Conservation keeps your home comfortable with less mechanical heating and cooling. Measures which help your home retain warmth in the winter and coolness in summer include: air sealing, insulation, sealed attic, insulated attic, insulated floor, radiant roof barrier, and efficient window upgrades including a low-e plus impact resistant window, interior window cover and low-e window film. A programmable thermostat allows you to turn down the heat and air conditioning when you are away or need it less.
The average cold weather home heating demand is very high: 97 million BTU’s per year. This is the equivalent to the heat delivered by a 78% efficient furnace burning 900 gallons of heating oil. For comparison, an average house in New York City might use 700 gallons of oil, and a house in San Francisco 450 gallons.
A typical home energy retrofit for a cold weather home could reduce heating requirements by up to 30%. A retrofit may include some combination of air sealing, insulation, duct sealing, heating zones, and programmable thermostat.
Super-insulating an old home could reduce heating requirements by up to 70%. This would include additional measures such as double-width insulated walls, very thick attic insulation, double-pane, low-e windows and a heat recovery ventilator. A heat recovery ventilator brings in fresh outside air while recovering 70% of the heat from the stale outgoing air.
As a benchmark, the tightest most well designed new cold weather home might use only 10% of the average fuel use!! Additional conservation features of new homes might include thick slab insulation, triple pane windows with low-e glazing, very tight air sealing, and south windows with solar mass. Such a home might use the equivalent of less than 100 gallons for annual heating.
On the conservation side you could spend $20,000 or more to reduce the heating demand. On the efficiency side you might spend up to $30,000 (for a geothermal system) to improve your heating efficiency. What is needed is a good combination of conservation and heating efficiency which delivers your desired return on investment.
This device controls the temperature settings for heating and cooling according to days of the week and hours of the day. Many people use this device to turn down the heat or air conditioning when they are away at work or at night.
This is the single most cost effective item a homeowner can install to save on heating and cooling bills. The federal Energy Star program estimates a programmable thermostat can save you about $180 every year in energy costs.
Leaks of heated or cooled air to the outside may account for one-third to one-half of heat losses in older homes. Sealing the numerous holes and cracks in and around walls, windows, ceilings, floors and foundations is like putting a windbreaker on your home. It keeps the warm or cool air in the house longer, by preventing heat loss by convection. A blower door test and thermal imaging camera can help to pinpoint leaks and measure air sealing improvements.
There is a huge variety of sealing techniques and materials available on the market today. These include caulking (including urethane foam guns), weather stripping for door and windows, door sweeps, and foam gaskets for electrical outlets and switch plates. Large holes can be filled with rigid foam board, aluminum flashing, foil faced bubble wrap, wood and even cardboard. Compacted cellulose, delivered under pressure, can fill large holes and also act as insulation.
Windows can be sealed by means of a plastic film or by manufactured interior window panels. Fireplaces drafts can be stopped by means fireplace doors, or by a fireplace balloon damper which is put in place only when the fireplace is not used.
Air sealing is best done before the installation of insulation in attics, walls, ceilings and floors. Insulation is more efficient when air is not blowing across it.
If “air sealing” is like a windbreaker, insulation is like a sweater. Insulation holds tiny pockets of air which prevent heat loss by conduction. Insulation also retards air leakage.
Air sealing is best done before the installation of insulation in attics, walls, ceilings and floors. Insulation is more efficient when air is not blowing across it.
In winter insulation keeps heat from escaping to the outdoors. In summer insulation keeps outdoor heat out. Insulation may used in attics, walls, floors, basements, crawlspaces, and foundations.
One study conducted by the Minnesota Department of Commerce found that insulation and air sealing helped reduce energy costs in pre-WWII homes by anywhere from 35-65 percent, depending on the level of investment undertaken. The Rocky Mountain Institute estimates that upgrading insulation in the attic, walls, and basement of the typical American home will reduce that home’s CO2 emissions by 4,147 lbs per year.
