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Energy Efficient Windows & Doors - U.S. DOE (supplemented) InspectAPedia®  -     Design and Update Suggestions for Energy Efficient Windows and Doors, U.S. DOE recommendations plus supplemental information Selecting and installing energy efficient doors and windows for new construction How to add storm windows, storm doors, or use caulk and weatherstripping to reduce air and heat leaks around older home windows and doors How to select and install skylights for energy efficient homes

Guide to energy efficient windows and doors to save on building heating and cooling costs - U.S. Department of Energy recommendations plus expanded and supplemental advice. This article combines, quotes from, supplements, and expands energy efficient building suggestions for new and older homes, provided by the U.S. Department of Energy. In the text we add commentary, photographs, and links to related material beyond the U.S. DOE content. Page top skech, U.S. Department of Energy.

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

How to Select & Install Energy Efficient Windows, Doors, and Skylights

According to the U.S. department of energy, energy-efficient windows, doors, and skylights—also known as fenestration—can help lower a home's heating, cooling, and lighting costs. Here we reproduce information from the D.O.E. along with additional commentary, advice, and links to more in-depth information about building energy efficient designs, heating cost savings, insulation and ventilation advice. Also see window and door energy performance ratings - definitions of heat gain and loss, U-factor, solar heat gain coefficient, air leak rate, sunlight transmittance etc. Readers should also see DEFINITION of Heating & Cooling Terms.

An exterior door can contribute significantly to air leakage in a home—as well as some heat transfer—if it's old, not properly installed, and/or not properly air sealed. This can result in energy losses. At this website we discuss air leaks, air leak detection, and air leak sealing in detail at these articles:
AIR BYPASS LEAKS
AIR LEAK DETECTION TOOLS
AIR LEAK MINIMIZATION
AIR SEALING STRATEGIES

Selecting & Installing Building Exterior Doors

New exterior doors often fit and insulate better than older types. If you have older doors in your home, replacing them might be a good investment, resulting in lower heating and cooling costs. If you're building a new home, you should consider buying the most energy-efficient doors possible. When selecting doors for energy efficiency, it's important to first consider their energy performance ratings in relation to your climate and home's design. This will help narrow your selection.

Types of Exterior Building Doors

One common type of exterior door has a steel skin with a polyurethane foam insulation core. It usually includes a magnetic strip (similar to a refrigerator door magnetic seal) as weatherstripping. If installed correctly and if the door is not bent, this type of door needs no further weatherstripping.

The R-values of most steel and fiberglass-clad entry doors range from R-5 to R-6 (not including the effects of a window.) For example: A 1-1/2 inch (3.81 cm) thick door without a window offers more than five times the insulating value of a solid wood door of the same size.

Glass or "patio" doors, especially sliding glass doors, lose heat much faster than other types of doors because glass is a very poor insulator. Most modern glass doors with metal frames have a thermal break, which is a plastic insulator between inner and outer parts of the frame. Models with several layers of glass, low-emissivity coatings, and/or low-conductivity gases between the glass panes are a good investment, especially in extreme climates. Over the long run, the additional cost is paid back many times over in energy savings. When buying or replacing patio doors, keep in mind that swinging doors offer a much tighter seal than sliding types.

Also, with a sliding glass door, it's impossible to stop all the air leakage around the weatherstripping and still be able to use the door. Also, after years of use, the weatherstripping wears down so air leakage increases as the door ages. If the manufacturer has made it possible to do so, you can replace worn weatherstripping on sliding glass doors.

Installation of New Energy-Savings Exterior Doors on Buildings

When you buy a door, it will probably be a pre-hung frame. Pre-hung doors usually come with wood or steel frames. You will need to remove an existing door frame from the rough opening before you install a pre-hung door. The door frame must be as square as possible, so that the door seals tightly to the jamb and swings properly.

Before adding the interior trim, apply an expanding foam caulking to seal the new door frame to the rough opening and threshold. This will help prevent air from getting around the door seals and into the house. Apply carefully, especially with a wood frame, to avoid having the foam force the frame out of square.

If needed, you'll also want to add weatherstripping. Check the weatherstripping on your exterior doors annually to see if it needs replacement.

Adding Storm Doors to an Existing Building Opening

Adding a storm door can be a good investment if your existing door is old but still in good condition. However, adding a storm door to a newer, insulated door is not generally worth the expense since you won't save much more energy.

Storm door frames are usually made of aluminum, steel, fiberglass, or wood (painted or not). Wooden storm doors require more maintenance than the other types. Metal-framed storm doors might have foam insulation within their frames.

High-quality storm doors use low-emissivity (Low-E) glass or glazing. Some doors have self-storing pockets for the glass in summer, and an insect screen for the winter. Some have fixed, full length screens and glass panels that slide out of the way for ventilation. Others are half screen and half glass, which slide past each other. Some are removable for cleaning, others are not. All of these features add some convenience and higher costs.

Never add a glass storm door if the exterior door gets more than a few hours of direct sun each day. The glass will trap too much heat against the entry door and possibly damage it.

Storm doors for patio doors are hard to find but they are available. Adding one to a new, multi-glazed, Low-E door is seldom economic. Insulated drapes, when closed for the night in the winter (or on sunny days in the summer) are also a good idea.

