Beyond a second pane, thermal performance is increased with additional panes of glass, and argon or krypton gas between the panes instead of air (argon and krypton are better insulators.) Additional thermal performance is achieved with "low-e" coatings. A typical double-paned "low-e" glass window with argon and a wooden, fiberglass or vinyl frame has an R-value of about 3. A similar triple pane window approximately R-4.5 and a quadruple pane window, approximately R-5. Triple and quadruple pane windows are rare in our climate but more common in highly efficient homes in colder areas.
"Low-E" stands for "Low Emissivity," referring to a micro thin metal film which is applied to the inner surfaces of one or more of the glass panes in a window. While the low-e coating is largely invisible to the human eye, it serves to reflect radiated heat back toward the source. A low-e coating on the outer surface of the inner pane reflects heat radiating outward from the home back inside, which helps to lower heating bills and increases comfort (clear glass windows "feel" colder because we radiate heat from our bodies out through them.) In the case of a low-e film on the inner surface of the outer pane, it serves to reflect solar heat away from the house, which is intended to reduce overheating of the house in the Summer. Unfortunately, this reflection occurs year-round, and can have the unintended consequence of reflecting away the sun's heat when it's desired...
Replacement Windows Can INCREASE Your Heating Load!!!
It is important to understand that installing sun-reflecting replacement "low-e" windows can actually INCREASE the heating load of a house because a significant amount of heat is gained in Winter through solar radiation. This lost "passive solar" energy is often not offset by the greater thermal performance of the window. This fact, unfortunately, is insufficiently acknowledged, making these windows difficult to find (see "Low-e Glass ... A Nation Divided.") A "low-e" coating on the outer pane is a blunt instrument for solar heat gain control, substantially reducing heat gain in Summer AND Winter.
A better approach (assuming the house has sufficient solar exposure and is well oriented) is to use windows on the South side with a high SHGC (Solar Heat Gain Coefficient) and rely upon properly sized roof overhangs, moveable awnings and/or deciduous plants to mitigate the heat gain in Summer. On the East and West sides of the house, solar heat gain is hard to control externally, so low SHGC windows generally make sense there. On the North side, thermal efficiency is the primary concern, as there is little direct sunlight through those windows.
"Old School" Window Insulation, aka "Curtains" Also worth noting is that tight fitting drapes, shades, or closed blinds increase the R-value of any window by about 0.3. Along with shutters, they can also be an effective means of controlling heat gain, as has been the practice for generations. "Solar film" can also be added "in place" to single pane windows with a solar heat gain problem, but a loss of passive solar energy will result.
Why isn't replacing all your windows an automatic "green slam dunk?"
It's expensive (to do it "right") and it often doesn't make that much difference in your energy efficiency. In many cases, the money for new windows is better spent first on other, more effective green options. Why? Well, despite the fact that a single pane wooden window has a low value of R-1, a "good" double pane, low-e window is "only" about R-3. Three times better, yes, but still a virtual "thermal hole" in even a code-minimum R-13 insulated wall. When evaluating the benefits of upgrading windows, it is important to consider window location, quantity, and size. These are factors in an HVAC Load Calculation, which is specific to each building's design and climate and vital in making decisions regarding the efficacy of energy efficiency upgrades, as well as sizing the HVAC system correctly. (See "When Size Matters: Proper HVAC Sizing Improves Efficiency, Health, Comfort" by Scott T. Shepherd, Professional Remodeler, September 1, 2006.)
Where's the "Low Hanging Fruit?"
As a general rule, it's best to "test not guess," as our friends at Renu Home Performance like to say. So, starting with a home energy audit is the logical first step. The results of the test, along with consultation with the tester, will help you identify the "best bang for the buck" in terms of improving your home's energy performance. These results vary from home to home but, as a general rule, replacing your windows doesn't rank at the top of the "to do" list.
Here are some general estimates:
The City of Boulder Office of Environmental Affairs "Greenhouse Gas Savings Actions Table, Personal Actions Checklist" determined the following average household greenhouse gas savings by improving building shell:
Note that both of these estimates are comparing new windows to leaky, non-weatherstripped single pane windows, so the savings from new windows are likely less when compared to existing retrofitted windows. As for old wooden windows, as much as 85% of air infiltration (or heat loss) is around the edges of the sash, not through the glass - weatherstripping is a HUGE energy efficiency improvement for a relatively low cost.
So, prior to signing that window replacement contract, commission a home energy audit, and determine which areas of your home need improvement. Adding insulation (especially in the attic), sealing leaks, and repairing leaky ducts are all relatively low cost measures that can lead to huge energy savings. Beyond that, an upgraded, properly sized HVAC system can cut energy use by 20% (see "A Guide to Energy-Efficient Heating and Cooling" by Energy Star for more info.)
What About Noise Control?
Another argument frequently used to justify window replacement is noise control. Again, there is some improvement (which varies with the type of window) from multiple panes, but the real benefit is realized by reducing air leakage, which weatherstripping can do well.
