This month's questions are answered by Angie Durhman, green roof program manager for Tecta America, spray foam consultant
Mason Knowles, and Brad Burdic, manager of sales and marketing for Ecostream. If you have a question specific to green roofing,
spray foam or solar roofing, please email RSI Editor-in-Chief Thomas Skernivitz at tskernivitz@questex.com
.
Durhman
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Q: Can native plants be used in a vegetated roof application? A: This question comes up frequently, especially as design teams attempt to specify materials grown (or manufactured) locally
and promote environmentally sound practices using native species.
Native plants can be tricky. Time must be devoted to understanding their natural environment and survival methods before taking
them to the roof. For example, some perennials and grasses have extensive root systems that can extend 2 feet to 3 feet in
search of water. This demand for deep soil and irrigation needs may not be suitable to replicate on a rooftop. However,
there are some native plants that would be ideal for a vegetated roof because they reseed themselves and are relatively small
plants that require minimal resources. When considering a vegetated roof, I'd recommend hiring a local ecologist to participate in choosing the right plants for
the right place, if native plants are desired for the roof. —AD
Knowles
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Q: I frequently am told by spray polyurethane foam (SPF) contractors that R-value of the product does not measure SPF's true
energy performance, and I don't need as much R-value of SPF to get the same results of traditional insulation. Is the SPF
community making this stuff up? Can I really get better energy performance with less R-value? A: They are not making it up, although some tend to exaggerate the energy-performance benefits.
There are three basic test methods identified by the Federal Trade Commission (FTC) to measure R-value of residential insulation
products. They are included in the American Society for Testing and Materials (ASTM) standard specification on SPF C 1029:
- ASTM C 177, Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the
Guarded Hot-Plate Apparatus
- ASTM C 518, Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the
Heat Flow Meter Apparatus
- ASTM C 1363, Standard Test Method for Thermal Performance of Building Assemblies by Means of a Calibrated Hot Box (recently
replacing ASTM C 963).
When tested according to FTC and ASTM C 1029 requirements, SPF's R-value ranges between 5.6 to 6.4 per inch for medium density
SPF (1.5-2.0 pounds [lbs] per foot [ft]3 ) and between 3.4 to 3.7 per inch for low density (0.04-0.06 lbs/ft3 ) SPF.
These test methods are fairly accurate and reproducible but only measure conduction within a narrow temperature range. (Median
temperature 75 degrees Fahrenheit [F], with a temperature differential of less than 50 degrees F.) The tests do not measure
convective or radiant heat. They do not take into consideration R-value reductions caused by thermal bridging, humidity, large
variations of temperature, air infiltration, and wind.
SPF applications installed properly can air-seal the building, reduce convective currents, reduce thermal bridging and radiant
energy heat gain into buildings. When tested in conditions that incorporate air pressure, convection, high and low temperatures,
SPF applications consistently perform 20 percent to 45 percent better than equal R-values of traditional insulation products.
The claims you have heard are most likely based on these tests and historical evidence of lower energy use with SPF applications.
The leakier the building and the more extreme the temperature, the more SPF can help save energy. — MK
Burdic
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Q: I want to install a solar system on my roof, but it is very steep. What do I need to do to make sure the system functions
properly? A: It is not the inclination of the roof that matters but the inclination or pitch of the solar module. By means of a support
structure, solar modules can be brought into an appropriate position even if the pitch of the roof or roof orientation is
unfavorable. To make the best use of the radiation from the sun, the angle of incidence of the sun rays should be 90 degrees.
Small deviations from the optimal angle of inclination have only a slight effect on the yield. If shadows are cast onto the
solar module, the effect on the yield is greater. As with typical roofing practice, we would focus our attention on the attachment
mechanism and ability to maintain the roof. Too many times we see panels being secured to the roof with 3/8-inch lag bolts
and roof cement. Creating an appropriate stand-off support structure with correct flashing will result in a watertight connection,
and a system that performs well over time. — BB
