The overall envelope approach treats envelope components as a system and offers the ability to make simple trade-offs between envelope components. §143(b) of the Standards describes the overall envelope approach. The overall envelope approach allows the performance of some building envelope components to be increased while the performance of others is reduced, as long as overall heat gain and loss are no greater than a building in minimum compliance with the prescriptive requirements.
The overall envelope approach permits tradeoffs between many building envelope components, but no tradeoffs are permitted with the indoor lighting system or mechanical systems. The performance approach is required in order to make these tradeoffs.
The overall envelope approach uses two measures of envelope performance: the overall heat loss and the overall heat gain. The overall heat loss is a measure of the insulating quality of all the envelope components together,including both opaque and glazing surfaces. The overall heat gain considers insulation, solar heat gain through windows and skylights, and the reflectance of the roof.
The code baseline for both heat gain and heat loss is determined using the insulation and solar heat gain coefficient values from the prescriptive requirements, applying them to the envelope surface areas of the proposed building (with some limits on glazing area). The proposed design’s overall heat loss and heat gain are calculated based on the installed insulation, fenestration
performance, and roof surface properties. If the proposed heat loss and heat gain are no higher than the standard heat loss and heat gain, then the envelope complies. See Section 3.7 for a more complete discussion of the overall envelope approach.
The overall envelope approach offers greater design flexibility. It allows the designer to make trade-offs between many of the building envelope components. For example, if a designer finds it difficult to insulate the walls to a level adequate for meeting the wall component U-factor requirement, then the insulation level in a roof or floor or the performance of a window component could be increased to offset the under-insulated wall. The same holds true for glazing. If a designer wants to put clear, west-facing glass to enhance the display of merchandise in a show window, it would be possible to use lower SHGC glazing on the other orientations to make up for the increased SHGC on the west.
The overall envelope approach has two parts, and both parts must be met: overall heat loss and overall heat gain. The overall heat loss accounts for the insulating qualities of the building and sets a maximum rate of conductive heat transfer through the building envelope. The requirements are more stringent in more extreme climate zones than in mild climate zones. The overall heat gain accounts for the area of windows and skylights and their ability to block solar heat gains, thereby reducing cooling loads on the building. Cool roofs are also accounted for in the overall heat gain calculations. The heat gain requirements are more stringent in warmer climate zones.
A standard design value and a proposed design value are calculated for both the overall heat loss and the overall heat gain. The standard design building complies with the exact requirements of the prescriptive approach. The standard values are compared to the proposed values calculated from the actual envelope design. If the proposed values do not exceed the standard values, then the overall building envelope requirements are met. While the overall envelope approach increases design flexibility, this comes at the expense of the complexity of the calculations.