Page 103 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

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for a minimum of 75% of the roof surface. (LEED 2009 provides a weighted rooftop SRI
average for these materials for varying SRI materials. Only the weighted average of the
included areas must meet the minimums.)
OR
OPTION 2
Install a vegetated roof for at least 50% of the roof area.
OR
OPTION 3
Install high albedo and vegetated roof surfaces that in combination meet the following
criteria:
(Area of SRI Roof/0.75) + (Area of Vegetated Roof/0.5) ≥ Total Roof Area.

Potential Technologies and Strategies
Consider installing high-albedo and vegetated roofs to reduce heat absorption. SRI is
calculated according to ASTM E 1980. Reflectance is measured according to ASTM E 903,
ASTM E 1918, or ASTM C 1549. Emittance is measured according to ASTM E 408 or ASTM
C 1371. Default values will be available in the LEED-NC v2.2 Reference Guide. Product
information is available from the Cool Roof Rating Council website, at www.coolroofs.org.

Calculations and Considerations
Note that this credit references solar reflectance which is different from visible reflectance,
as solar reflectance covers a wider range of wavelengths than visible reflectance does.
Therefore, any reflectance information from manufacturers should be checked to ensure
that the proper reflectance is used in the SRI calculation. Table 2.7.3 gives some example
SRIs for various roofing materials which have been taken from the referenced table in the
LEED-NC 2.2 and 2009 Reference Guides. Roofing materials with the higher SRI values are
sometimes referred to as cool roofs.
Also notice the big difference in the SRI requirement for low-slope versus steep-
slope roofs. Flat roofs have a much stricter requirement than sloped roofs. Therefore,
alternatives with some sloped roof surfaces may be preferable for obtaining the credit.
Also note that cool roofs may not be as energy efficient in the colder months due to the
impact of high emittance and low absorption on heating costs; however, on average, the
annual overall energy use may still be reduced. In addition, structures in very northern
climates tend to be highly insulated to keep warmth in during the summer and may not
have any air conditioning for summer months. Thus, darker roofs in the summer would
tend to overheat the buildings.
The equation for Option 3 is actually applicable for all three options. It is the only
one necessary to use as the limits of this equation for not using any of the appropriately
rated SRI roofing material in Option 1 default to the requirement in Option 2, and
conversely, not having any vegetated roof area defaults to the requirement in Option 1.
Therefore, the following write-up will only address the equation covering Option 3.
The first definition that must be understood is what is meant by roof area. As in land
areas used in all the other credit calculations, roof areas are assumed to be the horizontal
planar surface as would be projected on a site plan, not the actual slanted surface. [This
horizontal planar surface is the building footprint (BF) as previously defined.] Therefore,
2
a slanted shingled roof with an area as defined in this credit of 100 ft would actually