Windows and doors, being a large part of the building envelope, can contribute up to 30% of heat loss and gain because they can pose a triple threat -- heat moves through a window assembly by conduction through the glass, convection/leakage through gaps and cracks in the framing and assembly and joint with the wall, and by radiation by pulling heat from warm room-temperature objects including people – that’s why when you stand next a window you feel cold in the winter and warmer in the summer (heat moves to the colder space, and in the summer that would be you and the room you are occupying).  Addressing energy loss issues with windows and doors can have a dramatic impact on your energy consumption and heating and cooling bills.

Whether or not you need to replace your windows depends on the type and condition of your existing windows.  If you already have insulated glass windows and the sashes are working then you may not need to replace them, even fogged windows can often be repaired, but the windows should have a thermal break and frames of aluminum or wood usually have the best energy performance.  This Article will tell you how to improve the efficiency of existing windows and how to choose new windows if you need to replace what you've got, starting with the basics of window design and energy saving features.

Windows particularly can have an impact on energy consumption (heating and cooling costs) because they can pose a triple threat -- heat moves through a window assembly by conduction through the glass, convection/leakage through gaps and cracks in the framing and assembly and joint with the wall, and by radiation by pulling heat from warm room-temperature objects, including people, to the cold space – that's why when you stand next to a window you feel cold in the winter and warmer in the summer (in winter heat move from warmth, that would be you, toward the cold, and vice versa in the summer).  Understanding how windows are made and the technologies available can help you make the best choices for your window.


U.S. Department of Energy

The National Fenestration Rating Council is a voluntary organization that rates windows, skylights and doors for the following characteristics related to energy efficiency and other functions:

U-Factor or U-Value, and the use of  Low-E Coatings and Gas Fills
Solar Heat Gain Coefficient
Visible Transmittance
Air Leakage
Condensation Resistance

Energy Star labeled windows rely on these ratings.

This Article also covers:
Skylights
Daylighting
Putting it all together - Guidelines for your Windows and Skylight Project
Improving the Energy Efficiency of Existing Windows
Doors
New York City Landmarks Preservation Commission

“U-Factor” or “U-value” measures the rate of heat transfer and refers to how well a window prevents heat loss (the opposite of R-value which refers to insulating value). The values are usually between .20 and 1.20, and the lower the U-value the lower the amount of heat that transfers and therefore the greater a window's insulating value. Windows with lower U-value also have less of tendency form condensation. U-factor is affected by the airflow around the window and the emissivity of the glass. Emissivity is the ability of a product to absorb certain types of energy (specifically infrared) and radiate that energy through itself and out of a room. A product with high emissivity, such as one pane of clear glass, will transfer over 84 percent of the infrared energy from a warm room outside to the cold air. The lower the conductivity and emissivity of the glass, the lower the rate of heat loss and the lower the U-factor. It is now possible to obtain windows with low emissivity (low-e) coatings on the glass.

Low-e coatings. According to the Department of Energy, 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%. Keep in mind that not all low-e coatings are alike. 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 so it is important to choose a window based on your climate zone. In the northern climate of NYC the low-e coating should be applied to the inside pane of glass which will keep heat inside the house; in southern climates the low-E coating should be applied to the outside pane of glass to reduce solar heat gain. A Low-E coating can also reduce a window's visible transmittance unless you use one that is spectrally selective which means it can filter 40%-70% of the heat while still allowing a full amount of visible light to transmit through the pane.

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.

Gas fills. Windows are now readily available with an injection of argon or krypton between the panes -- inert gases (do not react readily with other substances) that have a higher resistance to heat flow than air and, consequently, help reduce the U-factor. Argon is inexpensive, nontoxic, nonreactive, clear, and odorless. Krypton is more expensive but has a better thermal performance. Gas fills increase the insulating properties, thus lowering the U-factor, of windows.

