EnergyRealist

One of our clients asked for advice on replacing skylights in the main bedroom.  The room has cathedral ceilings, a number of recessed lights and two skylights.  The combination makes for air-leakage, not great insulation and a general level of discomfort. Insulation in the cathedral ceiling can be upgraded, but with a lot of trouble, and not to the R50+ we aim for in a normal attic.  The recessed lights can be replaced.

But what about the skylights?

Here is an article from the California Energy Commission:

“Skylights are literally windows in the roof. They allow you to bring a bit of blue sky into your home. At night they can display the majesty of the stars and the moon.

Skylights can provide the interior of your home with the warmth and brightness of natural daylight. They enliven a house dramatically while cutting down the need for artificial light. They brighten up dark corners and let in lots of warmth. They can also provide much needed ventilation.

These are the positive aspects. Skylights also have the potential to cause drafts, making a room uncomfortable. They can leak in rainy weather. While they can reduce the amount of energy you might use for artificial light inside your home, they can let in extra, unwanted heat that will make your air conditioner work harder in the summer. In the winter they can allow precious heat to escape.

Fortunately, advances that have been made in the design of regular windows also apply to skylights. Skylights are labeled by the federal  EnergyStar program and the National Fenestration Rating Council. By considering such factors as heat losses and gains, solar gains, and infiltration and ventilation, you can minimize the problems skylights pose while enjoying the benefits.

The Problem of Heat Loss

A skylight in the roof of a house will typically lose 35 percent to 45 percent more heat during cold weather than the exact same window installed on the side of the house. That’s because heated air rises.

As the warm air in a house rises toward the ceiling, it comes in contact with the cold surface of the skylight. The air is cooled and falls, forming a large draft loop that can dramatically decrease comfort. The more nearly horizontal a skylight is, the more significant the effect is.

This type of heat loss is caused by convection . Even in dual-paned skylights, warm air between the panes rises upward to meet the large, cold, nearly horizontal upper surface. This warm air then falls and is replaced by more warm air from below. If there is more than a quarter inch of space between the dual panes of glass, the circulation of air is greater and the problem more intense.

Heat loss through convection is not as great through windows installed on the side of a house. That’s because the direction that heat flows through them is primarily horizontal, not vertical, and because side windows are not generally placed near the ceiling.

In addition to convection, skylights also lose heat through radiation. While normal windows are often protected by exterior shutters, overhangs or trees and shrubs, skylights are exposed to the cold night sky.

Most skylights have a curb — a supporting frame — which fits on between roof rafters. The curb increases the total surface area and also increases the potential for heat loss.

The Problem of Solar Heat Gain

More exposed to the high summer sun than vertical windows, skylights can let into your home as much as four times the amount of heat as a standard window. Unless you can effectively manage this heat gain, your home will be uncomfortable in hot weather and your air conditioner will work overtime.

Energy experts estimate that a 2-foot by 4-foot skylight made with a single pane of clear glass will allow enough heat into a home to make a typical air conditioner use an additional 240 kilowatt hours of electricity per year. Based on an average kWh rate of about 8 cents, that means that one clear skylight could boost your electric bills about $19 per year for extra cooling.

Overcoming These Drawbacks

Fortunately, it’s possible to enjoy the maximum benefit from a skylight while minimizing the drawbacks. To do so, you must consider how to properly position it. You must also take into account the unit’s design and the materials from which it’s made.

Positioning

By carefully choosing the location of your skylight, you can control the amount of heat — as well as the light — that it provides. Remember that a skylight can be an important source of passive solar heating.

When installed on a north-facing roof, a skylight will provide fairly constant but cool illumination. One on an east-facing roof would provide maximum light and solar heat gain in the morning. A west-facing skylight provides afternoon sunlight, but it can produce large amounts of unwanted heat in the late afternoon.

For passive solar heat gain in the winter, a south-facing skylight is the best choice. Unfortunately, it also produces the most heat gain in summer, unless you shade it somehow. One way to solve this problem is to shade the roof and its skylight with deciduous trees that lose their leaves in winter.

You can also add shade with movable shading devices, such as louvers, shades, or awnings on the inside or outside of the skylight, just as you can for vertical windows. In addition to regulating heat, shades can allow a more uniform source of light into the home. Most manufacturers of skylights now offer various types of interior and exterior shades, blinds or panels that you can easily control, and at least one manufacturer makes roller shades or closeable blinds that are installed between the double layers of glass.

The angle or slope of a skylight has a great effect on its solar heat gain. A low-slope will admit more solar heat in the summer when the sun is high, and less in the winter when the sun is closer to the horizon. (That’s unfortunately the opposite effect most of us try to achieve.)

To get the most from solar heat gain, a general rule of thumb is to position your skylight with a slope equal to your geographical latitude plus 5 to 15 degrees. For example, the optimum slope for a south-facing skylight in San Francisco, California, at 35-degree North latitude, would be between 40 degrees to 50 degrees. Some manufacturers now make prefabricated tilted bases that make it easy to increase the angle of a skylight above the roof to give the recommended slant.

Improved Designs and Glazing

Skylights come in a variety of shapes and sizes. Some are sealed units while others open to allow ventilation.

Most skylights are rectangular in shape. Units are also manufactured in circular, oval, diamond, triangular, and multi-sided configurations.

