Basics of Daylighting in a Green Environment
The use of natural sunlight, known as daylighting, to illuminate a building can save energy, reduce operating costs, create visual appeal, and enhance occupant health and productivity. The U.S. Green Building Council Leadership in Energy and Environmental Design (LEED™) rating system encourages the use of daylighting to achieve high performance buildings. This course provides an introduction to the use of daylighting in commercial spaces. The course objective is to show why daylighting should be considered, the basic guidelines of using daylighting and some words of caution when using certain daylighting techniques.

4.3 Daylight Feasibility

The following process from the Building Technologies Department at Lawrence Berkley National Laboratory will help to determine the feasibility of using windows to economically light an area.

Step 1: Calculate the predicted window-to-wall ratio (WWR) for a typical bay or office.

Net glazing area (window area minus mullions and framing, or 80% of rough opening) divided by gross exterior wall area (e.g., multiply width of the bay by floor-to-floor height) equals window-to-wall ratio (WWR).

Step 2: Make a preliminary glazing selection and note the visible transmittance (VT).

These are example visual transmittances. Be sure to use the actual manufacturer rating for VT for the glazing you are considering.

Generic Glazing type (1/4" panes) Typical VT
Single pane clear 0.89
Single pane tint - green or blue-green 0.70
Single pane tint - blue 0.57
Single pane tint - bronze 0.53
Single pane tint - gray 0.42
Single pane tint - extra dark 0.14
Single pane light reflective 0.35
Single pane medium reflective 0.25
Single pane high reflective 0.12
Double pane clear 0.80
Double pane tint - green or blue-green 0.65
Double pane tint - blue 0.51
Double pane tint - bronze 0.47
Double pane tint - gray 0.39
Double pane light reflective 0.30
Double pane medium reflective 0.20
Double pane high reflective 0.10
Double pane low-E clea 0.70
Double pane low-E tint - green or blue-green 0.63
Double pane low-E tint - blue 0.49
Double pane low-E tint - bronze 0.45
Double pane low-E tint - gray 0.37
Suspended low-E film products 0.27-0.60

* Double pane numbers also apply to laminates.

Step 3: Estimate the obstruction factor (OF).

Visualize a typical task location, 10 feet (3.3 m) in from a window and centered on the window. What is the view through the predicted window from desk height? Pick a location that represents an average view from the building. Sketch the window elevation and shade in anticipated objects that will obstruct sunlight from passing through the window. Select the obstruction factor as shown in the table below.

Percentage of window area obstructedObstruction Factor (OF)
< 50%1.0
>= 50% and < 70%0.85
>= 70% and < 90%0.65
>= 90%0.40

Step 4: Calculate the feasibility factor.

Window-to-wall ratio multiplied by visible transmittance multiplied by obstruction factor equals feasibility factor.

If the Feasibility Factor is 0.25 or greater, then daylighting has the potential for significant energy savings. If Feasibility Factor is less than 0.25, then consider removing obstructions, increasing window area, or increasing VT. If these modifications are not possible, it is unlikely that daylighting will be a cost-effective energy-saving strategy. However, windows can still be designed to provide views and to control glare.

Property of

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