Shades
Shades can be added to internal or external windows, glass doors, and skylight constructions.
Window shades can be on the inside of the window (“interior shades”), on the outside of the window (“exterior shades”), or between glass layers (“between-glass shades”). When in place, the shade is assumed to cover all of the glazed part of the window, including dividers; it does not cover any of the window frame, if present. The plane of the shade is assumed to be parallel to the glazing.
Shades can be used for diffusing materials such as drapery and translucent roller shades. For slat-type shading devices, like Venetian blinds, that have a strong angular dependence of transmission, absorption, and reflection it is better to use the Blinds material. The Screens material should be used to model wire mesh insect screens where the solar and visible transmission and reflection properties vary with the angle of incidence of solar radiation.
Transmittance and reflectance values for drapery material with different color and openness of weave can be obtained from manufacturers or determined from 2001 ASHRAE Fundamentals, Chapter 30, Fig. 31. Reflectance and emissivity properties are assumed to be the same on both sides of the shade. Shades are considered to be perfect diffusers (all transmitted and reflected radiation is hemispherically-diffuse) with transmittance and reflectance independent of angle of incidence.
Shades properties
Solar transmittance
This is the transmittance averaged over the solar spectrum. It is assumed independent of incidence angle.
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Default value:
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blank
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Min & Max:
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0 <= x < 1
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Typical Range:
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N/A
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Units:
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N/A
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Solar reflectance
This is the reflectance averaged over the solar spectrum. It is assumed the same on both sides of the shade and independent of incidence angle.
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Default value:
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blank
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Min & Max:
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0 <= x < 1
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Typical Range:
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N/A
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Units:
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N/A
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Visible transmittance
This is the transmittance averaged over the solar spectrum and weighted by the response of the human eye. It is assumed independent of incidence angle.
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Default value:
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blank
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Min & Max:
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0 <= x < 1
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Typical Range:
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N/A
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Units:
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N/A
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Visible reflectance
This is the reflectance averaged over the solar spectrum and weighted by the response of the human eye. It is assumed the same on both sides of the shade and independent of incidence angle.
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Default value:
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blank
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Min & Max:
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0 <= x < 1
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Typical Range:
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N/A
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Units:
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N/A
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Thermal emissivity
This is the effective long-wave emissivity. It is assumed the same on both sides of the shade. We can approximate this effective emissivity, εeff, as follows. Let η be the “openness” the shade, i.e., the ratio of the area of openings in the shade to the overall shade area. Let the emissivity of the shade material be ε. Then: εeff ≈ ε (1 − η). For most non-metallic materials ε is about 0.9.
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Default value:
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blank
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Min & Max:
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0 <= x < 1
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Typical Range:
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N/A
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Units:
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N/A
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Thermal transmittance
This is the effective long-wave transmittance. It is assumed independent of the incidence angle. We can approximate this effective long-wave transmittance, Teff, as follows. Let η be the “openness” the shade, i.e., the ratio of the area of openings in the shade to the overall shade area. Let the long-wave transmittance of the shade material be T. Then: Teff ≈ η + T (1 − η). For most materials, T is very close to zero, which gives Teff ≈ η.
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Default value:
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blank
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Min & Max:
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0 <= x < 1
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Typical Range:
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N/A
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Units:
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N/A
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Thickness
This is the thickness of the shade material (m). If the shade is not flat, such as for pleated pull-down shades or folded drapery, the average thickness normal to the plane of the shade should be used.
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Default value:
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6 mm; 0.25 in
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Min & Max:
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0 <= x < 1000
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Typical Range:
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3 to 30 mm; 0.01 to 1.5 in
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Units:
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mm; in
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Conductivity
This is the shade material conductivity.
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Default value:
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0.9 W/mK
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Min & Max:
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0 < x <= 100,000
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Typical Range:
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N/A
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Units:
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W/mK; W/m°C; Btu/hr•ft•°F
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