Using Weatherstripping to Stop Air Leaks at Windows and Doors

You can use weatherstripping in your home to seal air leaks around movable joints, such as windows or doors.

To determine how much weatherstripping you will need, add the perimeters of all windows and doors to be weatherstripped, then add 5%–10% to accommodate any waste. Also consider that weatherstripping comes in varying depths and widths.

Before applying weatherstripping in an existing home, take a look at the following articles on buliding air leaks, indoor air quality, and building ventilation:

• AIR BYPASS LEAKS
• AIR LEAK DETECTION TOOLS
• AIR LEAK MINIMIZATION
• AIR SEALING STRATEGIES
• Detect air leaks - U.S. DOE version
• INDOOR AIR QUALITY IMPROVEMENT GUIDE - detailed articles list
• VENTILATION in BUILDINGS - detailed articles list
• Assess your ventilation needs for indoor air quality - US DOE version

Selection of Weatherstripping

Choose a type of weatherstripping that will withstand the friction, weather, temperature changes, and wear and tear associated with its location. For example, when applied to a door bottom or threshold, weatherstripping could drag on carpet or erode as a result of foot traffic. Weatherstripping in a window sash must accommodate the sliding of panes—up and down, sideways, or out. The weatherstripping you choose should seal well when the door or window is closed while allowing it to open freely.

Choose a product for each specific location. Felt and open-cell foams tend to be inexpensive, susceptible to weather, visible, and inefficient at blocking airflow. However, the ease of applying these materials may make them valuable in low-traffic areas. Vinyl, which is slightly more expensive, holds up well and resists moisture. Metals (bronze, copper, stainless steel, and aluminum) last for years and are affordable. Metal weatherstripping can also provide a nice touch to older homes where vinyl might seem out of place.

You can use more than one type of weatherstripping to seal an irregularly shaped space. Also take durability into account when comparing costs. See this US DOE Weatherstripping Products Table for information about the common types of weatherstripping.

Selecting Energy Efficient Skylights

A skylight can provide your home with daylighting and warmth. When properly selected and installed, an energy-efficient skylight can help minimize your heating, cooling, and lighting costs. The US DOE suggests three steps in coming up with energy efficient skylights - summarized just below.

1. Skylight Design Considerations

Before selecting a skylight for your home, you need to determine what type of skylight will work best and where to improve your home's energy efficiency.

Skylight Energy Performance

First, it's a good idea to understand the energy performance ratings of skylights if you don't already. You can then determine what energy performance ratings you need for your skylight based on your climate and home's design.

For labeling energy-efficient skylights, ENERGY STAR® has established minimum energy performance rating criteria by climate. However, this criteria doesn't account for a home's design. Therefore, if you're constructing a new home or doing some major remodeling, you should also take advantage of the opportunity to incorporate your skylight design and selection as an integral part of your whole-house design — an approach for building an energy-efficient home.

Skylight Size and Position

The physical size of the skylight greatly affects the illumination level and temperature of the space below. As a rule of thumb, the skylight size should never be more than 5% of the floor area in rooms with many windows and no more than 15% of the room's total floor area for spaces with few windows.

You should also consider a skylight's position if you want to maximize daylighting and/or passive solar heating potential. Skylights on roofs that face north provide fairly constant but cool illumination. Those on east-facing roofs provide maximum light and solar heat gain in the morning. West-facing skylights provide afternoon sunlight and heat gain. South-facing skylights provide the greatest potential for desirable winter passive solar heat gain than any other location, but often allow unwanted heat gain in the summer. You can prevent unwanted solar heat gain by installing the skylight in the shade of deciduous (leaf-shedding) trees or adding a movable window covering on the inside or outside of the skylight. Some units have special glazing that can help control

Skylight Product Selection Advice

Consider the skylight's energy performance ratings that depend on glazing, skylight operation and use, and skylight shape.

Skylight Glazing Choices for Energy Efficient Buildings

When selecting a skylight for your home, it's important to consider what type of glazing you should use to improve your home's energy efficiency. Based on various skylight design factors—such as its orientation and your climate—you may even want different types of glazing for different skylights throughout your home.

Skylight glazing usually consists of either plastic or glass. Other glazing technologies may also be used for solar heat control.

Plastic Glazing on Skylights

Plastic glazing is usually inexpensive and less liable to break than most other glazing materials. However, these plastic surfaces scratch easily, and they may become brittle and discolored over time. Many plastics also allow most of the ultraviolet (UV) rays in (unless the glazing is coated with a special film), which increases fading damage to furnishings. Acrylics and polycarbonates are the most commonly used plastic glazing. Acrylics are weaker than polycarbonates, but cost less. Although polycarbonates offer high impact resistance, some yellow with age.

Glass Used on Skylights

Glass is usually found on the more expensive skylights. Glass is more durable than plastics and does not discolor. All glass used for skylights must be made of "safety glazing," a generic term for both tempered and laminated glass. Tempered glass is the most impact resistant. Laminated glass is fabricated with a thin layer of plastic embedded near the center of the glass. Both keep the glass from breaking into large, sharp pieces. Skylights are often made with a tempered glass on the exterior side and a laminated pane on the interior side. This arrangement gives maximum impact resistance while protecting occupants from falling shards of glass.