The Solar Heat Gain Coefficient (SHGC) measures how well a window blocks heat from sunlight. SHGC is expressed as a number between 0 and 1, 0 being the least amount of heat gain and 1 being the most, thus the lower a window's solar heat gain coefficient, the less solar heat it transmits. The NFRC notes that SHGC ratings express the performance rating for the entire window, not just the glass -- this means that SHGC ratings also include the ability of a window to absorb the heat from the sun and transmit it (conduct it) through the entire window and into the room. Therefore other aspects of a window such as framing, thermal spacers etc., as well as the glass (such as tinted or reflected), can affect the SHGC rating. Other characteristics that affect the SHGC are the use of spectrally selective glass, and tints or reflective coatings applied in conjunction with low-e coatings that together can dramatically reduce SHGC but may or may not be desirable depending on your climate zone. Heat-Absorbing, tinted window glazing contains special tints that change the color of the glass and are able to absorb a large fraction of the incoming solar radiation through a window, thus reducing the SHGC, visible transmittance (VT) and glare. Tint does not lower a window’s U-factor because some heat continues to pass through tinted windows by conduction and re-radiation.

Tinted glass and its affect on SHGC and VT: Tinted glass can affect VT and SHGC. Gray and bronze tinted windows reduce both light and heat by the same amount. Blue and green tinted glass allow more light to penetrate and slightly reduce heat transfer.

Visible Transmittance (VT) measures how much light, visible to the human eye, comes through the window. The solar spectrum is made up of ultraviolet (UV) light, visible light, and infrared (IR) light. VT is expressed as a number between 0 and 1 and the higher the VT the more visible light that’s transmitted. 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. Through the use of spectrally selective glass, and usually in conjunction with low-e coatings on the interior surface of the pane, windows can block or re-radiate infrared energy from the sun, thus reducing solar heat gain, while maintaining a high level of transmission of visible light. Daylighting can save 30%-60% in energy usage via reduced use of artificial light.

Tinted glass and its affect on SHGC and VT: Tinted glass can affect VT and SHGC. Gray and bronze tinted windows reduce both light and heat by the same amount. Blue and green tinted glass allow more light to penetrate and slightly reduce heat transfer.

Air Leakage (AL) measures how much outside air comes into a home or building through a product. AL rates typically fall in a range between 0.1 and 0.3. The lower the AL, the better a product is at keeping air out. AL is an optional rating, and manufacturers can choose not to include it on their NFRC labels. LEED for Homes does not have any requirements with respect to Air Leaking ratings.

Condensation Resistance (CR) measures how well a product resists the formation of condensation – water, water droplets, frost and ice that form on windows when the temperature of the window falls below the dew point temperature for that particular glass, and the dew point is also affected by the amount of moisture or humidity in the air and the ambient room temperature. CR is affected by the U-factor in that the lower the U-factor (the less the window transmit heat) the lower the CR. CR is expressed as a number between 1 and 100. The higher the number, the better a product is able to resist condensation. CR is an optional rating, and manufacturers can choose not to include it on their NFRC labels. In addition to U-factor, other characteristics of a window that can lower its CR are double and triple glazing, low-e coatings and non-thermally conductive framings such as wood or vinyl rather than metal, and non-thermally conductive spacers between window panes. LEED for Homes does not have any requirements with respect to Condensation Resistance ratings.

Properly sized and positioned skylights can help save energy by providing daylighting , consequently reducing the need for artificial lighting, and by providing warmth through passive solar heating in winter. Like windows, skylights have NFRC ratings for U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, Air Leakage and Condensation Resistance. The Department of Energy recommends that 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. Skylights that face south should be fitted with a covering or shade for use in the summer months to prevent solar heat gain. The LEED for Homes Guidelines that apply to windows should apply to skylights.

In addition to glass glazing, skylights are available in various plastics such as acrylics and polycarbonates, but these can scratch and yellow over time and they may allow UV light to penetrate causing upholstery and carpet fading. Glass skylights are usually more expensive but they do not discolor or scratch, and with glass you have the same glazing options that apply to windows in order to maximize energy efficiency and minimize winter heat loss – multi-panes, inert gas injection (gas fills), low-e coatings, and heat-absorbing tints if you are more concerned about passive solar summer heat.

In very cold weather, skylights are often prone to form condensation which, if significant enough, may cause water to drip into the room. Look for skylights that have an interior channel to collect the condensate so that it can evaporate later. As with windows, the better the thermal efficiency and U-Factor the less prone to condensation problems the skylight will be. You can also look for skylights that have a NFRC rating for condensation resistance.

Standard skylights, but not tubular skylights or “light tubes” described below, can also provide ventilation.

