Windows are selected for a variety of purposes but energy efficiency doesn’t always make the top of the list. Fortunately most all windows manufactured today must meet minimum efficiency standards. Efficiency is impacted by the number of panes (glass or interior non-glass films), what is between the panes (air/nothing or an inert gas charge), the size of the space between the panes, the coatings on the glass, the type of frame, and the design style (fixed, slider, casement, etc.)
Windows are not likely to be replaced simply to gain energy efficiency in a non-residential structure. However, when windows are replaced due to major renovation or other issues such as security, then energy efficiency should also be considered.
There are some window treatments that can be field-installed to existing windows. A common type application is adding reflective films to reduce solar gain.
In industrial facilities, windows are often covered over with insulating wall panels or spray on foam, leaving the windows in place. Older facilities with saw-tooth roofs originally designed for daylighting and ventilation are a common target for retrofit by covering them over from the outside.
The use of operable or fixed windows is sure to stimulate conversation between building occupants and designers. Fixed windows are less expensive, more secure and easier to control central HVAC systems. Operable windows are preferred by most occupants, but cost more to install and may be better or worse at managing energy, depending on the reliability of the occupants. It is more common to find operable windows that have been sealed. Buildings that have correctly functioning economizer systems will do better with fixed windows.
When modifications are made to window areas, be sure to consider the impact on heating, cooling, lighting, security, aesthetics and function.
Single pane, clear glass
Triple pane, coated, gas filled
Solar Gain is substantially influenced by the type of glass installed, as these two diagrams show. The SHGC is a factor of how much thermal energy gets through the window in the form of radiation from the sun. The U-Factor is how much heat can be conducted through the window. The SHGC is affected most by the coating on the glass and the U-Factor is influenced the most by the number of panes and the gas between the panes.
New coating technologies are being worked on to allow windows to change their properties based either on temperature or small amounts of electric current that provides a switch-able option. The idea is to allow solar gain during the winter months when it’s wanted and reflect it during the cooling months. Working versions have been around for many years. There is a problem with first cost and longevity with the coatings. This is a technology to keep track of because of its great potential.
This is a term that has a variety of meanings for technologies from rigid plastic sheeting to spray on coatings for any kind of surface. Translucent means that some form of light (wave lengths) get through, but that thermal energy flow is reduced. Most of the products returned on a web search of this technology are for non-glass window glazing replacement products that claim to do a better job of reducing heat loss and diffusing light and glare, making them also a better product for day lighting applications.
There are several advantages to using ‘Day Lighting’ (natural lighting as opposed to electric light fixtures) such as reduced electrical lighting costs and improved aesthetics. However, plain glass has a relatively low R-value compared to most building materials, and therefore creates other problems. High amounts of solar gain can also be a problem.
To take advantage of day lighting while minimizing the down side, consider:
- use glass block walls instead of glass windows for a higher R-value
- use “plastic” or other non-glass material designed for day lighting that has a higher R-value, yet a good light transmittance
- be sure to include proper day lighting controls on electric light fixtures so that lights are not on during the same time that day lighting is available
- consider the impact of direct solar gain that could over-heat spaces
For more information about Day Lighting see www.advancedglazings.com
Window infiltration is primarily a function of the type of window and/or how well it has been maintained. An operable window is likely to have more infiltration than a fixed window. However, an operable window with a quality seal can be better than a fixed window improperly installed or with poorly maintained sash seals.
Inspect the seals of operable windows and check both fixed and operable windows for caulking. Look for evidence of condensation damage and mold and mildew from dampness. Cheap caulks will crack and become ineffective in a short time, especially when installed on surfaces that move/shrink-swell with temperature and humidity – such as masonry next to wood or metal.
Areas adjacent to large window areas will be colder and may feel draftier even if the windows are well sealed. This is because of a factor called ‘Mean Radiant Temperature’. Window surfaces are colder than the surfaces of insulated and interior walls. This tends to draw heat away from the human body, making that area feel colder, even though a thermometer may say the temperature is the same as other, apparently ‘warmer’ areas. It is also possible that thermal currents may be set-up by warm room air hitting the windows and dropping towards the floor giving the impression of a ‘draft’ that is not really coming through the windows, but is along the face of the windows. HVAC installers try to minimize this affect by installing larger warm air supply registers in front of glass areas.
Practices and Measures
- Consider the building use and determine the appropriateness of operable versus fixed windows. Smaller buildings are more likely to have operable windows. Replacing old operable windows with new fixed windows will likely substantially reduced infiltration, even when the windows are closed.
- Keep weatherstripping in good condition on all operable windows
- Caulk and seal all window sashes
- Remove or cover over areas of un-needed windows, especially banks of single-pane, metal framed windows in older industrial facilities.
- Install reflective films to reduce solar gain on south and west facing glass
Source: Text Bob Fegan 1/2009; diagrams from www.efficientwindow.org web site 9/2005;