Solar Heat Control Glazing on Skylights

Because skylights are located on the roofs, they can result in large amounts of unwanted summer time solar heat gains and winter time heat losses. Manufacturers use various glazing technologies to reduce these impacts. The most common technologies include those also used for window glazing: [DOE links]

• Heat-absorbing tints
• Insulated glazing (double-glazed, triple-glazed)
• Low-emissivity (Low-E) coatings.

Some manufacturers even install a translucent insulation material between several glazing layers to create a more thermally efficient assemb

Skylight Operation and Use

Some skylights operate to maximize a home's daylighting , and others provide ventilation and moisture control.

Daylighting at Skylights

Recent "high tech" developments maximize skylights for daylighting. An "element" on the roof becomes an aperture for collecting sunlight. It may be a sun-tracking, open-sided cylinder; a large lens-like element; or merely a conventional skylight with a mirrored reflector mounted adjacent to it. This aperture may then connect to a mirrored pipe, or "light pipe," which has a diffusing lens that mounts on or is recessed into the ceiling of the room below. Most tubular skylights have this feature.

These skylight designs, relative to equivalent traditional skylights, effectively reduce daytime overheating and nighttime heat loss, but they do not provide views or ventilation.

Ventilation at Skylights

Skylights can provide ventilation as well as light. Ventilating a building through a skylight opening releases the hot air that naturally accumulates near the ceiling. Ventilating skylights usually open outward at the bottom, some more than others. Some units vent through a small, hinged panel. One design uses a swing-down inner sash with a protected vent strip above. This can reduce the potential for rain or snow entering the room if the vents are open. Skylights may be opened manually with a pole, chain, or crank. Automated units with electric motors or pneumatic devices are also available. Some models incorporate moisture sensors to automatically close the skylight when it rains.

Larger skylights that can be used as doors are sometimes called "roof windows." Roof windows are always located within a few feet of the floor.

Moisture Control Problems at Skylights - Leaks & Condensation

In very cold weather, skylights are often prone to water vapor condensing on the glazing. The accumulation of water may then drip into the room. Better skylights usually have an interior channel to collect the condensate so it can evaporate later. The most thermally efficient skylights are less prone to condensation problems.

Skylight Shapes

Skylights are available in a variety of shapes and sizes. The most common shapes include rectangular, circular, oval, diamond, triangular, multi-sided, and tubular.

Non-rectangular units usually use plastic glazing, but higher quality ones use glass. The glazing can be flat, arched, domed, pyramidal, or "warped plane"—flat on the low side and concave in section on the high side. Of these, the pyramidal, arched, and domed shapes offer flexibility for positioning. Their raised design allows light to enter from more extreme angles than flat or warped plane units. This allows more positioning options.

The slope or curvature of the glazing also helps to shed moisture and leaves. These skylight designs also do not require the additional framing needed to slope a flat skylight for proper drainage on flat or low-slope roofs.

Tubular skylights are smaller than most other skylights. They consist of roof-mounted light or solar collectors, which increase their daylighting potential without the need to increase their size. Because the rooftop solar collector has a small surface area, tubular skylights minimize heat loss in the winter and heat gain in summer. Their small size also minimizes their impact on a home's architecture.

Skylight Installation on Energy Efficient Buildings

Even the most energy-efficient skylight must be properly installed to ensure that its energy performance is achieved. Therefore, it's best to have a professional install your skylight.

In addition to following the manufacturer's guidelines when installing a skylight, it's also important to consider slope and moisture control.

Skylight Slope

The slope or tilt of the skylight affects solar heat gain. A low-slope will admit relatively more solar heat in the summer and less in the winter, exactly the opposite of what is desirable.

As a general rule of thumb, you want to achieve a slope equal to your geographical latitude plus 5 to 15 degrees. For example, the optimum slope for a south-facing skylight in Columbus, Ohio, at 40º North latitude, is 45º to 55º. At least one skylight manufacturer makes a prefabricated, tilted base that increases the angle of a skylight above the roof.

Moisture Control and Water Leaks at Skylights - how to avoid

Water leaks are a common problem with skylights. Take the following steps to avoid water leaks:

• Mount the skylight above the roof surface
• Install a curb (a raised, watertight lip that helps to deflect water away from the skylight) and flashing
• Thoroughly seal joints
• Follow the manufacturer's guidelines.

It is also prudent to apply a layer of sheet waterproofing over the flanges/flashing of the skylight. This is generally installed under the finish roofing material as an aid in protecting against ice dams. Avoid water diversion devices such as roof crickets or diverter strips, as they often create more problems than they solve.

Window Design Suggestions for Energy Efficiency - DOE

Windows provide our homes with light, warmth, and ventilation, but they can also negatively impact a home's energy efficiency. You can reduce energy costs by installing energy-efficient windows in your home. Some energy efficiency improvements to existing windows can also help.

Selecting New Energy-Efficient Windows

When properly selected and installed, energy-efficient windows can help minimize your heating, cooling, and lighting costs.

Energy Efficient Window Design Considerations

Before selecting windows for your home, you need to determine what types of windows will work best and where to improve your home's energy efficiency.

First, it's a good idea to understand the energy performance ratings of windows if you don't already. Then, you can determine what energy performance ratings you need for your windows based on your climate and home's design.

