Skylight, Molde, Norway (C) Daniel Friedman Design Issues for Skylights

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This article discusses design issues for skylights such as choosing the proper skylight size, solving glare problems, handling splayed skylight openings, and skylight ventilation.

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Skylight Window Design Issues

In 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. This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons. Our photo (page top) shows an older model skylight with factory-provided flashing and curbing. Despite lack of owner maintenance this unit is not leaking.

See WINDOWS & DOORS our home page for window and door information, and also see WINDOW TYPES - Photo Guide for a photographic guide to window and door types and architectural styles.

Skylights can add a dramatic element to any room in addition to providing high levels of pleasing natural daylight. In the case of operable skylights, they can provide effective ventilation as well. On the other hand, too many skylights can produce uncomfortable glare and significant summertime overheating. A few simple guidelines can ensure a successful installation.

Sizing Advice for Skylights

A small skylight can go a long way toward brightening a space. However, too much direct sunlight can produce very uneven room lighting, excessive glare, and localized overheating.

A rule of thumb developed by the Florida Solar Energy Center recommends that a skylight should be 4 to 6% of the illuminated floor area. This will provide a reasonable level of illumination (about 600 lux) during the morning and afternoon, and on days with overcast skies.

So, for example, 5 to 7 square feet of rooftop glazing (measured horizontally) will provide a reasonable level of illumination to a 10x12 kitchen. For skylights with frosted lenses or high up in cathedral ceilings, use a larger skylight; in regions with predominantly sunny skies, a smaller skylight should suffice.

Glare Problems at Skylights

Glare can be caused by a bright light source bouncing off a work surface into your eyes, or from a bright source directly striking your eyes from straight ahead or from an angle (Figure 3-19 below).

(C) J Wiley, S Bliss

Figure 3-19


The best way to reduce glare from skylights is to provide diffused, even lighting over a larger area. This can be accomplished with frosted glazing or interior shades.

However, both of these block views of the sky, and interior shades require maintenance. Another approach is to use a light-colored skylight well to reflect and diffuse the light over a larger area.

Deep skylight wells can also reduce unwanted solar gains by as much as 25% by absorbing heat into the walls of the well area.

Splayed Skylight Openings

To maximize the daylighting potential of a skylight, it is best to paint the light well (see Skylight Window Well) a light color and angle at least the top side of the well. Splaying the skylight well enlarges the opening at the bottom and brings the light deeper into the interior space, particularly in the winter when sun angles are lower.

(C) J Wiley, S Bliss

Typically, the top side of the well runs at 90 degrees to the roof slope, and the bottom side, if splayed, is framed plumb (see Figure 3-20).

On very steep roofs where the top of the skylight is near eye level, a horizontal angle works well for the top side of the well. The sides are usually left plumb to simplify the framing (see the "light well" at The Skylight Window Well. Also see Skylight Light Tubes.)


Ventilation Through Skylights

Ventilating through a skylight takes advantage of natural convection currents in a room that bring warm air toward the ceiling.

As the air is exhausted out the skylight, cooler air is drawn in through windows or other rooms. In rooms that generate a lot of moisture, such as kitchens and bathrooms, a venting skylight helps to control moisture levels as well. Some skylights are equipped with flap-style vents that can remain open during rain without leaking.

Also see
    Diagnose & Repair Skylight Leaks
    Rooftop Skylight Leaks
    Water Testing Skylights
    Repairing Skylight Leaks
    Avoiding or Preventing Skylight Leaks

Skylight Glass Breakage

Skylight Manufacturers & Product Sources

Andersen Windows and Doors Skylights and roof windows with exterior sash clad with glass-fiber-reinforced material

Milgard Windows and Doors Skylights with aluminum frames (thermal break optional) with vinyl subframes on operable models; optional motorized controls with rain sensor

Pella Windows and Doors Wood interior, aluminum exterior, optional motorized controls, and manual or motorized fabric-pleated shades

Roto Frank of America Wood interior, aluminum exterior, optional motorized controls, and manual or motorized fabric-pleated shades; Sweet16 model fits 16 in. o.c. framing

Velux America Inc. Skylights and roof windows with wood interior and aluminum-clad exterior. Options include insect screens, blinds, motorized controls and shades with rain sensor, electrochromatic glass, and flashing kits for metal and tile roofs and mulled units

Skylight Light Tube Manufacturers & Sources

SolaTube Light tubes from 10 to 21 in. in diameter; options include electrical lighting, daylight dimmer, and integral bath fan

Sun-Tek Skylights Light tubes from 10 to 21 in. in diameter; options include electrical lighting and multitube Spyder skylight

Velux America Inc. Sun Tunnel light tubes from 14 to 22 in. in diameter with flexible or rigid tubes

Industry Associations for Windows & Doors

American Architectural Manufacturers Association (AAMA)

Efficient Windows Collaborative

National Fenestration Rating Council (NFRC) Sustainable by Design

Shareware calculators for sun angles, solar heat gain, and shading

Window and Door Manufacturers Association (WDMA)

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


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