How your old windows and doors cost you comfort and money, and what you should know about replacements. People replace their windows and doors for a variety of reasons, but two big reasons are for increased comfort and energy efficiency. However, these concerns can be addressed simultaneously, as an energy efficient window or door can help increase the comfort of the room in which it is located. Below are the three ways energy is lost through windows and doors.
Air leakage: Air can leak between the window’s sashes and frame due to window design, because of aging and deterioration of window components, and from lack of maintenance. Doors can leak where they close into their frame (because of worn or deteriorated weather strip), or from poor fit (because of old sagging doors, poor installation, or building settling), Cold or hot outside air coming in through or around a window or door can affect your comfort and power bills in a number of ways. In the summertime, the outside air may be warm and humid, while inside you are paying good money for your air to be cool and dry. All modern windows must be rated for air leakage, and all have low rates compared to older windows. This test derived rating may be on the NFRC sticker, and if so will be reported as Air Leakage. Ask us for the information; we should be able to get it for you. Energy efficient windows
Conduction: Conduction is the transfer of heat through a solid. Window and door frames of different materials (aluminum, steel, wood, vinyl) can vary greatly in their conductive properties. Modern windows and doors generally utilize insulated glass to slow the conduction of heat through the glass. Insulated glass consists of two or more panes of glass to create dead air space to further insulate against the conduction of heat. A window’s ability to insulate against conduction is reported as its U-factor, a test derived insulation rating which will be on the window’s NFRC sticker. U-factor units are: BTUs (volume of heat) per square foot per hour per degree (temperature difference between the inside and outside of the house). Since the U-factor is actually the number of BTUs traveling through the opening over time, the lower the value the better.
Radiation: Heat is radiated through the glass of windows or doors in the form of infrared light, which goes through the glass just like visible light. This radiant heat can be experienced as the warmth we feel from the sun’s rays, but everything emits infrared energy based on its temperature. This infrared light travels through the air, through the glass, and into our homes and heats whatever it shines upon. It can be direct sunshine or heat radiated from the ground outside that the sun has warmed. Low-Emissivity, or Low-E, coatings are applied to the surface of the glass used in modern windows. Low-E coatings are manufactured by glass companies and vary in makeup and density. These coatings are engineered to selectively reflect the infrared light yet remain as clear as possible to visible light. The radiant heat a window allows to pass through it (or its ability to reflect it) is reported as its Solar Heat Gain Coefficient (SHGC). The SHGC is a test derived report of how much of the radiant heat (infrared light) passes through the window. The amount of radiant heat that hits the outside of the window or door is defined as 1.0, and the amount that passes through is the SHGC. It will be reported as a decimal, from 0.0 to 1.0. It is a “whole window” number, meaning it measures the amount of energy that passes through the window, compared to the amount that strikes the outside dimension (including the frame) of the window or door. Because frames do not pass infrared light, it will never be 1.0. The typical SHGC for a clear (no Low-E or tint) glass window is .6 – .7. Average SHGC for windows with “standard” Low-e glass is about .30 – .35, while we regularly see upgraded (but still very reasonably priced) Low-E windows with SHGC of .18. Especially here in famously hot Columbia, the lower the number the better. An interesting thing about radiation: since heat radiates from hot to cold, our homes radiate heat OUT in the wintertime. That is why Low-e glass is even effective at night in the winter, not just if the sun shines on the window in the summer. Energy efficient windows
Condensation Resistance (CR) Rating is a rating of how well a window resists moisture condensation on its inside surfaces during periods of cold weather. These low outside temperatures conduct through the various window components and cause cold surface temperatures on the inside of the window. This allows the relatively warm, humid inside air to condensate moisture onto the inside surfaces of the window. It is much more important in areas that have lower outside temperatures than ours in South Carolina. All modern windows have better CR than many older types of windows; particularly aluminum framed or single glazed windows. CR Rating is a number between 1 and 100, with actual window ratings generally between a very low of 15 to a very high of 72. Higher numbers are more resistant to condensation. CR may be reported on the NFRC sticker. Energy efficient windows
Visible Transmittance (VT) is the amount of visible light that passes through the window. Like SHGC, VT is a coefficient. It is a measure of how much of the visible light passes through the window. With 0.0 being no light transmitted and 1.0 being all light hitting the outside of the window. It is a “whole window” measurement, meaning it compares the amount of light that passes through the window to the amount that strikes the outside of the window (frame included). Because window frames do not allow light to pass, it will never be 1.0. VT will be reported on the NFRC sticker.
Sample NFRC sticker, courtesy of Energy.gov:
Click Here to learn more about the NFRC
What you need to know about energy efficiency Energy efficient windows
One way energy is lost is by air leaking through or around your windows. Air can leak between the window’s sashes and frame due to window design, aging and deterioration of window components, or from lack of maintenance. Cold or hot outside air coming through or around a window can affect your comfort and power bills in a number of ways. The dust and pollen coming through drafty windows can irritate allergies and make for a house that “needs another good dusting”. All modern, rated windows have reasonably low air infiltration rates compared to older windows. However, most manufacturers do not make these numbers readily available. Ask us for the information; if we do not already have it for the particular window brand and model you are considering, we should be able to get it for you.
A second way energy is lost through a window or door is through conduction. Conduction is the transfer of heat through a solid. Modern windows nearly always utilize insulated glass to slow the conduction of heat through them. Insulated glass consists of two or more panes of glass to create dead air space to further insulate against the conduction of heat through it. This dead air space can be filled with certain gasses, like Argon, to increase the insulation value compared to the dry air typically used. A window’s ability to insulate against conduction is reported as its U-value-a test derived insulation rating whose units are: BTUs (volume of heat) per square foot per hour per degree (temperature difference between the inside and outside of the house). Since the U-value is actually the number of BTUs traveling through the opening over time, the lower the value the better.
A third way energy is lost through windows is through radiation. Heat is radiated through the glass of windows or doors in the form of infrared light, which goes through the glass just like visible light. Infrared light is best envisioned as the warmth we feel from the sun’s rays, but everything emits infrared light based on its temperature. This infrared light travels through the air, through the glass, and into our homes and heats whatever it shines upon. It can be direct sunshine or heat radiated from something outside that the sun has warmed. Low-Emissivity coatings (generally known as Low-E coatings) are applied to the surface of the glass used in modern windows. Low-E coatings are manufactured by glass companies and vary in type, makeup, and density. These coatings are placed on the glass to selectively reflect the infrared light, yet (they attempt to) remain clear to visible light. The window’s resistance to radiant heat (or its ability to reflect it) is reported as its Solar Heat Gain Coefficient (SHGC). The SHGC is a coefficient of how much of the radiant heat (infrared light) passes through the window. It is a value ranging from 0.0 to 1.0, with 0.0 being no radiant heat transmitted, and 1.0 being all radiant heat transmitted through the window. It is a “whole window” number, meaning it measures the amount of energy that passes through the window, compared to the amount that strikes the outside dimension of the window. Because window frames generally do not pass infrared light, it will never be 1.0. The typical SHGC for a clear (no Low-E or tint) glass window is .6 – .7. Average SHGC for windows with Low-e glass is about .25 – .35, while we regularly see upgraded Low-E windows with SHGC of .18. Especially here infamously hot Columbia, the lower the number the better. An interesting thing about radiation: heat radiates from hot to cold, our homes radiate heat OUT in the wintertime. That is why Low-e glass is even effective at night in the winter, not just if the sun shines on the window in the summer. Energy efficient windows
We understand that all this information may be overwhelming. Please contact us today and we will help you put it all in perspective and find the right energy efficient doors or windows for your project.