For labeling energy-efficient windows, ENERGY STAR® has established minimum energy performance rating criteria by climate. However, this criteria doesn't account for a home's design, such as window orientation. For more information, see passive solar window design.

If you're constructing a new home or doing some major remodeling, you should also take advantage of the opportunity to incorporate your window design and selection as an integral part of your

Energy Efficient Window Selection

You'll find that you have several options to consider when selecting what type of windows you should use in your home.

When selecting windows for energy efficiency, it's important to first consider their energy performance ratings in relation to your climate and your home's design. This will help narrow your selection. [Illustration - U.S. Department of Energy]

A window's energy efficiency is dependent upon all of its components: the window frame, its glazing or glass, and the window's operation.

Types of Window Frames

A window frame can conduct heat, contributing to a window's overall energy efficiency, particularly its U-factor .

There are advantages and disadvantages to all types of frame materials. Overall, vinyl, wood, fiberglass, and some composite frame materials provide greater thermal resistance than metal.

Aluminum or Metal Frames

Although very strong, light and almost maintenance free, metal or aluminum window frames conduct heat very rapidly. Because of this, metal makes a very poor insulating material. To reduce heat flow and the U-factor, metal frames should have a thermal break—an insulating plastic strip placed between the inside and outside of the frame and sash.

Composite Frames

Composite window frames consist of composite wood products, such as particle board and laminated strand lumber. These composites are very stable, they have the same or better structural and thermal properties as conventional wood, and they have better moisture and decay resistance.

Fiberglass Frames

Fiberglass window frames are dimensionally stable and have air cavities (similar to vinyl). When these cavities are filled with insulation, they offer superior thermal performance compared to wood or vinyl (similar to insulated vinyl frames).

Vinyl Frames

Vinyl window frames are usually made of polyvinyl chloride (PVC) with ultraviolet light (UV) stabilizers to keep sunlight from breaking down the material. PVC is a very versatile plastic with good insulating value. Vinyl window frames also do not require painting and have good moisture resistance. However, at high temperatures, they may expand and warp; at extremely low temperatures, they may crack. Also, if sunlight hits the material for many hours a day, colors other than white may tend to fade over time.

Insulated vinyl frames are also available. Unlike standard vinyl frames, their hollow cavities are filled with insulation. This makes them thermally superior to standard vinyl and wood frames. Usually these high-performance frames are used with high-performance glazings.

Wood Frames

Wood window frames insulate well, but they also expand and contract according to weather conditions. They can also be quite heavy and thicker than other frames. This can make storage difficult, reduce the view out the window, and reduce the amount of natural light in the room. Wood frames also require the most maintenance. There are, however, aluminum- or vinyl-clad wood frames that reduce maintenance requirements.

Types of Window Glazing or Glass

When selecting windows for your home, it's important to consider what type of glazing or glass you should use to improve your home's energy efficiency. Based on various window design factors—such as window orientation, your climate, your building design, etc.—you may even want different types of glazing for different windows throughout your home.

There are many types of glazing available for windows, especially since many glazing technologies can be combined. These window glazing technologies include the following: [US DOE links:]

• Window Gas Fills

  To improve the thermal performance of windows with insulated glazing, some manufacturers fill the space between the glass panes with gas.

  For these gas fills, window manufacturers use inert gases—ones that do not react readily with other substances. Because these gases have a higher resistance to heat flow than air, they (rather than air) are sealed between the window panes to decrease a window's U-factor.

  The most common types of gas used by window manufacturers include argon and krypton. Argon is inexpensive, nontoxic, nonreactive, clear, and odorless. Krypton is more expensive but has a better thermal performance.

• Heat-absorbing tints
  

  Heat-absorbing window glazing contains special tints that change the color of the glass. Tinted glass absorbs a large fraction of the incoming solar radiation through a window. This reduces the solar heat gain coefficient, visible transmittance, and glare.

  Some heat, however, continues to pass through tinted windows by conduction and re-radiation. Therefore, the tint doesn't lower a window's U-factor. However, inner layers of clear glass or spectrally selective coatings can be applied on insulated glazing to help reduce these types of heat transfer.

  Gray- and bronze-tinted windows—the most common—reduce the penetration of both light and heat into buildings in equal amounts (i.e., not spectrally selective). Blue- and green-tinted windows offer greater penetration of visible light and slightly reduced heat transfer compared with other colors of tinted glass. In hot climates, black-tinted glass should be avoided because it absorbs more light than heat.

  Tinted, heat-absorbing glass reflects only a small percentage of light, so it does not have the mirror-like appearance of reflective glass.

  Note: when windows transmit less than 70% of visible light, indoor plants can die or grow more slowly.

• Insulated Window Glazing or Glass

  Insulated window glazing refers to windows with two or more panes of glass. They are also called double-glazed, triple-glazed, and—sometimes more generally—storm windows.

  To insulate the window, the glass panes are spaced apart and hermetically sealed to form a single-glazed unit with an air space between each pane of glass. The glass layers and the air spaces resist heat flow. As a result, insulated window glazing primarily lowers the U-factor, but it also lowers the solar heat gain coefficient. (See energy performance ratings for windows, doors, and skylights for more information on these terms.)

