The building envelope consists of those elements of a building that enclose conditioned spaces through which thermal energy may be transferred. Energy is saved when the heat exchange between the building and the outside environment is reduced and solar and internal heat gains are controlled. Many of the audits discussed in this chapter apply the principle of reducing the heat load or gain of the building. The envelope audit should record for each space the size, physical characteristics, Hours of operation and function. The assorted materials of construction, windows, doors, holes, percentage glass, etc. should also be recorded. Leakage or infiltration of air into a building is similar to the effect of additional ventilation. An alternate method to reduce infiltration losses through access doors is to provide an air curtain. Poor quality outdoor air dampers are another source of excess infiltration. Replacement with good quality opposed-blade dampers with seals at the blade edges and ends will reduce infiltration losses.