How to Measure Exterior Efficiency

Roofs, Walls, Windows, Energy Efficiency, Metrics   March 3, 2009

Today’s tip is about how to measure the efficiency of certain exterior elements. For new construction, the ASHRAE 90.1 standard is the baseline for measuring how efficient a building is. But there are several metrics for exterior building elements that can show how they contribute to, or are detrimental to, energy efficiency goals.

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 roofs, the two main measures are solar reflectance and infrared emittance. Solar reflectance, also known as albedo, measures a surface’s ability to reflect infrared, visible and UV light from the sun. 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.

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.


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