Sound transmission class (STC) is a rating that provides an estimate of the sound-transmission performance for general building design purposes. STC value represents an average of sound transmission losses between the 125 Hz and 4,000 Hz frequencies.
Typical STC ratings
Again, as in the case of thermal performance, the difference between windows types pales in comparison to the enhanced performance of a solid wall. Sound performance can be enhanced with such measures as thickened glass, laminated glass, "mismatched" pane thicknesses, larger air spaces, argon gas and storm windows, but again, the real culprit is air leakage. Single pane windows which are properly weatherstripped perform much better than the "leaky" single pane windows most comparisons are based upon.
The Environmental Cost of New Windows
Also frequently neglected in the discussion of window replacement is the environmental impact of the new windows themselves. While new windows surpass original wooden windows for energy efficiency, they certainly do not match the durability or serviceability of the latter, which were constructed from durable old-growth lumber, offering lifespans that dwarf their contemporary counterparts. A problem with even the best multi-paned windows is that, over time, the seals around the panes fail, and the windows "fog." In this circumstance or in the case of an errant baseball, the solution is the same - replacement, not repair. With the BEST window warranties at 20 years, what can result from window replacement is swapping windows which have lasted 80-100 years (and have plenty of life left in them with simple maintenance) with a product which cannot be repaired and wears out every decade - it brings an ironic new meaning to the term "replacement windows."
Of particular concern are vinyl-framed windows (many lower cost windows.) PVC (poly vinyl chloride) production has contributed a significant portion of the world's burden of persistent toxic pollutants and endocrine-disrupting chemicals - including dioxin and phthalates - that are now universally present in the environment and the human population. PVC cannot be recycled, and there are serious questions about the material's durability in sunlight and temperature swings.
It's the Architecture, Stupid!
There's a saying in architecture that "the windows are the eyes of a house." There is no surer way to destroy the beauty of a house than installing cheap, ugly replacement windows. Unfortunately, people are commonly convinced that they have "no choice" but to replace all their windows. Once brought to that point of view the cost issue looms larger. Replacement windows have improved markedly in recent years, as demand for windows that are historically compatible has increased, but the windows that look, and match, best are the most expensive. Thus can begin the downhill slide to ugly vinyl windows which are today's equivalent of the aluminum single-pane "maintenance-free miracle product" of decades past. Many a current buyer of an older home curses the decision to install these aberrations, which are less energy-efficient than the windows they replaced!
Note: For those Eichler/"Mid-Century Modern" homeowners, there are strategies and products to deal aesthetically with those "walls of glass." See "Windows, windows everywhere: the pride and peril of life in a house with all that glass" by Barry Brisco, Eichler House Doctor
If You ARE Buying New Windows
While we feel that the available replacements for existing wooden windows are not as good as is often advertised, we are not categorically opposed to window replacement, just uninformed window replacement. In many cases, when you're being sold on the benefits of a new windows, you're basically being sold on the benefits of weatherstripping. However, in new construction using salvaged historic windows is rarely practical (you have to hunt to find matching, complete windows and you have to design around what you can find.) If you are buying new windows, it is important to understand what you are buying, what to install where, and to purchase a quality product with as low an environmental impact as possible. As an alternative to vinyl, lower-cost windows with fiberglass frames are available. Fiberglass frames are energy efficient, cleaner to produce, recyclable and less prone to thermal expansion problems than vinyl.
It should come as no surprise that modern window technology is an improvement. After all, it was developed for a reason. What is unnerving (if not surprising) is that the quantitative value, appropriate application and environmental consequences of window replacement are so rarely discussed when these products are marketed. The prevailing wisdom is that "you must replace your single pane windows to be energy-efficient." It may be illustrative to note that a previous generation's "miracle" window product (the single pane aluminum window) is the WORST performing window (R-0.77), less efficient than the original wooden windows it replaced in many unfortunate circumstances. Aluminum is a worse insulator than glass and the frame is typically 20-30% of a window's area.
We do not regard window manufacturers or installers as sellers of "snake oil." Quite the contrary, there are numerous manufacturers of quality products and reputable installers of high integrity. Nonetheless, there is little profit in selling weatherstripping and window maintenance, so that side of the debate is less publicized. Whatever decisions you make regarding window replacement, know that it is a sufficiently expensive and architecturally significant decision to warrant careful consideration. If you're buying new windows, research the products you're considering to ensure you get maximum value for your investment. From an old house fanatic's standpoint, if you're replacing "replacement" aluminum frame windows, by all means do so, with an environmentally-friendly, period-appropriate product. If you have an old house with banks of double-hung or casement windows and you love the warp of light through the wavy glass, know that your windows can be improved markedly with weatherstripping and your home made energy efficient even if you keep the original windows. Regardless of the types of windows used, know that the size, position and shading of windows have an enormous impact on the energy performance of a house.
We are sorry if we appear the bearer of bad news, but we want to be "the bearer of important news." The good news is that many of these problems have accessible, affordable and pleasant solutions. Please stay tuned...
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