  Some window manufacturers use spacers—which separate two panes of glass—that conduct heat less readily than others. These spacers can further lower a window's U-factor.

  Other technologies window manufacturers use to improve the energy performance of insulated glazing include these:

  • Gas fills - discussed above
  • Low-emissivity coatings - discussed just below

• Low-Emissivity Window Glazing or Glass

  Low-emissivity (Low-E) coatings on glazing or glass control heat transfer through windows with insulated glazing. Windows manufactured with Low-E coatings typically cost about 10%–15% more than regular windows, but they reduce energy loss by as much as 30%–50%.

  A Low-E coating is a microscopically thin, virtually invisible, metal or metallic oxide layer deposited directly on the surface of one or more of the panes of glass. The Low-E coating reduces the infrared radiation from a warm pane of glass to a cooler pane, thereby lowering the U-factor of the window. Different types of Low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain. A Low-E coating can also reduce a window's visible transmittance unless you use one that's spectrally selective.

  To keep the sun's heat out of the house (for hot climates, east and west-facing windows, and unshaded south-facing windows), the Low-E coating should be applied to the outside pane of glass. If the windows are designed to provide heat energy in the winter and keep heat inside the house (typical of cold climates), the Low-E coating should be applied to the inside pane of glass.

  Window manufacturers apply Low-E coatings in either soft or hard coats. Soft Low-E coatings degrade when exposed to air and moisture, are easily damaged, and have a limited shelf life. Therefore, manufacturers carefully apply them in insulated multiple-pane windows. Hard Low-E coatings, on the other hand, are more durable and can be used in add-on (retrofit) applications. The energy performance of hard-coat, Low-E films is slightly poorer than that of soft-coat films.

  Although Low-E coatings are usually applied during manufacturing, some are available for do-it-yourselfers. These films are inexpensive compared to total window replacements, last 10–15 years without peeling, save energy, reduce fabric fading, and increase comfort.

• Reflective Window Glazing or Glass

  Reflective coatings on window glazing or glass reduce the transmission of solar radiation, blocking more light than heat. Therefore, they greatly reduce a window's visible transmittance (VT) and glare, but they also reduce a window's solar heat gain coefficient (SHGC). (See energy performance ratings for windows, doors, and skylights for more information on these terms.)

  Reflective coatings usually consist of thin, metallic layers. They come in a variety of metallic colors, including silver, gold, and bronze.

  Reflective window glazing is commonly used in hot climates where solar heat gain control is critical. However, the reduced cooling energy demands they achieve can be offset by the resulting need for additional electrical lighting, so reflective glass is mostly used just for special applications.

• Spectrally Selective Window Glazing or Glass

  A special type of low-emissivity coating is spectrally selective. Spectrally selective coatings filter out 40%–70% of the heat normally transmitted through insulated window glass or glazing, while allowing the full amount of light to be transmitted.

  Spectrally selective coatings are optically designed to reflect particular wavelengths but remain transparent to others. Such coatings are commonly used to reflect the infrared (heat) portion of the solar spectrum while admitting a higher portion of visible light. They help create a window with a low U-factor and solar heat gain coefficient but a high visible transmittance. (See energy performance ratings for windows, doors, and skylights for more information on these terms.)

  Spectrally selective coatings can be applied on various types of inted glass to produce "customized" glazing systems capable of either increasing or decreasing solar gains according to the aesthetic and climatic effects desired.

  Computer simulations have shown that advanced window glazing with spectrally selective coatings can reduce the electric space cooling requirements of new homes in hot climates by more than 40%.

Window Operating Types

When selecting windows for your home, it's also important to consider how they're operated. Some operating types have lower air leakage rates than others, which will improve your home's energy efficiency.

There are numerous window operating types to consider. Traditional types include the following: [Illustration: U.S. Department of Energy]

• Awning Windows: the window is hinged at the top and open outward. Because the sash closes by pressing against the frame, they generally have lower air leakage rates than sliding windows.

• Casement Windows: hinged at the sides. Like awning windows, they generally have lower air leakage rates than sliding windows because the sash closes by pressing against the frame.

• Fixed Glass Windows: fixed panes that don't open. They're airtight but not suitable in places where window ventilation is desired.

• Hopper type windows: hinged at the bottom and open inward. Like both awning and casement, they generally have lower air leakage rates because the sash closes by pressing against the frame.

• Single- and double-hung windows: both sashes slide vertically in a double-hung window. Only the bottom sash slides upward in a single-hung window. These sliding windows generally have higher air leakage rates than projecting or hinged windows.

• Single- and double-sliding: both sashes slide horizontally in a double-sliding window. Only one sash slides in a single-sliding window. Like single- and double-hung windows, they generally have higher air leakage rates than projecting or hinged windows.

Select energy-efficient windows or improve the energy efficiency of existing ones.

Window Installation

Even the most energy-efficient window must be properly installed to ensure that its energy performance is achieved and that it does not contribute to a home's moisture problems . Therefore, it's best to have a professional install your windows.

Window installation varies depending on the types of the following materials:

• Window Type
• House construction (wood versus masonry)
• Exterior cladding (e.g., wood siding, stucco, brick)
• Weather-resistive barrier.

Windows should be installed following manufacturing recommendations, along with the following additional guidelines (primarily for windows with fin mounting systems).

