Requirement Summary
This measure can be claimed if the solar reflectivity (albedo) of the roof is greater than the local base case as set out in the Key Assumptions for the Base Case in the Design section. EDGE will calculate the impact of any improvement beyond the base case.
Intention
Specifying a reflective finish for the roof can reduce the cooling load in air-conditioned spaces and improve thermal comfort in non-air-conditioned spaces. Due to the reduction in surface temperature, the service life of the finish also improves, and the impact on the urban heat island effect can be reduced.
Approach/Methodologies
EDGE uses the solar reflectivity (albedo) of the roof finish as the indicator of best practice. This is the part of the total incident solar radiation that is reflected back from a surface. Unlike Visible Solar Reflectance, it includes the full solar spectrum. It does not include the effect of emittance which is reflected in a metric such as Solar Reflectance Index (SRI).
The solar reflectivity for a specific roofing material and finish can be acquired from the product manufacturer. It is often indicated in the product data sheet or laboratory test results published on manufacturers’ websites. Solar reflectivity is expressed as a fractional value between 0 and 1, or as a percentage.
By subtracting the solar reflectivity from the total level of solar radiation that falls on the roof surface, EDGE calculates the portion of solar radiation that is transferred into the building.
To model more than one roof type, use weighted average values.
To specify a green roof, adjust the solar reflectivity of the roof (use the default of 70% if the actual value is not available), and the insulation of the roof (U-value) to define the green roof condition. Also select the insulation type used in the roof assembly in the Materials tab, under Roof Insulation.
Table 18 provides an indication of the values for different roof finishes, but is meant only as a guide. Manufacturers’ published values must be used in the EDGE assessment. If manufacturer data is not available, the EDGE reference values may be used.
Table 18: Solar reflectivity values for typical roofing materials
* mil is equal to .001 inches or .0254 millimeter
Potential Technologies/Strategies
The key factor in the solar reflectivity of the material or finish is its color. In warm climates a white finish is ideal to maximize reflectivity. If a white finish is not possible then the designer should select a very light color.
Relationship to Other Measures
The impact that the solar reflectivity of the roof has on the energy consumption of a building is dependent upon the insulation levels and the approach used to cool the building, as well as the efficiency of any cooling systems. The solar reflectivity of the roof finish has a decreased effect on the internal heat gains as the insulation levels are increased. Super-insulated buildings may not benefit significantly from roof finishes with a high solar reflectivity. Higher solar reflectivity values will have no effect on the energy consumption in passively cooled buildings, but may have an impact on virtual energy and, therefore, EDGE results due to occupant comfort. As the efficiency of the cooling system increases, the solar reflectivity will have a decreasing impact on energy consumption. If the roof area is a useable area (i.e. for roof activities), then the use of bright white colors is not recommended as they can cause glare and discomfort.
Assumptions
The base case for solar reflectivity may vary in different countries. The base case assumption can be seen in the Design tab under Advanced Settings: Key Assumptions for the Base Case. The typical default value is 30%. The improved case has a default value of 70% which can be adjusted by the user. The actual solar reflectance / reflectivity provided by the manufacturer must be provided for certification.
Compliance Guidance
At both the design and post-construction stage, it is important to ensure that the value obtained for the roof material/finish is the solar reflectivity of the finish rather than an alternative indicator of performance. Solar reflectivity is also referred to as solar reflectance (R). Other values that may be provided by a manufacturer include the solar reflectance index (SRI), visible solar reflectance, the emittance, or gloss units, which are not the same as solar reflectivity.
| Design Stage | Post-Construction Stage |
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At the design stage, the following must be used to demonstrate compliance:
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At the post-construction stage, the following must be used to demonstrate compliance:
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