Thee DOE breaks insulation into the following categories:
• Blankets: batts or rolls of fiberglass or rockwool. Cotton fiber batts are also available in some areas Blankets work well for do-it-yourself projects when space is relatively free from interior obstructions.
• Loose-fill or Spray-applied: Usually blown in or sprayed-in-place with professional equipment, available substances include rockwool, loose fiberglass, cellulose, or polyurethane foam.
• Rigid insulation: Any among several types of condensed foam board; best suited for maximizing R-value with minimal thickness.
• Reflective barriers: Foil-faced materials designed to be particularly effective in preventing heat transfer via radiation.
Cellulose and fiberglass are thought to occupy between 80 and 90 percent of the US insulation market. Emerging alternative insulation solutions include wool, hemp, straw bales, and NASA-developed aerogel.
In cold climates, walls account for one-third of a home’s heat loss. Insulating an old home’s underperforming walls may save up to 10-20% in heating or cooling bills, saving hundreds of dollar per year.
A home’s exterior wall is hard at work doing all sorts of stuff: It supports the roof and connects it to the ground. It protects the interior from fierce winds, wildfires, heat and cold, sunlight, rain, hail, humidity, wildfires, dust, insects and noise. An insulation retrofit is an opportunity to improve the wall’s multi-dimensional performance.
All of these wall functions vary in importance, depending where you live. Insulation blocks indoor heat from escaping in winter, and summer heat from intruding in summer. Regarding insulation, don’t forget that heat always flows from greater to lesser. The purpose of insulation is to inhibit that flow.
The greatest savings from wall insulation accrue in extremely cold or hot climates where there are big differences in the desired indoor and outdoor temperatures. The greater the temperature difference from inside to outside, the more important insulation becomes. Insulating a wall in Florida (75oF inside /95ooutside ) will have a lower return on investment than insulating a Minnesota wall (75oF inside /25oF outside).
When And Where To Insulate: When your siding needs replacement, is a good time to install exterior insulation. Usually, this consists of loose rigid foam boards, or siding with foam built in. Foam has a high insulating value per inch, and is water resistant. Placing foam on the outside of a masonry wall transforms it into an insulated thermal mass, which reduces interior day-night temperature swings.
Cellulose or spray-foam insulation may be blown into a wall cavity-between wooden studs or in the voids of a concrete block wall. An inch or two thick of foam board may be added to the interior wall. Sometimes, in very cold climates, the exterior wall width is doubled (by installing more framing) to hold more insulation.
Optimizing Total Wall Performance: When installing insulation, is very important also to control three interrelated factors: 1) Air movement between outside and interior; 2) Rain leakage into the interior; and, 3) Humidity and condensation. Porous insulation like cellulose or fiberglass works poorly if it becomes wet or is exposed to moving air.
1) Air movement between outside and interior: Air moving through walls can increase heating/cooling load, and transport troublesome humidity. Before adding insulation, the wall needs to be air sealed, including measures such installing house wrap, door sweeps, and electrical gaskets seals, and by caulking windows. Spraying foam insulation into existing wall cavities is a good fix for leaky wood-framed walls. The foam bonds to wood and can flow around wires and electrical boxes to create tight air seals.
2) Rain leakage into the interior: Verify proper installation of window, door and roof flashing, and an adequate drainage plane. A drainage plane is a 1/8- to 1-inch air gap located just behind exterior siding (e.g., stucco, brick, vinyl, cedar shakes). It is lined by water-repelling house wrap or building paper. Its main function is to drain the small fraction of rain that penetrates the siding, to the ground. It may also be used to drain condensation and exhaust humidity to the outside.
3) Humidity and condensation: One thing to avoid is a rendezvous between warm humid air and a cool wall surface; this causes condensation. Another is the trapping of humidity in the wall between two water vapor barriers so the wall cannot dry out–either to the inside or to the outside. House wrap is formulated to repel liquid water while allowing the passage of water vapor to the wall.