Window Installation Guidelines

• The window opening must be flashed and integrated into the home's weather-resistive barrier so that any potential water leaks do not cause damage. A weather-resistive barrier is a drainage plane that allows water that has penetrated past the siding to drain away from the wall system. Details will vary with siding, window type, the installation sequence for the window, trim, and weather-resistive barrier. It is advisable to install window head and sill flashing, whether it is metal, plastic, or a self-sticking, elastomeric membrane. Avoid relying on tapes or sealants to provide waterproofing, as these products may fail over time.

• It's common practice and recommended by some manufacturers to cut an "X" in housewrap placed over window openings, pull the material inside, and secure it by stapling. Other manufacturers require alternative methods, such as the modified "I"-cut, depending on the overall flashing approach. The "I"-cut allows the vertical leg of the head flashing to be placed under the weather-resistive barrier and then taped or sealed.

• It's best to divert drainage onto the face of the weather-resistive barrier. Do not tape down or seal behind the bottom nailing flange of the window, as doing so could accidentally trap in water.

• Windows must also be properly air sealed during installation to perform correctly. To air seal the window, caulk the backsides of the window mounting flanges (top and sides only) to the weather-resistive barrier during installation. The mounting flange (nailing fin) is an integral part of most window frames that laps over the conventional stud construction. Nails are driven through it to secure the frame in place. Also, from inside the house, seal the gap between the window frame and rough opening using backer rod and caulk or non-expanding latex-based spray foams that will not pinch jambs or void window warranties. Backer rod is a closed-cell foam or rope caulk that is pressed into cracks or gaps with a screwdriver or putty knife. Insulation stuffed into this crack does not stop air flow.

[Illustration: U.S. Department of Energh]

Improving the Energy Efficiency of Existing Windows

You can improve the energy efficiency of existing windows by taking the steps discussed just below: adding storm windows, or adding caulking and weatherstripping to existing windows. However, if your home has very old and/or inefficient windows, it might be more cost effective to replace them than to make these improvements. New, energy-efficient windows eventually pay for themselves through lower heating and cooling costs, and sometimes even lighting costs. [U.S. DOE articles]

Before buying new windows or even adding storm windows, take a look at your existing windows to see if simple adjustments can reduce air leaks around the window sash. For example some older homes (pre 1940) had window stops (the wood that holds the window sash in place) that could be adjusted by loosening screws and pushing the stop towards the sash. In any case, if you are keeping old windows, check for cold air drafts by feeling around the windows on a cold windy day.

Add Storm Windows

If you have old windows in your home, the best way to improve your home's energy efficiency is to replace them with new, energy-efficient windows. However, if you're on a tight budget, a less expensive option is to use storm windows. Some types of storm windows are also a good option for those living in apartments.

Even though storm windows add little to the insulating performance of single-glazed windows (that are in good condition,) field studies have found that they can help to reduce air movement into and out of existing windows. Therefore, they help reduce heating and cooling costs.

Types of Storm Windows

Storm windows are available for most types of windows. They can be installed on the interior or exterior of the primary window. They range from the inexpensive plastic sheets or films designed for one heating season, to triple-track glass units with low-emissivity coatings that offer many years of use. Mid-priced storm windows may use glass, plastic panels, or special plastic sheets that have specific optical qualities. Those made of polycarbonate plastic or laminated glass also offer a high degree of resistance to breaking during storms and/or from intruders.

For the most part, interior storm windows offer greater convenience than exterior storm windows. They're easier to install and remove; they require less maintenance because they're not exposed to the elements; and, because they seal tightly to the primary window, they're more effective at reducing air infiltration. Interior storm windows also are often the best choice for apartments and houses with more than one floor. If you can afford exterior storm windows, you can probably afford some newer, more energy-efficient windows, which will be a better investment.

Glass pane types offer better visibility and longer life than plastic pane types, but glass is heavy and fragile. In general, plastics are most economical for people with small budgets or who live in apartments. However, while inexpensive and relatively easy to install, they are easy to damage. Plastic panels, such as Plexiglas and acrylics are tougher and lighter than glass, but may scratch easily. Some may turn yellow over time as well. Some plastic films may significantly reduce visibility and degrade over time when exposed to sunlight.

Wood, aluminum, and vinyl are the most common storm window frame materials. There are advantages and disadvantages to all types of frame materials. Although very strong, light, and almost maintenance free, aluminum frames conduct heat very rapidly. Because of this, aluminum makes a very poor insulating material.

Wood frames insulate well, but they weather with age. They also expand and contract according to weather conditions. Wood-frame storm windows installed during the winter may not close easily during the summer, and those installed during the summer may fit loosely in the winter. They can also be quite heavy and thicker than metal frames. This can make storage difficult, reduce the view out the window, and reduce the amount of natural light in the room. Wood frames also require the most maintenance. There are, however, aluminum- or vinyl-clad wood frames that reduce maintenance requirements.

Vinyl frames are usually made of polyvinyl chloride (PVC) with ultraviolet light (UV) stabilizers to keep sunlight from breaking down the material. They, however, may expand and warp at high temperatures, and crack in extremely low temperatures. Also, if sunlight hits the material for many hours a day, colors other than white will tend to fade over time.

