Ashrae 90.1 Standard for Building Envelopes
First appearing in 1975, an updated Ashrae 90.1 energy standard for all but low-rise residential buildings comes out every 3 years. Taking into account current technology and life cycle cost, Ashrae 90.1 establishes minimum energy efficiency requirements for building design and operation including building envelopes (as of 2007).
Currently, the Ashrae 90.1 uses the 8 US climate zones identified by the International Energy Conservation Code (IECC) to define minimum R-values for continuous insulation of buildings in the region. Canada also uses climate (average temperature, humidity and moisture) to delineate its zones.
The best most economical way to increase the effective R-value of walls is exterior continuous insulation with or without interior insulation. However, when there are thermal bridges such as studs present, the R-value of the wall assembly will not achieve the R-value of the insulation between studs. In other words, the effective R-value of the assembly is less than its nominal R-value.
Effective air and moisture barriers exhibit three properties. They are continuous, preventing air leakage. They are permanently affixed to the underlying structure to avoid tearing or displacement from stack pressure, wind pressure and mechanically induced pressure. They are durable which minimizes maintenance and ensures long term performance.
Wind and stack air pressure on buildings cause air infiltration and exfiltration. Changes in wind pressure around a building encourages water infiltration at joints and penetrations unless the air barrier is effectively sealed in those areas. Stack pressure, the pressure difference between the top and bottom inside a building caused by rising of warm air, promotes air leakage.
Air barriers seal the structure, limiting air infiltration and exfiltration.
Modeling has shown that air barriers can have a substantial effect on the energy efficiency of a building, adding to the beneficial effect of continuous insulation. Adding an air barrier to a building with continuous insulation can save a significant amount of energy and money.
Durable air and moisture membranes that can withstand the stress of building creep, stack and wind pressure and which are puncture, mold and scuff resistant are best.