Depending upon climate, a vapor barrier, may be omitted or placed to block the dominant flow of humidity: In hot and humid regions, a vapor barrier may be placed just behind the exterior siding/drainage plane assembly which “steams” after summer showers. This “steam” might otherwise condense on the insulation or inner wall, which was cooled by air conditioning.
In cold climates, a vapor barrier may be placed under the interior wallboard to prevent warm, humid house air from entering the winter cold wall cavity where it can cause condensation. Vapor barriers of different degrees of vapor permeability may be used to fine tune the movement of wall humidity.
Siding replacement is also an opportunity to treat the wall with borates to deter insects and mold. Adding plywood can give the wall shear strength for protection from hurricanes, tornados, and earthquakes. It is cost effective to install a durable, fire-resistant siding such as concrete board painted with high performance paint (e.g., solar reflective).
A Cautionary Tale of “Bad” Stucco Walls: Uncontrolled wall moisture can have disastrous consequences. One instructive example, is a national epidemic of stucco wall failures. Failure means that the sheathing rotted away, the stucco fell down and the walls needed to be rebuilt. Martin Holladay, of Green Building Advisor traced the causes of the this puzzling phenomenon.
In Minnesota alone, the repair costs for stucco have been running hundreds of millions of dollar annually. In one locality Woodbury, Minnesota half of the stucco walls built in 1990s failed. The stucco walls at the Woodbury home of Steve and Debbie Long required $174,860 to fix. Tragically, some of the Minnesota repairs also failed through lack of proper diagnosis of the problem. Some repairs failed two times!!
Holladay’s forensic analysis distilled four lessons to prevent stucco wall failures:
1) Two layers of building paper: One layers of building paper placed behind the stucco to create a drainage plane was not enough protection. Two layers were needed. However, two layers of building paper alone did not prevent thousands of walls with oriented strand board (OSB) sheathing from failing.
2) Ventilated drainage plane: The biggie! Stucco absorbs water every time it rains and it dries out slowly. So it is essential to provide a true ventilated drainage plane (aka rainscreen), with an air gap of at least 3/8 inch, behind the stucco. Such a drainage plane both intercepts rainwater, and helps the wall dry in-between storms by exhausting humidity to the outside.
3) Follow best practices: Manage moisture with practices such as effective flashing details and wide roof overhangs to protect walls from rain.
4) Don’t heap soil around the stucco wall. The soil will keep the wet stucco soaking wet. And the wet stucco will rot the OSB sheathing behind it.
Figures 1 and 2 illustrate how stucco wall failure can occur, and be prevented.
Conclusion: If done correctly, a wall insulation retrofit conserves lots of heating and cooling energy. However, insulation is but one aspect of a high performance wall. When performing a wall insulation retrofit, it is essential to control moisture and air movement. The wall must have the capacity prevent moisture from wetting the interior wall cavity, and to dry out between rainstorms. The means to do this will vary by climate zone. An essential element for all climate zones is the provision of a drainage plane air gap just behind the exterior siding.
Siding replacement and interior remodeling offer opportunities to upgrade a wall’s multi-dimensional performance. Upgrade opportunities include increased durability and fortification against wildfires, tornados, hurricanes, earthquakes, insects and mold.
When it comes time to replace your doors, you have an opportunity to upgrade them. One important upgrade in every climate zone is insulated doors. Another upgrade for hurricane and tornado areas is impact resistance.
Most insulated doors consist of an exterior and interior skin and are insulated by a foam core. Insulated doors keep heat in winter and heat out in summer. An insulated garage door can make your garage usable as a workshop or exercise room in hot or cold weather.
Older doors and windows had poor thermal performance, and not enough impact resistance to survive wind-born debris, e.g., 2 X 4 boards. So when upgrading in hurricane and tornado areas, make sure your new door (and new windows) are impact resistant.
There are many kinds of doors—wood, steel, fiberglass, and vinyl. Like anything else, you get what you pay for. Get reliable advice when choosing a door for your climate zone.
