Windows and doors make up a significant part of most building shells. As a result, they often become a high priority for owners looking to cut down drafts, reduce energy bills, and make their building more comfortable. Replacing windows and doors can be a significant financial investment and is rarely as cost-effective as air sealing and insulating when it comes to saving energy. There are many reasons to replace windows and doors (for example, eliminating lead paint, addressing safety issues, condensation between the glass panes, rotting frames, aesthetics, etc.), but in general, energy savings alone do not usually justify the expense. However, energy efficiency improvements to existing windows may be worth considering and may offer return on investment.
Windows, doors, and skylights can gain and lose heat through:
- 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
Windows and doors are typically measured and rated in a variety of ways. The National Fenestration Rating Council (NFRC) is an independent, non-profit organization that establishes objective window, door and skylight energy performance ratings. The NFRC label appears on all new products. If you are shopping for new windows, the U-factor is the best indication of a window's performance: the lower this rating number, the more efficient the window. Check for information about Vermont Building Energy Standards and requirements when replacing windows. Information on Vermont Building Energy Standards is available at the Department of Public Service website.
In heating-dominated climates like Vermont, major glazing (glass) areas should generally face south to collect solar heat during the winter, when the sun is low in the sky. In the summer, when the sun is high overhead, overhangs or other shading devices can prevent excessive heat gain. Windows on east, west, and north-facing walls should generally be minimized, while still allowing for adequate daylight. Low-emissivity (low-e) windows can help control solar heat gain and loss.
There are many different frame materials used in windows today, including wood, vinyl, fiberglass, PVC and aluminum. In Vermont's cold climate, it is important to choose a material that minimizes the heat transfer through the frame, in order to minimize not only heat loss, but also excess condensation that can occur on the inside of a cold window frame. The NFRC label takes into account the overall window performance, including glass and frame.
You may be able to improve your existing windows through simple, cost-effective steps that can make a big difference in your comfort level. Here are some ideas:
Tape a heavy-duty clear plastic sheet or clear plastic film to the inside of your window frames to reduce drafts.
Install tight-fitting, insulating window shades on windows that feel drafty.
In the winter, close your curtains and shades at night to protect against radiant heat loss and cold drafts, then open them during the day to let in warming sunlight. In the summer, closing your curtains during the day can help reduce the amount of heat gain from the sun.
Install exterior or interior storm windows. Depending on the type of windows already installed, this measure may significantly reduce heat loss through windows. Storm windows should typically have weatherstripping at all movable joints, be made of strong, durable materials and have interlocking or overlapping joints.
Repair and weatherize your current storm windows, if necessary.
New exterior doors often fit and insulate better than older doors. If you have older doors in your building, replacing them may result in lower heating and cooling costs. However, as with windows, there are good reasons to replace doors, but rarely will energy savings alone justify the expense.
Your building's exterior doors can contribute significantly to air leakage, and can also waste energy through conduction, especially if they are old, uninsulated, improperly installed, and/or improperly air sealed. Weatherstripping may be one way to reduce the energy losses due to air leakage. Adding a storm door may also 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, because you won't save much additional energy. Be aware that adding a glass storm door if the exterior door gets more than a few hours of direct sun each day can cause issues, as the glass can trap heat against the entry door and can cause damage.
Some Benefits of Window & Door Improvements
Replacing your windows and doors may be something you want to consider, regardless of the potential for energy savings (e.g., if they are no longer operable or if you think it may be appreciated by potential homebuyers).
New, energy-efficient windows may pay for themselves eventually through lower heating and cooling costs.
Improving your existing windows and doors can often be done at a minimal cost.
Requires a significant financial investment to have much—if any—impact on the energy efficiency of your home
There is typically a long return on investment period
Proper weatherstripping is typically needed to minimize air leakage around new windows and doors
Some Next Steps
Replacing windows and doors can make a difference in comfort and fuel use, but may not be the most effective starting place. You may want to determine your priorities for energy upgrades and develop a plan before making investments. Consider consulting a trained energy auditor to help you create a plan and decide where to start with your building.
You can learn more about the National Fenestration Rating Council (NFRC) label for windows and doors by visiting their webpage.
You can learn more about the U.S. Environmental Protection Agency's (EPA) ENERGY STAR® Exterior and Interior Storm Windows Specification by visiting their webpage.
Visit the pages below to learn more about Weatherization: