Windows
Smaller houses are more directly and dramatically affected by changes to the envelope than large ones. Due to this fact, the choice of windows can dramatically improve the energy efficiency of a home, help to minimize operating costs, and increase comfort. The following comparative analysis identifies the relative economic, energy, and environmental implications of three high performance windows: double-glazed low-E windows with high solar heat gain (0.5 SHGC), double-glazed low-E windows with low solar heat gain (0.3 SHGC), and triple-glazed low-E windows with moderate solar heat gain (0.4 SHGC). All windows discussed in this section meet or exceed the minimum energy code requirements for Minnesota.
General Recommendations
To balance cost and performance, select an NFRC-rated, double-glazed, low-E, argon fill window that meets the 2010 ENERGY STAR guidelines. Windows should have a whole window U-value of 0.32 or less, solar heat gain coefficient (SHGC) between 0.30 and 0.60, and a visible light transmittance (VT) of 0.50 or more. These values should be listed on the window's NFRC label. Another NFRC measure used is air leakage, which should be 0.3 or less. Selecting high-performance, ENERGY STAR-rated windows will reduce energy costs, improve comfort, reduce fading of furnishings and interior finishes, and reduce window condensation under most conditions. To learn more about these benefits, visit the Efficient Windows Collaborative website and navigate to the "Benefits" page.
Traditionally, the incremental performance improvement offered by triple-glazed windows was not worth the added expense, compared to double-glazed low-e options. However, triple-glazed windows are now readily available from Canadian manufacturers with substantially improved performance. These windows typically incorporate insulated fiberglass frames, with whole window U-values around 0.18 and SHGCs of 0.4 and higher. Such windows are frequently used in homes designed to take advantage of passive solar gain. Passive solar is difficult to implement using windows with low solar heat gain coefficients (less than 0.4). At this time, most American window manufacturers only offer windows with SHGCs less than 0.4.
Options and Analysis
| Window Alternatives | |||||
| alternatives | double hung | casement | awning | ||
| vinyl | $163 | $236 | NA | ||
| wood | $188 | $265 | $315 | ||
| wood, vinyl clad | $246 | $295 | $328 | ||
| wood, metal clad | $282 | $271 | $369 | ||
| fiberglass | $318 | NA | NA | ||
Costs based on Means CostWorks 2009, not including installation.
All windows approximately 30" x 36", with double-glazed, low-e glass: U-value=0.35, SHGC=0.35
Cost
Frame types:
Windows with vinyl frames cost less than comparable windows with wood or wood-clad frames, while achieving similar energy performance. However, aesthetics and home values should also be considered. For most window replacement projects, wood windows retain their value better than vinyl windows1 . Vinyl-clad and metal-clad wood windows are more expensive than traditional wood windows, but have the benefit of requiring less maintenance. Fiberglass windows are typically the most expensive, but combine superior energy performance, durability, and low maintenance.Glazing types:
In terms of first cost, double-glazed, low-E windows are frequently cheaper than triple-glazed options. Until recently, the often greater expense of triple pane windows was not easily recouped with energy savings. However, some Canadian window manufacturers currently supply fiberglass framed, triple-glazed windows at prices comparable to many double-glazed, wood frame options from American manufacturers. High performance triple-glazed windows can prove economical if they are incorporated into a holistic energy strategy that also reduces the size and expense of a home's heating and cooling system. Typically, this kind of strategy also employs passive solar heating, which is a no or low-cost option.
alternatives cooling cost/sf-habitable heating cost/sf-habitable energy cost/sf-habitable yearly energy cost double low-E high solar gain
U-value=0.33
SHGC=0.5$0.07 $0.57 $1.50 $1,295 double low-E low solar gain
U-value=0.33
SHGC=0.3$0.06 $0.61 $1.53 $1,318 triple low-E mod. solar gain
U-value=0.18
SHGC=0.4$0.07 $0.51 $1.43 $1,236 Energy modeling was conducted using REM Rate 12.3 based on an 864sf Minnesota code base house, with wood siding, 15% window-to-floor area, unshaded windows distributed equally with regard to orientation, U-value and SHGC as noted, 80 AFUE furnace, and 10 EER air conditioning.