Storm Window Installation

No matter what type you choose, the storm window frame must be hung square with the primary window and sealed to the opening. You should also consider the fact that they should be easy to move to allow for cleaning and ventilation.

Exterior-mounted storm windows must have "weep holes" at the bottom of the frame to allow any moisture that collects between the primary window and the storm window to drain out. Even though these drainage holes subtract from energy savings, not having them will eventually cause the primary window frame to rot, and possibly make them impossible to operate.

Window Caulking and Weatherstripping

Most experts agree that caulking and weatherstripping—two simple air sealing techniques—will pay for themselves in energy savings within one year. Applying these techniques will also alleviate drafts and help your home feel warmer when it's cold outside.

Before you use any air sealing technique in your home, you need to do the following:

• Detect air leaks
• Assess your ventilation needs for indoor air quality.

Then, you can select and apply the materials:

Window Caulking

Caulk forms a flexible seal for cracks, gaps, or joints less than 1-quarter-inch wide. You can use a caulking compound to seal air leaks in a variety of places throughout your home, including around windows and door frames.

In addition to sealing air leaks, caulking can also prevent water damage inside and outside of the home when applied around faucets, ceiling fixtures, water pipes, drains, bathtubs and other plumbing fixtures.

Before caulking any air leaks in an existing home, you need to do the following, if you haven't already:

Window Weatherstripping

You can use weatherstripping in your home to seal air leaks around movable joints, such as windows or doors.

To determine how much weatherstripping you will need, add the perimeters of all windows and doors to be weatherstripped, then add 5%–10% to accommodate any waste. Also consider that weatherstripping comes in varying depths and widths.

Before applying weatherstripping in an existing home, you need to do the following (if you haven't already):

Energy Performance Ratings for Windows, Doors, and Skylights

You can use the energy performance ratings of windows, doors, and skylights to tell you their potential for gaining and losing heat, as well as transmitting sunlight into your home.

Window or Door Heat Gain and Loss Sources

Windows, doors, skylights can gain and lose heat in the following ways:

• Direct conduction through the glass or glazing, frame, and/or door
• The radiation of heat into a house (typically from the sun) and out of a house from room-temperature objects, such as people, furniture, and interior walls
• Air leakage through and around them.

These properties can be measured and rated according to the following energy performance characteristics:

• U-factor

  The rate at which a window, door, or skylight conducts non-solar heat flow. It's usually expressed in units of Btu/hr-ft2-ºF. For windows, skylights, and glass doors, a U-factor may refer to just the glass or glazing alone. But National Fenestration Rating Council U-factor ratings represent the entire window performance, including frame and spacer material. The lower the U-factor, the more energy-efficient the window, door, or skylight.

• Solar heat gain coefficient (SHGC)

  A fraction of solar radiation admitted through a window, door, or skylight—either transmitted directly and/or absorbed, and subsequently released as heat inside a home. The lower the SHGC, the less solar heat it transmits and the greater its shading ability. A product with a high SHGC rating is more effective at collecting solar heat gain during the winter. A product with a low SHGC rating is more effective at reducing cooling loads during the summer by blocking heat gained from the sun. Therefore, what SHGC you need for a window, door, or skylight should be determined by such factors as your climate, orientation, and external shading. For more information about SHGC and windows, see passive solar window design.

• Air leakage

  The rate of air infiltration around a window, door, or skylight in the presence of a specific pressure difference across it. It's expressed in units of cubic feet per minute per square foot of frame area (cfm/ft2). A product with a low air leakage rating is tighter than one with a high air leakage rating.

Sunlight Transmittance

A window's, door's, or skylight's ability to transmit sunlight into a home can be measured and rated according to the following energy performance characteristics:

• Visible transmittance (VT)

  A fraction of the visible spectrum of sunlight (380 to 720 nanometers), weighted by the sensitivity of the human eye, that is transmitted through a window's, door's, or skylight's glazing. A product with a higher VT transmits more visible light. VT is expressed as a number between 0 and 1. The VT you need for a window, door, or skylight should be determined by your home's daylighting requirements and/or whether you need to reduce interior glare in a space.

• Light-to-solar gain (LSG)

  The ratio between the SHGC and VT. It provides a gauge of the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains. The higher the number, the more light transmitted without adding excessive amounts of heat. This energy performance rating isn't always provided.

Energy Performance Testing, Certification and Labeling

The National Fenestration Rating Council (NFRC) operates a voluntary program that tests, certifies, and labels windows, doors, and skylights based on their energy performance ratings. The NFRC label provides a reliable way to determine a window's energy properties and to compare products.

The NFRC label can be found on all ENERGY STAR® qualified window, door, and skylight products, but ENERGY STAR bases its qualification only on U-factor and SHGC ratings.