Daylighting is a primary advantage of skylights. In adition to standard window-style skylights, advances in the design of tubular skylights, or light tubes, allow sunlight to be channeled to rooms all over the house, and some manufacturers claim that they can carry light to stories below the top floor but the amount of light that is transmitted will diffuse the further it travels. Tests performed at the Alberta Research Council, Canada, indicated that one 13-inch tubular skylight had equivalent light output in December of up to one 700-watt incandescent bulb, and in June of one 1,200-watt bulb.

Tubular skylights consist of a dome-shaped light collector mounted on the roof that is designed to direct sunlight down a metal tube that is lined with a highly reflective coating. At the end of the tube the light passes through a diffuser lens that is mounted on the interior ceiling surface; this diffuser lens spreads sunlight evenly throughout the room. The tube is relatively easy to install between the roof rafters. Some manufacturers have designs that include electric lighting as well so there is no need for different ceiling fixtures. Tubular skylights do not result in unwanted solar heat gain. They can be much less expensive than standard skylights, but they do not provide exterior views, ventilation, or passive solar heat gain which may be desired in the winter. Given their relative discrete size and light weight they do not require structural changes, they can be used in situations where other skylights may not be appropriate and do not have a big impact on the exterior aesthetic appearance of the home.

If you are planning to completely replace your windows, meaning removing all parts including the frame down to the building rough opening, or to install replacement windows or sashes with a sash replacement kit into the existing frames, then the following characteristics should factor into your window choice:

1. U-Factor and SHGC. LEED for Homes certification requires that homeowners maximize the energy performance of windows by installing windows that meet certain minimum Energy Star windows requirements for U-factor and Solar Heat Gain Coefficient factor depending on the climate zones in which the house is located. LEED for Homes sets out the following window requirements and are good guidelines to follow whether or not you are going for LEED certification:

Northern Climate (including NYC, Boston and Chicago): Select windows with a U-factor of 0.35 or less and any SHGC.
North/Central Climate (including Washington DC): Select windows with a U-factor of 0.40 or less and SHGC of .45 or less.
South/Central Climate (including San Francisco): Select windows with a U-factor of 0.40 or less and SHGC of .40 or less.
Southern Climate: Select windows with a U-factor of 0.55 or less and SHGC of .35 or less.

2. Number of panes. The greater the number of panes, the higher the insulating value and sound control offered. Choose double or triple panes.

3. Gas Fills such as argon or krypton, as discussed earlier in this Article. 

4. Low-e coating, as discussed earlier in this Article. 

5. Tinted coatings, as discussed earlier in this Article.

6. Frame type. If your brownstone or townhouse is in a landmarked area in NYC then the type of windows you are able to use will be strictly regulated by the New York City Landmarks Preservation Commission for any windows visible to the street (in the public domain). Typically windows will need to be the same size and style as existing windows with wooden frames.  If you have windows that are not regulated by the NYC Landmarks Preservation Commission, then when making your choice you can take into consideration how the type of framing you choose may affect the window’s energy efficiency. Aluminum frames are lightweight and strong and virtually maintenance free, but aluminum conducts heats rapidly and can have a negative impact on U-factor and CR. Wood frames insulate well and expand and contract less than other materials but may require more maintenance because they weather with age, however choosing a vinyl or aluminum clad exterior (if not landmarked) can reduce maintenance. Vinyl frames are also lightweight but may expand and warp in high temperatures and can discolor over time. Fiberglass offers good insulation value and a relatively low degree of expansion and contraction, and fiberglass windows can be made with a wood veneer. Vinyl windows can also be obtained with a fiberglass core.

But remember, replacing the windows themselves is not enough if you don’t also ensure adequate insulation around them. See our Article Insulation and the Building Envelope: Controlling Heat Loss and Gain  for an in depth analysis of insulation issues.

If you are not replacing windows there is still plenty you can do to increase your current window energy efficiency. 

Apply a low-e film to your existing windows. Although Low-E coatings are usually applied during the manufacturing process, some are available for do-it-yourselfers. These films are inexpensive compared to total window replacements and have a useful life of 10–15 years. 

Reduce the air leakage around your current windows. Most heat is lost thoguth convective heat loss -- leaks.  By plugging all holes with caulk or using weather stripping on window sashes or other things that move (window sashes, doors), you can substantially reduce the amount of air infiltration and leakage around windows. See our Article Insulation and the Building Envelope: Controlling Heat Loss and Gain for a full discussion of caulking and weatherstripping. 

