold seasons and cut cooling costs in hot seasons.

Insulation Counts

Certain elements of the building envelope play a major role in determining a building’s energy use and consequent costs and greenhouse gas emissions, according to the Pew Center website. One important element is insulation, which resists heat flow between indoors and outdoors and thereby cuts heating and cooling costs. Insulation effectiveness is rated by R-value. The higher the R-value number, the more resistant the insulation is to heat flow.

Airtight

Adding insulation to the building envelope is only half the battle, said the Pew Center website. The owner also needs to seal any air leaks with caulk, foam or weatherstripping. Proper insulation and air sealing of walls, roof and floors can cut energy bills by 12 percent.

Roofs and Walls

Proper roof design can substantially reduce air conditioning costs by increasing the amount of solar heat that’s reflected rather than absorbed. Roofs that meet Energy Star standards can cut cooling costs by up to 15 percent. Roofs also present opportunities to install photovoltaic systems that can cut utility bills by producing some or most of the electrical energy to run the building. The right wall materials can increase a building’s thermal mass, meaning the walls will gain heat slowly but also lose it slowly. This reduces indoor temperature fluctuations and reduces overall heating and cooling requirements.

Windows, Doors & Skylights

Windows, skylights and exterior doors essentially are holes in the walls and roof that admit light and people. Placement of these elements can affect lighting costs by taking maximum advantage of natural daylight. Energy efficient windows, doors and skylights reduce heat flow between indoors and outdoors. The Pew Center website says optimum window and door placement with energy efficient glazing can reduce energy consumption by 10 percent to 40 percent.

Local Climate’s Effects

The local climate plays a major part in selecting building envelope design features that result in the greatest reduction in energy usage. For instance, you may want large south-facing windows to capture passive solar heat in cool climates, but shading of walls and windows to minimize heat gain in hot climates.

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