Insulation’s effectiveness depends on three factors: 1) Trapping of air bubbles; 2) The thermal resistance of the insulation material itself; and, 3) Its ability to retard the movement of air through it (convection).
For example, fiberglass insulation is made of small strands of glass which trap innumerable air bubbles. Most heat flow is blocked by these air bubbles. However, the glass fibers themselves block some heat. Glass is a poor conduction because of its atomic disorder. Furthermore, fiberglass strands that have minimal contact with each other so there is little opportunity to transfer heat.
The effectiveness of loose insulation such as fiberglass is limited because some air can flow through the material. Moreover, loose insulation does not perform well when wet or compacted. By contrast, closed cell foam insulation resists air flow, compaction and moisture penetration.
The performance of insulation is as part of a whole systems home climate control may be measured by its HERS Index, a standard measure of house energy efficiency. The thermal performance of insulation, and its presence or absence, may also be indicated by a thermal imaging camera. The air-sealing impact of insulation may be assessed by a blower door.
Insulation is rated by its ‘R” value per inch.. The higher the value the higher is the performance of the insulation. The following table comparative value for several insulation types.
The United States Department of Energy (DOE) recommends different amounts of insulation for recommended for different climate zones and parts of the house. http://www.simplyinsulate.com/savings/index.html
If “air sealing” is like a windbreaker, insulation is like a sweater. Insulation holds tiny pockets of air which prevent heat loss by conduction. Insulation also retards air leakage
The attic is the zone of greatest heat loss is cool climates. Hot air rising exerts a upward pressure which carries hot air up through the attic floor and exhausts it to the outside.
In a cool-climate vented attic, the insulation is placed on the attic floor. In older homes the attic insulation is most often thinner than current recommendations. Adding more insulation, or insulating from scratch, is a top value. Often insulation (usually fiberglass or cellulose) is blown in upon the attic floor.
In hot humid climates, the attic is a major entry point of heat and humidity into the home. In these climate zones, it is very cost effective to seal the attic with foam insulation. The insulation is sprayed between the rafters. The foam insulation adds hurricane protection: it strengthens the roof and provides a secondary water barrier.
Adequately insulating and air sealing the access to an attic— especially to unconditioned attics—will help lower your heating and cooling bills.
A home’s attic access, which could be an attic hatch, pull-down stairs, or a knee-wall door, often goes uninsulated. This gap in the attic insulation increases heat loss in the winter and heat gain in the summer.
These accesses also often aren’t sealed properly. A 1/4-inch gap around the perimeter of an attic access can potentially leak the same amount of air supplied by a typical bedroom heating duct.
Before insulating your attic access, you should first determine the recommended insulation R-value for your area and climate.
A sealed attic multi-tasks in hot, humid climates. The foam insulation, used to seal the attic, deters heat, humidity, mold and insects. A sealed attic often reduces air conditioning costs by 20% or more. A cooler, dryer sealed attic and sealed air conditioning (AC) ducts work together to prevent mold, and provide a better storage location.
The foam insulation also strengthens the roof. Closed cell insulating form acts like a secondary water barrier. These Fortified Home features may qualify the homeowner for a hurricane insurance discount.
In hot, humid climates, the attic is a major entry point of heat and humidity into the home. Most attics in hot, humid climates are vented to the outside air. These often have inadequate attic floor insulation and contain AC ducts. Unfortunately, condensation can occur when cool air, leaking from AC ducts, air mixes with hot, humid outside air. The resulting moisture favors mold growth. Also, hot, humid air from the attic leaks into the living quarters below and increases the AC load.
The conversion of a vented attic to a sealed one requires blocking the outside air vents, removing the old insulation, and spraying of about 4 inches of foam insulation on the roof sheathing between the roof rafters.
The foam insulation is usually a better insulator than the old insulation on the attic floor. The impact of any AC duct leakage is mitigated because the attic has now become part of the air conditioned space. It is a good idea to seal the AC ducts when the attic is sealed.
