Consider the snow

Designers need to be aware of roof systems' structural limitations in cold climates

When you are asked to replace a roof system that has exhausted its serviceable life, you know the building owner probably will be looking for ways to enhance the roof system's energy performance. And, of course, energy codes require minimum R-values that frequently necessitate increasing insulation within walls and roofs to reduce energy consumption. Although a seemingly worthwhile venture, there are consequences of adding insulation in colder climates, such as accounting for the additional dead load placed on the structure by the insulation, as well as snow loads.

Start with R-value

To illustrate the implications of adding additional R-value to an existing roof system, it is necessary to review the design criteria in effect when the building was constructed. As recently as 30 years ago, the building industry seemed to pay little attention to the need for insulation in roof systems. Before the 1970s, no specific insulation requirements were imposed on buildings. From the 1960s through the late 1970s, the Uniform Building Code (UBC) only stipulated the use of combustible insulation was permitted provided it was covered with an approved material in all types of construction, but no R-value requirements were noted.

Energy use first became a concern for building code organizations after the 1973 oil embargo. About this time, the National Conference of States on Building Codes and National Bureau of Standards (NBS) developed a draft performance standard for energy-efficient buildings. NBS issued an interim report, 74-452, which was further developed by ASHRAE Inc. ASHRAE subsequently released consensus standard 90-75, "Energy Conservation in New Building Design," in conjunction with the U.S. model building code organizations. ASHRAE's 90-75 document ultimately was published by the Council of American Building Officials and became known as the Model Energy Code (MEC) in 1977. However, there was resistance in the building industry, and it was not until the late 1980s that building codes began to include requirements to improve energy performance.