(C) J Wiley, S Bliss Guide to Methods of Window Shading to Reduce Solar Gain & Glare
     


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In this article series we discuss methods for shading windows to reduce excessive solar gain or glare.

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Window Shading to Reduce Solar Gain

(C) J Wiley, S BlissIn this article series we discuss the selection and installation of windows and doors, following best construction and design practices for building lighting and ventilation, with attention to the impact on building heating and cooling costs, indoor air quality, and comfort of occupants.

We review the proper installation details for windows and doors, and we compare the durability of different window and door materials and types. Also see   SOLAR SHADES & SUNSCREENS and see  SUNGAIN, FILMS, LOW-E GLASS

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As described in Best Practices Guide to Residential Construction Chapter 3, BEST PRACTICES GUIDE: WINDOWS & DOORS:

Shading of glass with overhangs, plantings, or shades will reduce cooling loads and increase comfort in any climate with significant cooling loads.

Shading of windows will also reduce glare (Figure 3-19, below), fading of furnishings, and localized overheating in rooms with south- or west-facing glass. Window overhangs, plantings, or exterior shades that block the sun before it strikes the glass are the most effective approach since the heat never gets into the building. But light-colored interior shades can also substantially reduce heat gains.

Spectrally Selective Window Glazing

From a shading standpoint, using spectrally selective glass (SHGC below .40) is like having shades or blinds on standard low-E glass.

However, adding good shading to spectrally selective glass can reduce cooling costs by another 10 to 15%. This would make sense in very hot climates or on houses with large expanses of glass on the south or west side. In many cases, the shading adds no cost or serves other design needs. For example, a porch on the east or west side of a house provides effective shading as well as outdoor living space.

Plantings for Window Shading

Deciduous trees can provide very effective summer shade on the south side but, depending on the type of tree, may block 20% or more of the solar radiation in winter. Because trees follow the local seasons rather than the calendar, the shading tends to occur when needed most. For example, leaves appear earlier in the spring and last longer in the fall in warmer climates, which need spring and fall shading. Trees also cool the area around them by their natural evaporative cooling—as water evaporates from the leaves.

Other options for shading south-facing windows include trellises with dense foliage or evergreen trees. Evergreens should be tall enough to block the summer sun but trimmed so their canopies allow the low winter sun to reach the windows.

On the east and west sides of the house, trees or large shrubs can provide very effective shading, since the problem times are morning and afternoon when the sun is low in the sky and easily blocked by a well-placed planting, either deciduous or coniferous. Fixed Overhangs. These are commonly used on the south side of homes with clear glass or high-solar-gain glass. To be effective, the overhangs must be sized correctly to reject the high summer sun but allow in the low winter sun. In most temperate climates, a 1 1/2 - to 2-footwide overhang is adequate for average size windows. However, to provide full shading from March to September in hot climates may require a 3-foot or wider overhang.

(C) J Wiley, S BlissOne limitation of fixed overhangs is that the shading will be the same on March 21 and September 21, although the heating and cooling needs at these times may be very different. The following guidelines for shading south facing glass strike a balance between summer and winter performance:

  • Cold climates: Above 6,000 heating-degree days (HDD), locate the shadow line at mid window, based on the June 21 noon–sun angle (see Figure 3-11 below). This will shade the window 50% in mid-summer and provide full sun penetration from late September to late March. If more shading is required in summer, locate the shadow line closer to the window sill.
  • Moderate climates: In climates with less than 6,000 HDD and less than 2,600 cooling-degree days (CDD), locate the shadow line at the window sill based on the June 21 sun angle at noon. This will allow full sun exposure from late October to mid-February.
  • Hot climates: Above 2,600 CDD, locate the shadow line at the window sill using the March 21 sun angle at noon. This will provide full shading from late March to late September and about one-third shading in mid-winter.

Figure 3-11

Guide to Window Awnings and Shutters

(C) J Wiley, S Bliss

Old-fashioned awnings are very effective at blocking solar gain—up to 65% on south facing windows and up to 80% on east and west windows.

Light-colored awnings are more effective, since they will reflect more solar radiation.

To be most effective, the awning’s “drop” should cover 65 to 75% of an east or west window and 45 to 60% of a south-facing window (see Figure 3-12 at left).

Other low-tech, but very effective exterior options for windows that are difficult to shade include wooden shutters, bamboo shades, and rolling shutters.

  • Light-colored interior shades can reduce heat gain by 15 to 20%, depending on the density and reflectivity of the material.
  • Interior blinds typically reduce heat gain by 10 to 15%, depending on reflectivity.
  • See SOLAR SHADES & SUNSCREENS for details.

Solar Screening for Windows

This dark plastic or fiberglass screening is mounted on the exterior of the window in a frame or retractable roller or, in some cases, applied directly to the glass. Depending on the weave, it can reduce a window’s solar heat gain by 40 to 70%. Solar screening reduces glare and fading, but it also reduces daylight and obscures views.

Solar screens are best used on difficult to shade areas, such as west-facing glass or skylights. Some are designed to also serve as insect screens.

Interior Shades for Windows

Though not generally as effective as exterior shading, light-colored drapes, shades, or blinds can reduce solar gain by 10 to 50% (for typical curtains or blinds) to as high as 70% for an insulated, reflective shade that seals tightly around the window perimeter.

The performance of an interior shade depends on the reflectivity of its outer face, the density and R-value of the material, and whether it seals around the window. Between-the-glass pleated shades or mini blinds, available from Pella and some smaller manufacturers, are more effective in blocking solar gain than interior shades, because they block the heat before it enters the building interior.

-- Adapted and paraphrased, edited, and supplemented, with permission from Best Practices Guide to Residential Construction.

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