2 FM quick reads on Roofs
1. How to Measure Exterior Efficiency
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.
2. Your Garden Variety Roof
Hello. This is Greg Zimmerman, executive editor of Building Operating Management magazine.
Today’s topic is green roofs, specifically what to be aware of if you’re considering a green roof for your next project.
By green roofs, I mean REAL green roofs – the ones with plants, foliage and other greenery, as opposed to cool roofs, which are sometimes labeled green in the sense of being environmentally responsible because they can reduce energy use.
With that distinction made, here’s another one: There are two varieties of green roofs. Intensive green roofs are the fancy kind, most often recognized as roof gardens that allow people to walk around and relax. They often include flowers, bushes, and even small trees. Extensive green roofs are usually less involved, with a thinner planting medium, and usually include smaller plants with shallower roots, like grass and flowers.
Two important considerations when selecting a green roof are waterproofing and weight. Green roofs are much heavier than traditional roofs, so selecting a green roof means making sure the facility has sufficient load-bearing capacity. Because of their much thicker planting medium, intensive green roofs can weigh upwards of 120 pounds per square foot.
The waterproofing system for a green roof can be either a sheet system, a built-up system or a fluid-applied system. Many experts recommend a waterproofing system without seams to reduce possible water-entry points. Waterproofing is certainly the one area where you don’t want to value-engineer.
If a green roof is waterproofed and installed correctly, it can last many years longer than a traditional single-ply roof. Other potential benefits of green roofs include better aesthetics, reduction of the urban heat island effect, energy savings, and reduction in stormwater runoff.
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