The Florida Division of Emergency Management approves the use of polyurethane foam adhesive, and insulating foam to strengthen the attachment of the the roof deck to rafters or truss http://www.floridadisaster.org/mitigation/rcmp/hrg/content/roofs/reroof_later.asp.
The foam creates a strong bond between all the structural members and the roof sheathing, and can keep your roof from blowing away in a hurricane.
Insulating foam significantly increases the stiffness and rigidity of the roof, including the deck. Closed cell foam acts as a secondary water barrier which reduces penetration of wind-driven hurricane rain.
The purpose of a radiant roof barrier is to keep your home cool on hot sunny days. It keeps heat waves, radiating from your hot roof and attic, out of your air-conditioned living space.
For new construction, the radiant barrier consists of highly reflective aluminum foil laminated on one side of roof decking. The decking, typically plywood or OSB (oriented strand board), is installed with the foil side facing down.
The foil requires an air space of at least three-quarters of an inch to function. The foil is usually perforated to allow moisture to escape from the building.
In existing homes the radiant barrier is a tarp-like material, laminated on both sides with foil (ref. http://en.wikipedia.org/wiki/Radiant_barrier ). The radiant barrier foil material can be either stapled to the bottom of the roof rafters or laid out over the existing insulation.
The U.S. Department of Energy estimates that a radiant barrier can reduce cooling load by 2 to 10 percent. Not bad for a thin sheet of aluminum foil!
Super-insulated houses were designed to minimize heating requirements in cold regions. If an ordinary house uses 1,000 gallons of heating oil per year, a super-insulated one would use 100 to 250 gallons.
The design features of a super-insulated house work together to minimize heat loss. A super-insulated house has a continuous layer of insulation from top to toe. The foundation, walls, and ceilings have thicker-than-code insulation. The windows are also efficient, often with triple panes and low-e coatings.
Cold air penetration is well controlled with a continuous air barrier. Exterior air exchanges may be 10% of those in an average home. Since outdoor ventilation is so limited, a mechanical heat recovery ventilator (HRV) is used to exhaust stale air containing moisture and air pollutants. The HRV can recover 75% of the heat contained in the stale outgoing air. The mechanical system has to be able to efficiently ventilate all the living spaces.
In tight super-insulated homes, it is critical to minimize air contaminants. This is done by measures such as low VOC paints and finishes, low VOC rugs, cabinets and furnishings, and eco-friendly pest control.
Due to air quality and energy efficiency concerns, only sealed combustion furnaces, stoves, or fireplaces are specified for super-insulated homes: Valuable pre-heated interior air is not wasted for combustion. The combustion air is piped in from outside. The fire chamber is sealed from interior air. This prevents pollutants, such a carbon monoxide, from entering the home’s living spaces.
The measurement of a windows performance is a highly technical subject. Nevertheless, as informed consumers, we need to be able to compare a window’s ability to:
• Block or transmit solar heat
• Transmit visible light
• Insulate from heat and cold
• Resist wind-borne debris carried by hurricane winds
• Muffle sound
What follows is a brief description of how these aspects of window performance are measured.
Solar Heat Gain Coefficient (SHGC)
In warm climates such as Florida, solar heat gain, through windows, is responsible for a third of the heating load. The Solar Heat Gain Coefficient (SHGC) is the fraction of sunlight which is admitted through a window and released as heat indoors. It is expressed as a number between 0 and 1, which translate to between 0 to 100%.
A conventional single pane window has a SGHC of 0.86 , and admits 86% of the sun’s heat. A low-e window may have a SGHC of 0.32 and admit only 32% of the sun’s heat.
The lower the SHGC, the less solar heat that the window transmits through the glazing, and the greater its shading ability. In general, south-facing windows in houses designed for passive solar heating (with a roof overhang to shade them in the summer) should have windows with a high SHGC to allow in beneficial solar heat gain in the winter.
