What Do the Experts Say?
“Exterior or interior storm windows; storm windows can reduce heat loss through the windows by 25% to 50%. Storm windows should have weatherstripping at all moveable joints; be made of strong, durable materials; and have interlocking or overlapping joints. Low-e storm windows save even more energy. ”
“Installing new, high-performance windows will improve your home’s energy performance. While it may take many years for new windows to pay off in energy savings, the benefits of added comfort and improved aesthetics and functionality may make the investment worth it to you.” ” You can reduce your home’s heating and cooling costs by as much as 30 percent through proper insulation and air sealing techniques. These techniques will also make your home more comfortable. Reducing your home heating and cooling bills begins with conducting a home energy audit to assess where your home may be losing energy through air leaks or inadequate insulation.”
“If you don’t have storm windows or double-pane windows consider purchasing them. Both of these types of windows can reduce your heat loss through the windows by 25% to 50%. Double-pane windows with low-e coating on the glass reflect heat back into the room during the winter months. If you already have storm windows be sure that the weatherstripping is still tight. An extra pane of glass or plastic added to a window will reduce air infiltration and boost the insulation value of the window. If you are considering adding storm windows, you should compare the costs to installing new energy-efficient windows. You can also install tight-fitting, insulating window shades on windows that feel still feel drafty after you have tried your best to seal them.”
The common misconception that replacing windows will save as much as 50% in energy costs is simply not true. The windows in many historic buildings have functioned for more than 100 years and, with regular maintenance, will usually survive longer and work better than any replacement window. A replacement window does not generally pay for itself in a reasonable length of time. Unlike historic windows, new window assemblies cannot be repaired; they can only be replaced once again. The sustainable choice is to repair historic windows whenever possible.
Any storm window manufacturer can use low-E glass for their storm windows. By switching their glass orders to low-E, manufacturers can quickly make that change. Yet not all storm window manufacturers offer low-E because traditionally there hasn’t been much awareness of this option. But as demand increases due to larger awareness, manufacturers may easily include low-E in their repertoire.
Time for new windows? Not necessarily. High-quality storm windows may be all you need to banish leaks, at a fraction of the cost of replacement windows. “A good storm stops air infiltration about as well as most replacement windows, and the upfront costs are much lower,” says This Old House general contractor Tom Silva. “It’s like putting money in your pocket.”
Michigan State University
The term “heat loss” refers to the movement of heat from warm areas or surfaces to cold areas or surfaces. Heat can be lost from a room by passing through building materials, air spaces, and cracks or openings. Insulation itself does not completely stop the passage of heat; it merely reduces the rate of heat movement.
Loss of heat occurs in one of the following ways or by a combination of conduction, convection and radiation heat transfer.
Heat movement occurs by conduction whenever there is direct contact between the hot and the cold areas. For example, if one surface of the material is heated, the heat will be conducted through the material to the colder surface.
Heat movement by convection depends on some medium, usually air or water, to convey heat from the warm areas to the cold areas. When air moves across a hot surface it becomes warm and rises, thus carrying heat away from the surface.
Heat loss by radiation occurs when there are two separate bodies or surfaces at different temperatures. The warmer body or surface will radiate heat to the colder body or surface without heating the air between them.
“Storm windows save considerable heat and reduce window condensation. They are necessary in Michigan’s climate.”
-Cooperative Extension Service, Michigan State University; Departments of Human Environment and Design, Agricutureal Engineering, Urban Planning and Landscape Architecture.
Windows provide daylight, ventilation, and solar heating to the inside of our homes, but they are also potential sources of energy loss that can lead to excessive air conditioning and heating costs. The National Association of Home Builders Research Center estimates that 43 percent of American homes have single-pane windows that would benefit from cost-effective improvements. Fortunately, advances in window technology offer far more solutions to energy loss than ever before. As evidence of the choices in window technologies available to satisfy a range of climate demands, we need only note the 34 different generic window types — various glazing materials and designs combined with four frame types — that have been tested by the Center for Sustainable Building Research at the University of Minnesota in each of the four ENERGY STAR® climate zones.1 Researchers evaluated window performance in these tests based on annual energy costs, peak demand, winter and summer thermal comfort, and condensation resistance.
The U-factor (or U-value) of a window assembly is a major characteristic related to performance. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating value.
The Efficient Windows Collaborative (EWC)—part of the University of Minnesota’s Center for Sustainable Building Research—defines U-factor as a “measure of the rate of non-solar heat loss or gain through a material or assembly. It is expressed in units of [Btu hour per square foot for F°] (W/sq m-°C). Values are normally given for NFRC/ASHRAE winter conditions of 0°F (-18°C) outdoor temperature, 70°F (21°C) indoor temperature, 15 mph wind, and no solar load. The U-factor may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials.”
Advances in window technology over the past 30 years, such as low-emittance (Low E) coatings, have reduced the impact of this temperature transfer. Low-e is described by the EWC as “microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow.”
S.Craig Drumheller (ASHRAE), Christian Kohler (ASHRAE), Stefanie Minen. “Field Evaluation of Low E Storm Windows”
“Low E glass incorporated into a storm window has the potential of achieving nearly equivalent thermal performance as new windows at a much lower cost”
click here to download the full “Field Evaluation of Low E Storm Windows” PDF.
Efficient Windows Collaborative
Low-emittance (Low-E) coating are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. The principal mechanism of heat transfer in multilayer glazing is thermal radiation from a warm pane of glass to a cooler pane. Coating a glass surface with a low-emittance material and facing that coating into the gap between the glass layers blocks a significant amount of this radiant heat transfer…(read this article)
Lawrence Berkeley National Laboratory (LBNL)
In 2002, the Lawrence Berkeley National Laboratory compared the efficiency of different prime/storm window combinations with that of a replacement window treated with a low-emissivity (low-E) coating of metal or metallic oxide to reduce heat loss or gain. This transparent coating not only reduces heat emissivity but also blocks out the ultraviolet light that fades and damages home furnishings. Under simulated conditions, net heat flow comparisons were made based on various prime/storm combinations (prime alone, prime/low-E external storm, prime/ low-E internal storm, prime/regular external storm) with the low-E replacement window. The research team found that the “addition of low-E storm windows to the prime window provided performance very similar to that of the replacement window, and expected differences in performance were only detectable through a sensitive fitting procedure (essentially, a long-term averaging)”
iVillage Garden Web
click here to enter an informative forum about storm and Low E windows!