By being a bitmore radical without opening walls can have a big impact:  for example, removing shoe molding or baseboards to seal all joints between the floor and wall wiht stop a substantial amount of leaks.  Also, many old brownstone windows are surrounded by lovely wooden decorative moldings and often there is no insulation between these moldings and the sheathing. It is possible to have a carpenter remove, in tact, units of molding so that insulation can be placed or blown into cavities and then the molding replaced.

It also may be possible, depending on your window sash depth and the overall state of your windows, to take your existing old brownstone windows to a window fabricator who can use the original sashes and add an additional pane of glass, low-E coating and argon or krypton gas. This may be substantially more expensive than simply caulking and weatherstripping and/or adding storm windows, but it may be less expensive than complete replacement particularly if you have true mullioned divided lights or huge floor-to-ceiling parlor floor windows often found in brownstones, and you will keep your historic windows and will have a higher-end look than weatherstripping and storm windows may yield. 

Install Storm Windows. Storm windows are a viable option even in historic landmarked areas because they are available to install on the inside as well as the outside of windows. Storm windows do not increase the insulating properties of single pane windows but they prevent air leakage and seepage and consequently reduce draughts, increase comfort and will help reduce heating costs. They range from the inexpensive plastic sheets or films designed for one heating season, to triple-track glass units with low-e coatings meant to be used for many years. The panes can be of glass, plastic such as plexiglass or acrylic. Glass provides greater visibility and does not deteriorate with age so will have a longer life but it is more fragile, whereas plastic options are lighter and stronger but may discolor and scratch. Interior storm windows are easier to install and remove and are better at reducing air leakage and infiltration than exterior storm windows because they make a tighter seal. Exterior mounted windows should be installed with a small “weep” hole at the bottom to allow moisture and condensation to escape, and although this reduces the energy efficiency it is necessary to avoid a moisture build that will cause a wooden exterior window frame to rot over time. Framing options present all the choices discussed earlier in this Article  under frame choices for new windows. 

Window Treatments are not just for show. The right kind of window treatment, used properly, can dramatically affect heating and cooling loads. Window coverings will, of course, reduce the amount of light transmitted into the room and therefore may increase the need for artificial light thus reducing energy savings. You can also combine window treatments to increase insulating properties.

Shades, Blinds and Draperies can be quite effective in keeping the heat in or out. Honeycomb shades, for example, which consist of honeycomb-shaped pockets (one, two or three cells deep) that trap air and act as an insulating barrier, typically have R-values in the range of 3.0 to 3.9 and should be inside-mounted for greatest insulation. The Department of Energy notes that honeycomb blinds reduce air infiltration only slightly, however our personal experience with honeycomb blinds is that they make a noticeable improving in reducing draughts thereby increasing room comfort, and they substantially reduce solar heat gain in the summer which is key in those top floor south-facing brownstone bedrooms! Draperies should be mounted close to the window. Blinds do not help greatly with retaining heat but can help reduce solar heat gain in the summer. Blinds and shades can also be made with special liners to make them more or less reflective of solar heat. In winter coverings should be open during the day when the sun is shining on that side of the house to allow solar heat to enter, and closed at night; in summer do the opposite and close shades during the sunny part of the day.

Insulating window panels are essentially pop-in shutters that clip into the interior of a window frame and have R-values ranging from 3.5 to 7. No hardware or latches are required and seals can be made with Velcro or magnetic tape.
Mesh window screens, mounted on the exterior of the window frame, can diffuse solar radiation.

Awnings and roof overhangs/eaves can shade windows and reduce solar heat gain. Awnings on west and south windows can reduce solar heat gain between 65 to 77 percent.  Eaves are simply not part of the brownstone architectural ethos, and awnings may be an option on the rear of your brownstone if you are in a landmarked zone but they will change the character of the building because these appendages were not original to brownstones.

Window Shutters. If you are lucky enough to have the original shutters that many brownstones were built with, or the space to retrofit new ones, then take advantage of their energy saving properties. Solid wood shutters reduce heat loss and summer solar heat gain; louvered shutters do not insulate against heat loss but help prevent unwanted solar heat gain.

Federal, state, local and utility incentive programs exist to help homeowners afford the costs of increasing window energy efficiency. For a complete discussion of incentive programs see our Article Incentive Programs for Green Renovation. 