Visible Light Transmittance (VT)
Visible Light Transmittance (VT) is similar to the SHGC. A single-pane window has a VT of 0.90 which means that it transmits 90% of the sun’s visible light. Modern windows can be purchased with a VT as low as 0.44 or 44%.
The visible transmittance (VT) refers to the percentage of the visible spectrum (380-720 nanometers) that is transmitted through the glazing. When daylight in a space is desirable, as in showrooms and studios, high VT glazing is a logical choice. However, low VT glazing such as bronze, gray, or reflective-film windows are more logical for office buildings or where reducing interior glare is desirable. A typical clear, single-pane window has a VT of 0.90, meaning it admits 90% of the visible light.
Light-To-Solar Gain Ratio (LSG)
The ratio between SHGC and VT is called the light-to-solar gain ratio (LSG). LSG provides a gauge of the relative efficiency of different glass types in transmitting daylight while blocking heat gains. The higher the window’s LSG, the brighter the room is without adding excessive amounts of heat. A high performance low-e window can have a LSG of 1.65. An ordinary double pane window has a LSG of 1.05.
“R” Value
A window’s ability to insulate, to keep heat out on a hot day, or keep cool air in on a hot day, is measured by the “R” value. The higher the “R’ Value, the higher is the window’s performance.
A single-pane window as “R” value of 1; a double-pane window has an “R” of 2. A roof in New England may have an “R” value of around 30. Modern high-performance windows may have an “R” value of 3.
“U” Value
A window’s ability to insulate, to keep heat out on a hot day, or keep cool air in on a hot day, is measured by the “R” value. The higher the “R’ Value, the higher is the window’s performance.
“U” is the mathematical inverse of “R.” The “U” of a single-pane window is 1; the “U” of a double pane window is ½ or 0.5; a New England roof may have a “U” of 1/30 or 0.03; and, a high performance window may have a “U” of 0.3.
Impact Resistance
A window’s ability to insulate, to keep heat out on a hot day, or keep cool air in on a hot day, is measured by the “R” value. The higher the “R’ Value, the higher is the window’s performance.
The most stringent standard for window impact resistance is embodied in the Miami-Dade Building Code. The Miami-Dade Building Code requires that every exterior opening– residential or commercial–be provided with protection against wind-borne debris caused by hurricanes. Such protection could either be shutters or impact-resistant products.
There are two types of impact-resistant products: large-missile resistant and small-missile resistant. If you live in a building where doors and windows are located 30 feet or less above grade (e.g. above ground level), then the products must pass the large-missile test. If the doors and windows are more than 30 feet from the ground, then they must be either large or small missile resistant.
A product is tested for large-missile resistance by exposure to various impacts with a piece of lumber weighing approximately 9 pounds, measuring 2″ x 4″ x 6’ (no more than 8′) in size, traveling at a speed of 50 feet per second (34 mph). Then the product must pass positive and negative wind loads for 9,000 cycles, with the impact creating no hole larger than 1/16 x 5″ in the interlayer of the glass.
A product is declared small-missile resistant if it has withstood various impacts with 10 ball bearings traveling at a speed of 80 feet per second (50 mph). The product is then subjected to wind loads for 9,000 cycles. This test simulates the impact of gravel on a window.
Also See: Impact Windows
Acoustic Properties
The ability of a window to block sound is measured by Sound Transmission Class (STC). This is roughly the decibel reduction in noise a window can provide, abbreviated ‘dB’. The dB scale is a logarithmic one and the human ear perceives a 10dB reduction in sound as roughly halving the volume–a 40 dB noise subjectively seems half as loud as a 50 dB one. An ordinary 1/8 inch window has a STC of 27, which means that it reduces outside noise by 27 dB. Insulated double-pane glass has a STP of 31 dB.
An impact resistant window consisting of an outside sandwich of two panes of glass enclosing a vinyl membrane and an inner pane has a STC of 42. It is roughly twice as quiet as a double-pane insulated window.