Higher quality skylights use glass, but plastic is also used because it can be formed into arched, pyramidal, and domed shapes that allow for more placement options on the roof. These raised designs allow light to enter from more extreme angles, and they help to shed moisture and leaves.

When used as glazing in a skylight, plastic is less expensive and less liable to break than other glazing materials like glass. On the downside, however, plastic surfaces scratch easily and may become brittle, discolored or stained over time. Unless they are specially coated with a film to filter out ultraviolet (UV) rays, plastic glazings can also contribute to the damage and fading of your furnishings because of the sun.

Unlike plastic, glass does not discolor. It is also much harder and durable than plastic. In skylights, however, manufacturers must use what’s called “safety glazing” — a generic term for both tempered and laminated glass. Tempered glass, the most impact resistant, is often used on the exterior side of a dual-pane skylight. Laminated glass, made with a thin layer of plastic sandwiched between glass layers — much like the glass used in automobile windshields — is used on the interior side. This combination gives maximum impact resistance while keeping the glass from breaking into large, sharp pieces which could endanger those standing underneath the skylight.

We mentioned earlier how skylights can lose large amounts of heat in the winter and add unwanted heat from the sun in the summer. Today, manufacturers of skylights have developed ways to reduce these impacts. Just like energy efficient windows, skylights are often designed with multiple layers of glazing that can have inert gases like argon or krypton gas between their layers. They also may be covered with a low-emissivity or spectrally selective films to help block solar heat gain and reduce heat loss through radiation. These films can also reduce the level of light the skylight allows into the room, however, and can cut beneficial amounts of solar gain so beneficial in the winter.

Since hot air rises, installing a skylight that opens will allow warm air rising toward the ceiling to escape. This ventilation provides natural cooling on hot days.

Ventilating skylights usually open outward from the bottom to prevent rain or snow from entering the room when the vents are open. They can be opened manually with a pole, chain, or crank, or they can be controlled remotely with electric motors. Fancier models include moisture sensors to automatically close the skylight when it rains.

A recent innovation in skylight design is the tubular skylight. Looking much like a piece of ductwork, it is a tube lined with a reflective material that channels daylight into a diffusing grid at the ceiling. Inside the room the result looks like a round, flush-in-the-ceiling light fixture that doesn’t come on at night.

Made by several manufacturers, tubular skylights usually less expensive than regular skylights. They are easy to install because they involve very little framing, drywall or painting work. Because the tubes are flexible and can bend around rafters and roof supports, they can bring daylight into areas where normal, larger skylights aren’t practical.

Because these bendable light tubes have a diffuser at the ceiling level, however, you can’t view the sky through them — a major drawback for many true skylight lovers. At the same time, since tubular skylights are sealed units with a light-gathering dome at the roof level and a diffuser at the interior end of the tube, they let in daylight without adding much in the way of heat gain or heat loss. Unfortunately, their design prohibits them from producing much solar heat gain in the winter when it could be of benefit. You also can’t open them in the summer to provide needed ventilation.

Finally, new high-tech, high-performance skylight designs are entering the marketplace. Some of these use innovative sun-tracking reflectors or large lens-like elements to maximize the amount of light captured by skylights. At the same time, new thermochromic coatings automatically change their glass from clear to diffused in response to heat. This transformation reduces the amount of solar heat transmitted by the skylight and significantly reduces your air conditioning costs in hot weather.

Make the Wise Choice

The price of a skylight can vary widely depending on options. Expect to pay more for special glazing materials, venting, shading devices, or other options. Consider not only the up front cost of the unit but how much it will cost you over the years in extra energy bills.

The Energy Star® program labels many consumer appliances and residential building products, including skylights. Energy Star® approved skylights exceed a defined minimum energy performance requirement for three broad climate regions of the country — sometimes by as much as 40 percent!

All Energy Star® qualifying windows and skylights also will carry the label from the National Fenestration Rating Council (NFRC). NFRC labels indicate specific energy performance characteristics such as U-Values and Solar Heat Gain Coefficients that will help you to make a wise energy choice.”

(The California article ends here. A detailed description of the NFRC label is here and a comparison paper here.

A more technical and older, but still relevant article is here

None of the above articles mention the obvious; which is that skylights need to be well installed!  The number of leaks we have seen through skylights, even in relatively new houses, shows just how much poor installation work is still being done.  In one place we could poke a pencil through gaps in the flashing, on all four corners, to the outside air.  The owner told us that the kitchen felt drafty.  Yes, indeed!

And finally here is a table on calculating R values of various assemblies, from ColoradoENERGY.   The R values of glass, even the most advanced, is far below that of a comparable wall or ceiling; which is why windows (and skylights) are always a compromise.

The typical 2×4 wall has an R value of about 15, a 6” ceiling about R 20.  By comparison glass has approximate R-values as follows:

Type	R value	Comments
Single Pane	1
Double Pane	1.5 – 2.5	Depending on framing, spacers etc.  The R value is for the whole assembly, not just the glass
Double Pane, low-e coating and argon gas	3-4	Depending on number and type of coatings
Triple Pane	2.5-3	This doesn’t make sense.  Triple pane R values, comparing like to like, should be slightly higher than double pane

Some futuristic thinkers suggest we build houses without any windows at all and rely on artificial light and flat panel displays to provide our contact with the outside.  The realist’s opinion: No thanks, I like skylights….