Exterior doors can contribute to heat loss and solar heat gain in the same manner as windows – if the door contains windows then it is subject to heat loss through conduction, convection and radiation, and is also subject to air infiltration and leakage through gaps and cracks in the door framing.

The NFRC rates doors for U-Factor and Solar Heat Gain Coefficient, just as it does for windows. Doors may also be rated by the manufacturer with an R-Value.

Guidelines for your Doors Project

Working with your existing exterior doors. With a brownstone you may be working with the original wood front entrance single or double doors and the interior vestibule door. These doors, being antiques, are usually much thicker than modern doors and made from old growth wood with very good thermal properties, and being architecturally desirable it is well worth the effort to restore them and to work on the framing to mitigate air leakage. If your doors have lites, consider adding a low-e coating or an additional pane on the inside. Weatherstripping and caulking is essential to eliminate gaps through which air leaks. See our Article Insulation and the Building Envelope: Controlling Heat Loss and Gain for a full discussion of caulking and weatherstripping.

Replacing exterior doors. If you completely replace an existing door, right down to the building rough opening, then you would likely buy a pre-hung door that comes already hanging in a frame and is installed within the rough opening. New doors are made with thermal cores such as polyurethane foam insulation. Be sure to fill all gaps between the new pre-framed door and the building with insulation. Add weatherstripping, and check it annually to see if it needs to be replaced. link to insulation article

Glass sliding doors pose the biggest challenge to energy efficiency. If you must have sliding doors over swinging glass doors, which typically make a much better seal, be sure the door you choose has a thermal break between the inner and outer frame, low-e coating and gas fills between the panes.

Storm doors, like windows, come with all types of framing (wood, steel, fiberglass, aluminum), and glass options (low-e, tinted) with an array of options for operating the windows and insect screens. The Department of Energy recommends that a glass storm door not be used if the door receives more than a few hours of direct sunlight a day because the heat build-up between the storm door and the entry door could damage the entry door. Storm doors are only relevant for the back of the  building when dealing with brownstones in a landmarked zone. 

The Landmarks Preservation Commission would rather have a building owner repair existing, original windows by filling with wood fillers and epoxies etc., than replace them. There are, however, newly manufactured windows that the Commission usually will approve. See our upcoming blog entry on the analysis of window options for 168 Clinton St. where we discuss the current appropriateness of the bias towards this approach and the decision making process for 168 Clinton St.

Below is a reprint from The City of New York Landmarks Preservation Commission Guidelines and Materials Checklists for Performing Work on Landmarked Buildings:

Ordinary Maintenance (No permit required)
The following are examples of the types of work that are considered ordinary maintenance and do not generally require a permit from the Commission:
• weather stripping;
• caulking;
• puttying;
• replacing broken glass;
• repairing suspension systems (cords, pulleys, etc.);
• repairing or replacing window hardware; repairing window components by partial replacement,
scraping, filling, or sanding;
• painting window sash or frames the same color;
• installing interior storm windows or panels;
• installing interior security gates or grilles; and
• installing regulation child guards.

Approval Required for Work
The following are examples of the types of work that require a permit from the Commission:
• painting window sash or frames a different color;
• installing new window sash or frames;
• installing exterior storm windows and exterior storm window frames;
• installing or removing exterior shutters;
• installing window awnings;
• repairing or altering window enframements;
• installing or removing exterior security window grilles or bars;
• changing the shape or design of window openings;
• blocking in existing windows or creating new ones;
• restoring original or architecturally appropriate window openings; and
• replacing extensive amounts of original window materials or consolidating windows with epoxies or other plastics.

National Fenestration Rating Council Fact Sheet on Windows

National Fenestration Rating Council Fact Sheet on Doors

The National Fenestration Rating Council maintains a database of rated windows, skylight and door manufacturers.

Department of Energy Consumers Guide to Windows, Doors and Skylights

Department of Energy Energy Efficiency and Renewable Energy Consumer’s Guide Improving the Energy Efficiency of Existing Windows

Energy Star Consumer Information Residential Windows, Doors, and Skylights

Energy Star 2007 Partner Resource Guide Windows, Doors and Skylights 

Efficient Window Collaborative

Efficient Windows collaborative Fact Sheet for choosing windows in New York including NYC