Energy costs based on average 2007 U.S. residential rates from EIA: $0.106/kWh for electricity and $1.301/therm of natural gas.Energy
Windows can affect heating and cooling energy use, primarily through two characteristics: insulation value (expressed by the window's U-value) and solar heat gain (expressed by the window's SHGC value). Low U-values (0.32 or lower) are desirable in cold climates to reduce heat loss in the winter and prevent window condensation. SHGC values are more complex. Low solar heat gain windows block heat from the sun, lowering cooling costs in the summer, but increasing heating costs in the winter. High solar heat gain windows allow heat from the sun to pass, increasing cooling costs in the summer, but reducing heating costs in the winter. In cold climates where heating costs greatly outweigh cooling costs, it is more advantageous to reduce heating costs so high SHGC windows have a greater impact on reducing total energy costs. Based on energy model results, this is illustrated by comparing the high solar gain and low solar gain windows in the chart above. Although high solar gain windows increase the annual cooling cost by 1 cent per square foot of floor area, they reduce the annual heating cost by 4 cents per square foot of floor area.Alternatively, the placement of windows with differing SHGC's can be optimized depending on their orientation and the design of the home. Low solar heat gain windows are especially valuable in east and west-facing walls that are exposed to long periods of direct solar gain in the summer, when heat is least wanted. Use of low solar heat gain windows in these locations lowers cooling costs without substantially increasing heating costs. High solar heat gain windows are especially valuable in south-facing walls that are exposed to direct solar gain in the winter, when heat is desired the most. Using high solar gain windows in south-facing orientations lowers heating costs without substantially increasing cooling costs. North facing windows simply require very low U-values. For a conventional house, optimizing windows in this manner has a small net improvement in energy costs, but if passive solar gain is optimized with proper house shape and orientation, roof overhangs, and thermal mass, the advantage is significantly greater. A passive solar house design in Minnesota can reduce heating costs by 20% or more with no or minimal additional cooling costs.
Durability
Exterior maintenance and durability are issues that are frequently considered when selecting windows. In general, the key to window durability is good design (protection from the elements), proper installation, and adequate maintenance (caulking and periodic painting or resealing). Generous roof overhangs to shield windows from strong summer sun and rain, combined with flashing details and window sills with drip edges to redirect water around window openings, are important elements of design. For proper installation, consult the Water Management Guide, by Joe Lstiburek, a book that provides details on the proper installation of windows in a wide variety of walls and configurations.Vinyl windows are commonly perceived as low-maintenance as compared to solid wood windows. However, vinyl exposed to UV rays from sunlight has historically been subject to problems, such as bleaching and material degradation. These issues have largely been addressed in current vinyl window products. However, compared to other frame materials like wood and fiberglass, vinyl frames are less stiff, more brittle in cold weather, and are subject to increased thermal expansion and contraction. To reduce thermal expansion, most vinyl windows are offered only in lighter shades, which absorb less heat. Nevertheless, repeated expansion and contraction can warp vinyl window frames and decrease the lifespan of the product.
Wood window frames require more maintenance than other frame materials. Maintenance includes repeated applications of wood sealers and finishes (i.e. paint) to maintain moisture resistance. Without proper maintenance, wood windows are subject blistering, peeling, and ultimately rot. However, if properly maintained, wood windows have a long track record of durability.
Increasingly, options that blend the properties of wood and vinyl to create long-lasting, low-maintenance windows are becoming available. For example, wood windows clad with exterior vinyl or aluminum can create low-maintenance exteriors. Alternatively, wood/plastic composites and fiberglass windows offer increased strength and durability with low maintenance. Solid metal frame windows, even those that incorporate a thermal break, are not recommended for cold climates due to lack of insulation and potential issues with condensation on the frames during the winter.
Materials
Vinyl is made from non-renewable petroleum products. The production and eventual disposal of vinyl is associated with environmental pollutants such as dioxins, PCBs, and phthalates. Despite reductions in the release of these chemicals during manufacturing, a lack of recyclers currently accepting post-consumer vinyl products means that most discarded vinyl is landfilled or burned, leading to increasing levels of toxins in the environment. In general, lifecycle analysis shows that wood has significantly lower environmental impacts than most other materials, including vinyl. These impacts are further lessened if the wood is harvested from certified forests. Several large window manufacturers have committed to using various levels of FSC-certified wood in their window frames. Although metal has a larger environmental impact than wood or vinyl, it can be recycled easily and many recyclers currently accept post-consumer metal. Recycling reduces the environmental impact of metal substantially. The largest concern with fiberglass is the release of VOCs during the production process.Future Recycling
In theory, most types of windows are recyclable. The glass itself is recyclable, though specialized chemicals like the low-e coating make this more difficult. For example, many glass manufacturers only accept recycled glass from their own product streams. Most types of window frames, including aluminum, fiberglass, wood, and vinyl, are also recyclable in theory. Exceptions include composites such as wood/plastic and fiberglass/plastic. In practice, windows are fairly difficult to recycle because they are assemblies of several different materials including glass, frame material, metal, silicon, and sometimes rigid foam insulation. In addition, no networks of recyclers currently exist to handle post-consumer vinyl, wood, aluminum, or fiberglass windows.Practice
All the window products discussed here follow standard practice.
Other Resources
Publications
Residential Windows: A Guide to New Technologies and Energy Performance, by John Carmody, Lisa Heschong, Stephen Selkowitz, and Dariush Arasteh
Fact Sheet: The Efficient Windows Collaborative Builder Toolkit
Efficient Windows Collaborative
Fact Sheet: Improving Energy Performance in Low-Income Housing
Efficient Windows Collaborative
Fact Sheet: Selecting Energy Efficient Windows in Minnesota
Efficient Windows Collaborative
Water Management Guide, by Joe Lstiburek
Web Sites
Efficient Windows Collaborative
1 "Cost vs. Value Report 2008-09 Minneapolis, MN." Remodeling Magazine. 2009. Accessed November 2009. www.remodeling.hw.net/2008/costvsvalue/division/west-north-central/city/minneapolis--mn.aspx
