Choose the most appropriate lighting control strategies
  
  
 

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 0.89 Double pane tint - bronze 0.47
Single pane tint - green or blue-green 0.70 Double pane tint - gray 0.39
Single pane tint - blue 0.57 Double pane light reflective 0.30
Single pane tint - bronze 0.53 Double pane medium reflective 0.20
Single pane tint - gray 0.42 Double pane high reflective 0.10
Single pane tint - extra dark 0.14 Double pane low-E clear 0.70
Single pane light reflective 0.35 Double pane low-E tint - green or blue-green 0.63
Single pane medium reflective 0.25 Double pane low-E tint - blue 0.49
Single pane high reflective 0.12 Double pane low-E tint - bronze 0.45
Double pane clear* 0.80 Double pane low-E tint - gray 0.37
Double pane tint - green or blue-green 0.65 Suspended low-E film products 0.27-0.60
Double pane tint - blue 0.51
* Double pane numbers also aply to laminates.
 
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:
 
Situation % obstructed OF
1 less than 50% 1
2 between 50% and 70% 0.85
3 between 70% and 90% 0.65
4 more than 90% 0.40
 

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)

 

  • Variable occupation: occupants spend part of their time in the space and part elsewhere, eg where workers spend only part of their time as a working practice.
  • Intermittent scheduled occupation: people occupy the space for relatively short but defined periods, eg a school classroom or a sports hall.
  • Full occupation: occupants are in the space for the entire working day, eg a factory space where occupants are at their machines the entire time.
  • Intermittent occupation: an area visited only occasionally for short periods during the working day, eg storerooms or a warehouse.

 

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