STEP 1. Look if daylighting is sufficient for significant energy savings
(extract from Daylighting Manual, Public Works, Canada, March 1990)
1.1 Calculate the predicted window-to-wall ratio (WWR) for a typical bay or office
The window-to-wall ratio (WWR) is equal to the net glazing area divided by the gross exterior wall area. If unknown, use 0.35 for a typical, moderately strip-glazed building. If windows are larger, use 0.50. For smaller punched windows, use 0.25.
The net glazing area is the window area minus millions and framing (or approximately 80% of rough opening). The gross exterior area can be obtained for instance by multiplying the width of the bay by the floor-to-floor height.
1.2 Note the visible transmittance (VT) of your glazing
Generic Glazing Type (6 mm panes) | Typical VT | Generic Glazing Type (6 mm panes) | Typical VT |
Single pane clear |
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Double pane tint - bronze |
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Single pane tint - green or blue-green |
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Double pane tint - gray |
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Single pane tint - blue |
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Double pane light reflective |
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Single pane tint - bronze |
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Double pane medium reflective |
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Single pane tint - gray |
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Double pane high reflective |
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Single pane tint - extra dark |
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Double pane low-E clear |
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Single pane light reflective |
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Double pane low-E tint - green or blue-green |
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Single pane medium reflective |
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Double pane low-E tint - blue |
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Single pane high reflective |
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Double pane low-E tint - bronze |
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Double pane clear* |
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Double pane low-E tint - gray |
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Double pane tint - green or blue-green |
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Suspended low-E film products |
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Double pane tint - blue |
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1.3 Estimate the obstruction factor (OF)
At desk height, look 3 meters in from a representative window and centred on the window. Select the obstruction factor as shown below:
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1.4 Calculate the feasibility factor
Feasibility factor = WWR x VT x OF
If the feasibility factor is superior or equal to 0.25, then daylighting has the potential for significant energy savings.
STEP 2. Refer to the chart below
(extract from the EC Joule-Thermie Maxibrochures on energy-efficient lighting)