Window System Design Guide

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  • Overall Window Design Guidelines
    • Window -to-Wall Ratio (WWR) < 27%
    • U Factor (Thermal): we'll do life-cycle costs for 0.35, 0.27 and 0.10. After which we will decide.
    • Solar Heat Gain Coefficient (SHGC): E = .30; W = .30; N = .30 ; S = .55; Skylight = .55.
    • Visual Light Transmittance (VLT or VT); E = .50; W = .50; N = .50; S(up) = .70; S (view)= .50; Skylight = .70.
    • Daylighting Goal = 0.9 W/ft2 goal.
    • Maximize south windows (because radiation is high in winter and low in summer)...minimize east and west windows they are difficult and expensive to control daylighting that maximizes winter heat gain and minimizes summer cooling loads.
  • All Windows
    • U < 0.30
    • WWR = 0.20 to 30 percent ratio of window area to wall area are preferred. Minimize windows on the north side. Glazing the wall areas below desk height (0-30 in. above the floor) offers little or no benefits for daylighting an office or view for the occupants.
    • Use ratio of VLT/SHGC to determine window performance. Poor < 1.00 ... Excellent >1.55
    • Effective Aperture (EA) = WWR * VLT; Desired values = 0.30; Poor < .15 ...unlikely that sufficient day lighting savings or user acceptance will be realized below this level.
    • Thermal gains and losses associated with windows should be balanced with daylight-related savings achieved by reducing electric lighting consumption.
    • Use neutral colored glass, avoid tinted glass (e.g. green or bronze)
    • Use high-performance and selective low-e glazing excellent way to let in light, but reduce solar gain.
    • Angle the vertical walls along side the window opening to allow more light in....more pleasing visual transition from bright window to darker wall.

Two Window Design
Wall Section - Two Window Design
  • View Windows (<6ft from floor)
    • Use VLT = 0.35 - 0.50 (note: below 0.35 will look dim or tinted)
    • Locate bottom of windows no higher than 48 inches (for view)
  • Upper Windows (> 6ft from Floor)
    • High, continuous windows are more effective than individual or vertical windows.
    • Locate the top of windows close to ceiling line to distribute light deeper into the space and provide greater comfort for the occupants.
    • Use VLT = 0.60 - 0.70 to let as much sunlight through as possible, especially in overcast climates
  • Ceiling
    • Greater daylight savings can be achieved by increasing ceiling heights to 11 feet or higher.
    • Use of light-colored materials (>80% reflectance) and matte finishes in all daylighted spaces increases efficiency through interreflections and greatly increases visual comfort.
  • Overhangs
    • Typically 45 degrees from horizontal from the top of the window
  • Skylighting (more details on p.13 - 14)
    • Typically the worst energy choice for daylighting. Vertical are better than horizontal skylights.
    • Use either north- or south-facing clerestories for skylighting but not east or west. E
    • Reduce summer heat gain as well as winter heat loss by using skylights with a low overall thermal transmittance. The overall U-factor includes the glazing as well as the frame and/or curb. Use a skylight frame that has a thermal break to prevent excessive heat loss/gain and winter moisture condensation on the frame. Insulate the skylight curb above the roof line with continuous rigid insulation.
    • Use light reflecting baffles and/or diffusing glazing to control direct sun
    • North facing clerestories are more effective than skylights to bring daylight into the building interior.
    • Use light colored roofing in front of the vertical skylight windows.
  • Light Shelves and Sills
    • South side only
    • Interior or exterior light shelves between the daylight window and the view window. These are effective for achieving greater uniformity of day lighting, and for extending ambient levels of light onto the ceiling and deeper into the space
    • Can use the window sill as a light reflector

Office Layout for Daylighting
  • Office Placement (more details)
    • North and South => Open Plan - Daylighting is more cost effective if open plan workstations are located on the north and south side of the building since open plan areas are more continuously occupied and achieve lower savings from occupancy sensors. The open configuration also absorbs less light, and inter-reflections provide a more uniform distribution of light deep into the space.
    • East and West => Private Offices - The control of heat and glare on the east and west facades is difficult, because daylight and views are blocked in an effort to properly control the low sun angles. By placing private offices on the east and west, occupants can individually control their blinds, and thereby control thermal discomfort and glare.

  • Interior Sun Control
    • Similar to exterior sun control, horizontal blinds on the south windows and vertical blinds on the east and west are most effective. In northern latitudes, low angles of sun can enter the north windows on summer mornings and afternoons. Vertical blinds that retract fully for the middle of the day are recommended for these conditions. Perforated blinds and translucent shades may cause glare when hit by direct sunlight.
  • Lighting
    • Use of local articulated task lights (desk lamps that can be adjusted in three planes) in daylighted spaces increases occupant satisfaction and is an effective supplement for daylighting.

Glossary: VLT - Visual Light Transmission SHGC - solar heat gain coefficient WWR - Window Wall Ratio EA - Effective Aperture