PATH - A Public Private Partnership for Advancing Housing Technology
Natural Lighting Strategies
A well designed home maximizes the use of natural lighting without compromising energy efficiency. Consider the following steps.
Locate Windows with Daylighting in Mind
Good lighting begins with the effective use of sunlight. Consider the size, location and primary function of the windows. Will the window frame a view, capture prevailing breezes, allow adequate light, provide architectural balance?
There is a tradeoff between the benefits of windows, such as daylighting and good views, and the major drawback, reduced insulation value. Because even new windows have a much lower insulation value than walls, they should not exceed 15% of the wall area, unless the home is specifically designed to take advantage of passive solar heating. With proper window placement, there should be more than enough daylighting in the home with 15% or less window-to-wall area.
Low-emittance (low-E) windows keep the heat inside in winter and outside in summer and - as a side benefit - protect furnishings from fading. They have a thin film applied to the glass that improves the performance of the window.
The two most important indicators of a window's performance are the U-factor and the Solar Heat Gain Coefficient (SHGC). The U-factor measures how well the window insulates. The lower the U-factor, the better the insulation value. The SHGC measures how well the window blocks heat from the sun. The lower the SHGC, the less solar heat the window lets in.
Each climate has optimum values for both the U-factor and SHGC. See the Energy Efficient Window Collaborative window selection tool (click on Selection Tool) to choose the right windows for the climate. Make sure to specify ENERGY STAR qualified windows, which meet or exceed your region's criteria.
Remember, the better the window, the more comfortable - and more energy efficient - the home.
Seasonal Window Shading
Provide for shade at south- and west-facing windows with an appropriate roof truss design, architectural features, landscaping plan, and decorating plan.
In warm climates, sunlight can overheat the home through windows on the south and west facing sides of homes. Roof overhangs can provide shade for these windows as well as improve the home's long-term ability to withstand rain. They also allow homeowners to open windows on a rainy day. Specify 24" eave and 12" rake overhangs when possible.
Where overhangs will not throw shade, like first floor windows in 2-story homes, architectural features like lanais or awnings can offset any potential heat gain that windows on southern and western facades might be subject to during the cooling season. Deciduous trees and vine shading can also help keep the home cool.
Window treatments like window films can also be added to windows to keep heat from the sun from entering the home, although they have the disadvantage of darkening the home throughout the day. Shades and blinds can also be closed during peak solar hours to prevent overheating.
Where windows are not practical due to elevation or a room's layout, tubular skylights can provide natural light while minimizing the size of the roof penetration.
Tubular skylights have a roof-mounted light collector that reflects light through a metal or plastic tube with a highly reflective interior coating. The reflective tube guides the sunlight to a diffuser lens mounted on the interior ceiling surface that spreads light evenly throughout the room.
They are frequently used in windowless bathrooms and closets where natural light is desirable and where skylights aren't possible because of attic space above the ceiling. They are also often installed in other dark spots throughout the house, such as hallways and stairwells. In single-story homes they can direct natural light to basements.
Content updated on 8/7/2006
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