A related measure to STC is Outside Inside Transmission Class (OITC) which accounts for low frequency sound such as trucks rumbling past.
Window glass is manufactured from quartz sand which consists of silicone dioxide. Manufacturing melts the quartz crystals in the sand, which cool into a rigid, transparent substance. Most window glass is manufactured in the range 1/8 to 1/4 inch thick.
The plain-Jane single-pane window, pictured below is a single sheet of glass set in a frame.
Windows serve to admit the suns heat and light. They protect buildings from rain, wind, dust, exterior noise, animals and flying debris. They stabilize the temperature in a building by preventing infiltration of air. They also heat buildings through the greenhouse effect.
A single-pane window is most suited to a warm climate where the annual temperature approximates room temperature. However, these windows are gradually being replaced in warm locations, due to their vulnerability to hurricane damage and rising energy costs.
Double pane windows are commonly used in cool climates for their higher insulation value.
The physical and chemical properties of window glass enable it to perform many functions. Its rigid nature helps keep it in place; this property also makes it easy to shatter. The silicone dioxide in windows is impermeable to air, water, and dust. It is chemically inert so that it is very slow to degrade in use through the action of sun and rain.
A window warms buildings through “the greenhouse effect.” As illustrated below, sunlight enters the clear window. When the sunlight strikes the interior surface, it is converted to heat waves. The heat waves are larger and vibrate more slowly than the light waves in the original sunlight. For this reason they are not able to penetrate the glass, and remain trapped inside.
Thermopane insulated window glass was invented by C. D. Haven in the United States, in 1930. As the diagram below shows, insulating glass consists of two sheets of glass, mounted together with one-half inch of air in the space between them. Often this space is filled with argon.
A double pane window has double the insulating value of a single pane window. Thus, it has a greater capacity to protect the interior from outside extremes of cold and heat. It also dampens noise 3 decibels (dB) more than a single plane window.
Insulating glass transmits about 76% of total solar heat gain and 81% of visible light. This is compared than a single-pane which transmits about 86% of total solar heat and 90% of visible light.
The extra pane of glass and the gap between the panes act as extra insulation. The use of argon in the gap between the panes can increase the insulating quality of the window by 6%. Argon is less dense than air and acts as a better insulator.
A low-emissivity (low-e) window consists of a double-pane, insulated window equipped with a very thin metallic coating. This coating functions like a mirror which reflects long-wave heat radiation.
Any low-e coating is roughly equivalent to adding an additional pane of glass to a window. Low-e coatings reduce heat transfer by long-wave radiation by 5 to 10 times.
A low-e window, increases summer comfort and lowers cooling bills, especially in hot climates. Before innovations in glass, films, and coatings in the past decade, a typical residential window, with one or two layers of glazing, allowed roughly 75-85% of the solar energy to enter a building. By contrast, a low e-window only allows 30-35% of the sun’s heat to come inside.
The diagram below shows the use of a low-e window in blocking the sun’s energy. The low-e window transmits most of the visible light, but blocks the longer-wave infrared light. Low-e windows are especially useful on east and west exposures to block unwanted hot morning and afternoon sun.
The three divisions of the solar spectrum–ultraviolet, visible and infrared– are illustrated in the diagram below. Low-e windows block the large proportion of solar energy, which is in the form of invisible infrared radiation. Infrared waves have relatively low energy. Low-e windows also block high-energy ultraviolet light waves, which fade carpets and produce suntans.
A low-e window also has application in keeping a cool-climate home warm in winter. The window configuration, shown below, is the reverse of that of a tropical climate: The heat waves from the house are reflected from the window back into the house. Sunlight (visible and infrared) is allowed in to heat and light the house.
Emissivity is a measure of a material’s ability to radiate absorbed energy. Emissivity factors range from 0.00 to 1.00. In general, the duller and blacker a material is, the closer its emissivity is to 1. Similarly, an ordinary clear, uncoated single glass pane absorbs and transmits most of the sun’s energy; its emissivity is 0.84
The more reflective a material is, the lower its emissivity. Highly polished silver has an emissivity of about 0.02. A low-e coating gives a window a mirror-like property to reflect long-wave infrared radiation. The lower a window’s emissivity, the less solar heat it transmits.
