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).See next page for 'typical' VTs .
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 obstructed||Obstruction Factor (OF)|
| >= 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.