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Windows, Doors, and Skylights

With the recent cold temperatures that we have experienced, you may have noticed that the area around your doors and windows to be drafty and cold. Or possibly that there is condensation on, or between the panes. These are all very good indicators that your windows and/or doors are leaking that expensive warm air outside. This could be due to the age of them, or they might just be in need of some general maintenance. So this week’s article is going to be about the energy and performance of your windows, doors, and potentially skylights.  

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

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 solar heat gain coefficient ratings, which are described below.

Heat Gain and Loss

Windows, doors, skylights can gain and lose heat through:

  • Direct conduction and convection heat transfer through the glass or multi-layer glazing and framing

  • Thermal radiation into a house and out of a house from room-temperature objects, such as exterior walls and windows, people, equipment, furniture, and interior walls

  • The solar radiation into a house, which is converted to heat when absorbed by building surfaces

  • Air leakage through and around them.

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

  • U-factor is the rate at which a window, door, or skylight transmits non-solar heat flow. For windows, skylights, and glass doors, a U-factor may refer to just the glass or glazing alone. NFRC U-factor ratings, however, 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) is the 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 during the winter. A product with a low SHGC rating is more effective at reducing cooling loads during the summer by blocking heat gain from the sun. Your home’s climate, orientation, and external shading will determine the optimal SHGC for a particular window, door, or skylight. For more information about SHGC and windows, see passive solar window design.

  • Air leakage is the rate of air movement around a window, door, or skylight in the presence of a specific pressure difference across it. A product with a low air leakage rating is tighter than one with a high air leakage rating. Note that air leakage also depends on proper installation of a window, which is assumed in all ratings.

NFRC labels on window units give ratings for U-factor, SHGC, visible light transmittance (VT), and (optionally) air leakage (AL) and condensation resistance (CR) ratings.

National Fenestration Rating Council

Sunlight Transmittance

The ability of glazing in a window, door, or skylight to transmit sunlight into a home can be measured and rated according to the following energy performance characteristics:

  • Visible transmittance (VT) is a fraction of the visible spectrum of sunlight (380 to 720 nanometers), weighted by the sensitivity of the human eye, that is transmitted through the glazing of a window, door, or skylight. 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)is the ratio between the VT and SHGC. 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.

Recommended Performance Ratings by Climate Zone

Review the state fact sheets or use the window selection tool for new construction or existing homes from the Efficient Windows Collaborative to determine the desired performance ratings for your climate.

Tips To Improve Energy Efficiency

Replacing your old windows with new, energy efficient windows is the most impactful way to improve your home’s energy efficiency. However, there are ways that you can improve your old windows while you prepare for your window replacement. Don’t let your old windows drain your wallet and compromise your comfort— try one of, or a combination of these 10 simple DIY tips!

1. Caulk Around Window Seams

Over time, the caulk around your windows can begin to break down. Caulk secures your windows from the elements, insects and even pollen. If your caulk is severely worn down, it can let air pass through your home and make your HVAC system work harder to keep your home at its set temperature.

Replacing your caulk will help insulate your windows and protect your home from insects and allergens. Plus, the new caulk makes a big aesthetic difference! Most people match the caulk color to the paint color of their trim, or there are clear options available.

2. Replace Or Add Weather Stripping Around Window Seams

Weather stripping is another simple way to improve the insulation of your windows. It is an added layer of protection on top of the caulk. Weather stripping can be purchased at most big box stores and can easily be applied yourself. Weather stripping can help reinforce caulking and ensure that all tiny holes are reinforced.

3. Replacing Old Windows With Inserts

You may be able to save some money on your window replacement by purchasing window inserts instead. If your frame is in good shape (meaning it’s not bent or misshapen), this may be a viable alternative for your home.

A window insert is simply replacing the glass in your window. They are less expensive than a full window replacement. You can purchase an average size window insert for around $200-$400 and install it yourself, or you can hire a contractor.

It should be noted that if you have significant wood rot around your window or if your window frame is misshapen, this may not be an option for you. 

4. Install Storm Windows

Storm windows are another DIY option to improve the energy efficiency of your windows. Storm windows are installed on top of your current windows. They can be put on the exterior and interior of your windows.

According to the U.S. Department of Energy, low-e exterior or interior storm windows can save you 10-30% on heating and cooling costs, depending on the type of window you currently have. Storm windows also help soundproof your home.

5. Install Outdoor Awnings Above Your Windows

Installing awnings on the exterior of your home will help block sunlight from entering. Awnings are especially popular on windows overlooking porches and other outdoor areas. Adding shade to your exterior and indoor spaces will help keep heat from penetrating your home, and it will also be less wear on your glass. Whether you use awnings to help improve energy efficiency of your old windows or as a preventative measure, they are good for both aesthetic and function!

6. Install Exterior Functional Shutters

Another solution that improves the aesthetics and energy efficiency of your home is installing exterior, functional shutters on the outside of your windows. If you’re handy, exterior shutters can be a fairly easy DIY weekend project. Being able to close the shutters and shield your home from harmful solar rays will help improve the energy efficiency of your windows and extend the life of your window films and coatings.

7. Try Out New Window Coverings

If your windows are nearing the end of their useful life or you don’t currently have any window coverings, consider installing new blinds, shutters or drapes. This will help provide more insulation to your existing windows.

8. Add Energy Efficient Curtains To Your Space

Any easy interior project that can be accomplished in a few hours (or less!) is installing new black-out curtains. Blackout curtains help block sunlight and solar heat in your home. This will certainly help keep your home cooler in summer months.

However, this is merely a Band-Aid fix. If your windows are past their useful life, it’s important to replace them in a timely manner.

9. Add Solar-Blocking Window Film

Window films are extremely easy to install and come in a variety of options. While they are not as durable or as strong as ones made by the manufacturer, window films can be purchased on Amazon or other home improvement stores. They can help provide your windows with more functionality and energy efficiency.

10.  Add Some Type Of Decorative Window Film

Window films are a fun and functional way to block sunlight. They come in a variety of options such as stained glass, tinted and more. Decorative window film options won’t yield as much energy efficiency as solar blocking ones, but they can be a fun way to block sunlight and improve the aesthetics of your home.

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