Most window manufacturers now use one or more layers of low-e coatings in their product line. Low-e window coatings usually consist of a very thin layer of metal, a few molecules thick. Below are some emissivity values for a single pane of glass with and without low-e coatings:
• Clear glass, uncoated: 0.84
• Glass with single hard coat low-e: 0.15
• Glass with single soft coat low-e: 0.10
The installation of low-e window film can greatly enhance a windows performance, at a relatively low cost. In hot climates, the metallic coating of the film is positioned to reflect solar radiation. The high performance film can reflect half or more of solar radiation. In cool climates the film is installed to reflect interior heat back inside.
In warm climates low-e film is especially useful to block solar radiation coming through large, sunny east- or west-facing windows. Low-e film prevents overheating which greatly enhances room comfort and lowers your air conditioning bill.
Double and triple pane, low-e windows are the aristocrats of the glazing world. When used to replace old single-pane windows, they confer tremendous energy savings and quality of life benefits.
An innovation for aluminum and steel windows is a thermal break, between the inside and outside of the window frame. A thermal break prevents heat loss or gain through the frame itself. Newer vinyl and fiberglass window frames are poor heat conductors.
Triple-Pane Pros and Cons: Triple-pane, low-e windows inhabit super-insulated homes in cold climates. Their thermal rating of R-4 is stellar. R- 2.5 is typical of double-pane, low-e windows. Single-pane windows with no low-e coating rate R-1.
Triple-pane windows may cost 25% more than double panes. Consequently, in cold climates, window replacements with triple-pane windows have a lower rate of return (energy savings versus costs) than that of double-panes. However, triple-panes do provide a hedge against high future energy prices.
Triple-pane windows have a substantial advantage over double-panes for comfort. (Visualize a winter’s day in Minnesota, high temperature 10oF.) On a very cold day, the interior surface of triple-pane windows is warmer than that of double-panes. Their higher radiant temperature makes them feel noticeably warmer than double panes. Also triple panes minimize cold drafts arising from the convective cooling of interior air. The warmer panes also minimize condensation problems. A final advantage is that triple panes provide greater soundproofing.
Things not to like: Thicker triple-pane windows are less pleasing to the eye. Their additional weight can make them harder to open and close.
In cold climates, old-fashioned drafty single-pane windows make rooms uncomfortable and are energy hogs. Modern storm windows enable the homeowner to dramatically upgrade window energy performance without the high cost of window replacement.
The function of a storm window is to add an extra “skin” of cold weather protection to an existing window. They seal out cold drafts and keep interior heat inside.
Modern storm windows are manufactured in both single-pane and double-pane designs, and offer the option of low-e coatings. They also offer the option of conveniently converting into a screened window to catch warm weather breezes.
Storm windows may be placed inconspicuously on the windows exterior enabling a valuable energy upgrade while preserving the look of traditional windows. For example, a period reproduction of Victorian or Colonial window, updated for energy efficiency, would be far too expensive to be practical. Storm windows can also be conveniently installed on the interior of the window.
The U.S. Departments of Energy and Housing and Urban Development sponsored a storm window energy conservation study, in which homes with single-pane windows were retrofitted with single-pane storm windows. The study confirmed the tremendous potential benefits of storm windows in tightening the house and reducing heating bills.
The study concluded: “Based on the results from the field monitoring, storm windows should be considered as an energy efficiency improvement measure for homes with single-pane windows in northern climates. The data gathered from six homes in Chicago indicate that there is consistent benefit to using storm windows. Clear glass storm windows reduced the heating load by 13% with a 10-year simple payback. Low-e storm windows also showed an additional improvement on top of the clear glass benefits amounting to 21% heating savings and an average payback of less than five years.
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