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  SLOPED GLAZING DETAILS
  VERTICAL GLAZING DETAILS
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• Insulating and Air Sealing, U.S. Department of Energy, http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11220 - the website discusses air sealing, building insulation, moisture control, and building ventilation.
• Exterior Doors, contribution to building air leakage, U.S. Department of Energy - PDF file
• Exterior Door Selection and Installation, U.S. Department of Energy - PDF file
• Storm Doors as energy savers, U.S. Department of Energy - PDF file

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
• "Weather-Resistive Barriers", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
• Masonite Woodruf® Roofing or Masonite OmniWood® Siding Lawsuit Settlement Notice - PDF file
• ASHRAE resource on dew point and wall condensation - see the ASHRAE Fundamentals Handbook, available in many libraries. The following three ASHRAE Handbooks are also available at the InspectAPedia bookstore in the third page of our Insulate-Ventilate section:
  • 2005 ASHRAE Handbook : Fundamentals : Inch-Pound Edition (2005 ASHRAE HANDBOOK : Fundamentals : I-P Edition) (Hardcover), Thomas H. Kuehn (Contributor), R. J. Couvillion (Contributor), John W. Coleman (Contributor), Narasipur Suryanarayana (Contributor), Zahid Ayub (Contributor), Robert Parsons (Author), ISBN-10: 1931862702 or ISBN-13: 978-1931862707
  • 2004 ASHRAE Handbook : Heating, Ventilating, and Air-Conditioning: Systems and Equipment : Inch-Pound Edition (2004 ASHRAE Handbook : HVAC Systems and Equipment : I-P Edition) (Hardcover)
    by American Society of Heating, ISBN-10: 1931862478 or ISBN-13: 978-1931862479
    "2004 ASHRAE Handbook - HVAC Systems and Equipment The 2004 ASHRAE HandbookHVAC Systems and Equipment discusses various common systems and the equipment (components or assemblies) that comprise them, and describes features and differences. This information helps system designers and operators in selecting and using equipment. Major sections include Air-Conditioning and Heating Systems (chapters on system analysis and selection, air distribution, in-room terminal systems, centralized and decentralized systems, heat pumps, panel heating and cooling, cogeneration and engine-driven systems, heat recovery, steam and hydronic systems, district systems, small forced-air systems, infrared radiant heating, and water heating); Air-Handling Equipment (chapters on duct construction, air distribution, fans, coils, evaporative air-coolers, humidifiers, mechanical and desiccant dehumidification, air cleaners, industrial gas cleaning and air pollution control); Heating Equipment (chapters on automatic fuel-burning equipment, boilers, furnaces, in-space heaters, chimneys and flue vent systems, unit heaters, makeup air units, radiators, and solar equipment); General Components (chapters on compressors, condensers, cooling towers, liquid coolers, liquid-chilling systems, centrifugal pumps, motors and drives, pipes and fittings, valves, heat exchangers, and energy recovery equipment); and Unitary Equipment (chapters on air conditioners and heat pumps, room air conditioners and packaged terminal equipment, and a new chapter on mechanical dehumidifiers and heat pipes)."
  • 1996 Ashrae Handbook Heating, Ventilating, and Air-Conditioning Systems and Equipment: Inch-Pound Edition (Hardcover), ISBN-10: 1883413346 or ISBN-13: 978-1883413347 ,
    "The 1996 HVAC Systems and Equipment Handbook is the result of ASHRAE's continuing effort to update, expand and reorganize the Handbook Series. Over a third of the book has been revised and augmented with new chapters on hydronic heating and cooling systems design; fans; unit ventilator; unit heaters; and makeup air units. Extensive changes have been added to chapters on panel heating and cooling; cogeneration systems and engine and turbine drives; applied heat pump and heat recovery systems; humidifiers; desiccant dehumidification and pressure drying equipment, air-heating coils; chimney, gas vent, fireplace systems; cooling towers; centrifugal pumps; and air-to-air energy recovery. Separate I-P and SI editions."
  • Principles of Heating, Ventilating, And Air Conditioning: A textbook with Design Data Based on 2005 AShrae Handbook - Fundamentals (Hardcover), Harry J., Jr. Sauer (Author), Ronald H. Howell, ISBN-10: 1931862923 or ISBN-13: 978-1931862929
  • 1993 ASHRAE Handbook Fundamentals (Hardcover), ISBN-10: 0910110964 or ISBN-13: 978-0910110969
• Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Wiley.com and also at Amazon.com. See our book review of this publication.
• Energy Savers: Whole House Systems Approach to Energy Efficient Home Design - U.S. Department of Energy
• "Energy Savers: Whole-House Supply Ventilation Systems", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11880?print
• "Energy Savers: Whole-House Exhaust Ventilation Systems", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11870
• "Energy Savers: Ventilation", U.S. Department of Energy
• "Energy Savers: Natural Ventilation", U.S. Department of Energy
• "Energy Savers: Energy Recovery Ventilation Systems", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11900
• "Energy Savers: Detecting Air Leaks", U.S. Department of Energy
• "Energy Savers: Air Sealing", U.S. Department of Energy
• "Insulation: Adding Insulation to an Existing Home," U.S. Department of Energy - tips on how to do your own check for the presence of absence of insulation in a home
• Insulation: Selecting Insulation for New Home Construction, U.S. Department of Energy - "Your state and local building codes probably include minimum insulation requirements, but to build an energy-efficient home, you may need or want to exceed them. For maximum energy efficiency, you should also consider the interaction between the insulation and other building components. This is called the whole-house systems design approach."
• Insulation Types, table of common building insulation properties from U.S. DOE. Readers should see INSULATION R-Values & Properties our own table of insulation properties that includes links to articles describing each insulation material in more detail.
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