Shade data tab on Window Blinds Dialog.
Shade blinds 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 Slat option.
You can specify the properties of window shade materials. 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. 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.
Transmittance and reflectance values for drapery material with different colour and openness of weave can be obtained from manufacturers or determined from 2001 ASHRAE Fundamentals, Chapter 30, Fig. 31.
A diffusing blind can be applied inside, outside or mid-pane.
Thickness of the shade material. 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.
Note: If you plan to apply the shade mid-pane then you should enter the minimum 0.001m as the thickness.
Shade material conductivity.
Transmittance averaged over the solar spectrum. Assumed independent of incidence angle.
Reflectance averaged over the solar spectrum. Assumed same on both sides of shade and independent of incidence angle.
Transmittance averaged over the solar spectrum and weighted by the response of the human eye. Assumed independent of incidence angle.
Reflectance averaged over the solar spectrum and weighted by the response of the human eye. Assumed same on both side of shade and independent of incidence angle.
Effective long-wave emissivity. Assumed same on both sides of 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 (see Air-Flow Permeability, below). Let the emissivity of the shade material be ε. Then
ε eff ≈ ε .(1 − η)
For most non-metallic materials ε is about 0.9.
Effective long-wave transmittance. Assumed independent of incidence angle. We can approximate this effective long-wave transmittance, τeff, 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 τ. Then
Teff ≈ η +T.(1 − η)
For most materials τ is very close to zero, which gives
Teff ≈ η
Distance from shade to adjacent glass. This is denoted by s in Figures below. If the shade is not flat, such as for pleated pull-down shades or folded drapery, the average shade-to-glass distance should be used. (The shade-to-glass distance is used in calculating the natural convective air flow between glass and shade produced by buoyancy effects.). Note used for between-glass shades. In the following, H is the glazing height and W is the glazing width.
Effective area for air flow at the top of the shade divided by sW, the horizontal area between glass and shade (see Figures below).
Effective area for air flow at the bottom of the shade divided by sW, the horizontal area between glass and shade (see Figures below).
Effective area for air flow at the left side of the shade divided by sH, the vertical area between glass and shade (see Figures below).
Effective area for air flow at the right side of the shade divided by sH, the vertical area between glass and shade (see Figures below).
The fraction of the shade surface that is open to air flow, i.e., the total area of openings (“holes”) in the shade surface divided by the shade area, HW. If air cannot pass through the shade material, Air-Flow Permeability = 0. For drapery fabric and screens the Air-Flow Permeability can be taken as the “openness” of the fabric (see 2001 ASHRAE Fundamentals, Chapter 30, Fig. 31), which is 0.0 to 0.07 for closed weave, 0.07 to 0.25 for semi-open weave, and 0.25 and higher for open weave.
Vertical section (a) and perspective view (b) of glass and interior shade layers showing variables used in the gap air flow analysis. In (b), the air-flow opening areas Abot, Atop, Al, Ar and Ah are shown schematically. See Engineering Manual for definition of thermal variables.
Examples of air-flow openings for an interior shade covering glass of height H and width W. Not to scale.
(a) Horizontal section through shade with openings on the left and right sides (top view).
(b) Vertical section through shade with openings at the top and bottom (side view).
In (a) Left-Side Opening Multiplier = Al /sH = min(l/s,1) and Right-Side Opening Multiplier = Ar /sH = min(r/s,1). In (b) Top Opening
Multiplier = Atop /sW = t/s and Bottom Opening Multiplier = Abot /sW = b/s.