Patched skylight flashing (C) Daniel FriedmanEnergy Efficiency of Skylights

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Skylight energy efficiency guide:

This article explains the energy efficiency of skylights and skylight energy efficiency ratings and factors such as solar gain and the use of low-E skylight glazing. We also discuss the effect of skylight orientation - which roof slope etc., and skylight condensation problems.

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Skylight Energy Efficiency Guide

Sloped windows on barn silo (C) Daniel FriedmanArticle Contents

This article group discusses the selection and installation of skylights, including choosing a skylight, how to install the skylight, constructing the skylight well or chute, skylight controls, and skylight shades or screens.

We also discuss skylight condensation and special skylight products such as light tubes. 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. Our photo (page top) shows a skylight with a history of leaks and blob-on patching with roof cement, caulk, and chewing gum.

This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

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. Ourlinks listed at the "More Reading" links at the bottom of this article provide in-depth articles on window and door selection, inspection, installation, problem diagnosis, and repair.

Over glazing with skylights [putting in too many skylights] can increase heat loss in cold, cloudy climates, but the biggest impact of excessive roof glass in most climates is overheating, increasing both peak and annual cooling loads and making people uncomfortably hot.

Solar Heat Gain at Skylights

Because the sun is high in summer and low in winter, skylights tend to capture solar gains at just the wrong time. For example, an unvented skylight set at a 30- to 40-degree slope captures four times as much solar-heat gain in midsummer as a similarly sized vertical window.

To put this in perspective, at noontime on June 21, two single-glazed, 6-square-foot skylights facing south produce nearly as much heat as a 1,000-watt electric space heater. West-facing skylights produce almost as much heat in summer.

Low-E Glazing Solutions for Skylights

Fortunately, the new spectrally selective coatings (see Spectrally Selective Low-E Windows), cut solar gains by nearly 50% compared with clear double glazing, greatly reducing the cooling penalty of skylights.

Still, there is no sense in using more skylights than is needed to produce comfortable light levels, typically 5 to 7% of the floor area being lit. Where overheating is a possibility, use a venting skylight to change the glass angle and exhaust any heat buildup.


Skylight Orientation

While high-tech glass can significantly reduce solar gains, it is still possible to over glaze and overheat a room. For that reason, south- and west-facing skylights should be kept to a minimum.

Also, south-facing skylights produce a lot of glare from direct summer sun. North-facing skylights consistently provide even, diffused lighting with little heat gain. East-facing skylights generate moderate heat gain on summer mornings, which could pose a problem in hot climates if overused.

Skylight U-Values

The same double-glazing has a higher U-value (lower R-value) when used in a skylight than in a vertical window. Increased convection between the two panes of glass in a horizontal or low-slope orientation causes the greater heat loss in skylights.

Starting in mid 2004, all skylight manufacturers using NFRC labels started reporting U-values measured at a slope of 20 degrees. For older skylights rated at a 90-degree angle, multiply the U-value by 1.2 to get the 20-degree equivalent. This results in a 20% higher U-value.

Avoid Condensation Problems on Skylight Windows

Because warm moist air is carried upward by convection currents, skylights are often one of the first places to develop condensation. This can lead to dripping and staining of the frame, well, or furnishings below.

The best defense against skylight condensation is to choose high R-value glazing with warm-edge spacers. Good insulation of the skylight well also helps by keeping the surrounding area warmer. Several manufacturers offer skylights with integral condensation gutters, a helpful feature in cold climates.

Condensation Resistance of Windows: Measurements

The NFRC (National Fenestration Council) in discussing solar heat gain at windows, describes the Condensation Resistance of Windows (CR) as follows:

Condensation Resistance (CR) measures how well a product resists the formation of condensation. CR is expressed as a number between 1 and 100. The higher the number, the better a product is able to resist condensation. CR is an optional rating, and manufacturers can choose not to include it on their NFRC labels.


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.

Skylight Articles


Continue reading at SKYLIGHT INSTALLATION PROCEDURE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see SKYLIGHTS, GUIDE - home

Or see WINDOWS & DOORS - home

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