What Metrics to Look at For Exterior Energy Efficiency
December 22, 2010
Today's tip is about the important measures facility managers should be aware of when trying to quantify the quality and energy efficiency of exterior building elements. For new construction, the ASHRAE 90.1 standard is the baseline for measuring how efficient a building is. Soon, though, ASHRAE 189.1, a new green building construction code, and the ICC's International Green Construction Code, will be the standards. These standards reference several metrics for exterior building elements that exterior components must meet when choosing the prescriptive path to compliance.
For roofs, the two main measures are solar reflectance and infrared emittance. Solar reflectance measures a surface’s ability to reflect infrared, visible and UV light. Generally, the higher the solar reflectance, the more energy efficient the roof will be. Infrared emittance measures a surface’s ability to re-emit any energy absorbed back into the atmosphere. A so-called "cool roof" generally has an emittance value higher than .90 and a reflectance value of .65 or higher. Finally, solar reflectance index, or SRI, is a combination of the two via a standard calculation. SRI is the new standard used in LEED.
For windows and skylights assemblies, U-factor measures heat loss. The lower the number, the better the performance. Solar Heat Gain Coefficient measures how well a product limits radiant heat gain from sunlight. Visible Transmittance measures how much light, but not heat comes through. The higher the number, the more light is transmitted. Finally, Condensation Resistance indicates a product’s ability to resist the formation of condensation on interior surfaces. The higher the rating, the better.
For wall assemblies, the best measure for efficiency is R-value - or the thermal flow resistance. The higher the R-value, the better the insulating effectiveness. When calculating the R-value of a wall assembly, the R-values of the wall, insulation and any other layers are totaled to arrive at the R-value of the total assembly.