Published on April 28th, 2018 | by James Ayre0
Insulated Windows 101 — Double Glazing, Triple Glazing, Thermal Performance, & Potential Problems
April 28th, 2018 by James Ayre
Double glazing. Triple glazing. Insulated windows. Double-glazed windows. Triple-glazed windows. You’ve probably heard those terms before, but if you aren’t or haven’t been involved in construction and design work, or in the retrofitting of your own house, then you may not know exactly what they mean.
If you are unfamiliar with these terms, I’m going to provide a basic overview of the terminology and what exactly it relates to in practice, as well as a brief discussion of the problems that can arise with such tech.
Insulated Windows & Glass — Basic Overview
Insulated windows and glass are essentially an insulation solution whereby multiple layers (panes) of glass are used in place of a single layer/pane — with the spaces between these panes generally being turned into a vacuum or filled with a gas with lower thermal conductivity and heat capacity than “air” (such as argon, or sometimes krypton).
Double glazing refers to the use of two panes of glass with an evacuated space between or a high-performance fill-gas, and triple glazing refers to three panes of glass with two evacuated spaces or gas-filled spaces.
Generally speaking, the glass panes used in such systems vary between 1/8″ to 3/8″ in thickness (3-10 mm) — with both/all of the panes being of the same thickness in the same window, though there is some variance when sound-insulation is a desired quality as well.
The two or three glass panes in question (in double- or triple-glazed windows), are turned into a cohesive unit through the use of what’s termed a “spacer” — which is usually made out of foam, or a combination of foam and aluminum, nowadays. Care must be taken when materials are selected so as to reduce the occurrence of condensation — though desiccants are also generally relied upon in spacers as a further “insurance policy” to protect against condensation. The reason why condensation is a sensitive issue for multiple-pane windows is because there is no access to the inner spaces to wipe down the window as one would easily do with a single pane window.
Insulated or double-glazed windows date back to at least 1865, when an American by the name of Thomas Stetson patented a system whereby two panes of glass were sealed with a glass-edge. Further patents were filed in the 1930s by other parties, and followed quickly by products including the “Thermopane” branded ones — that word is now in common use to denote any insulated window products.
All of that said, the basic idea for double- and triple-glazed windows clearly builds upon earlier “storm window” designs. Storm windows essentially just being (generally removable) second-layers outside the building used to provide further insulation during cold weather. The primary downside to conventional storm window designs is the space required, and storage during summer months. What double-glazed and triple-glazed windows bring the table are their relatively compact footprints (which allows them to easily take the place of conventional windows, while also allowing for the use of screens during open-window weather) and better air-seals.
Double-glazed or triple-glazed insulated windows are one of the key components in passive solar home and building design.
Thermal Performance, R-Values, & Soundproofing
The thermal performance of double-glazed and triple-glazed windows will vary quite a bit based on: the thickness of the space between the glass panes; the gas used as fill (or the degree to which vacuum evacuation was effective); the number of glass panes involved; and the spacers used, amongst other things.
Generally speaking, the thicker the gas-filled space is the more effective it will be with regard to limiting thermal conductivity. Glazed windows utilizing evacuated vacuum spacing are a somewhat different matter — with effectiveness mostly depending upon the effectiveness of the manufacturing process. An issue with evacuated glazing though is that the temperature-difference between the two panes can become extreme and thus lead to cracking — owing to lack of convective thermal transfer between panes of glass (leaving only radiative transfer and conduction through the spacers.
As noted before, when vacuum glazing isn’t used, reliance upon gases that offer better thermal performance than air are generally used — in particular, argon is used a lot, and krypton is used occasionally. The thermal conductivity of argon is just 67% that if air, and the thermal conductivity of krypton is itself only around half of that of argon. Both gases are considered to be non-toxic, chemically inert, relatively easy to source, transparent, and odor-free — hence their widespread use.
Occasionally the potent greenhouse gas sulfur hexafluoride is also used — partly due to its effective sound-insulating qualities, and partly due to thermal conductivity somewhat better than that of argon — but this isn’t particularly widespread in many regions.
With regard to the R-values of different types of double-glazed and triple-glazed windows, ranges of between R-3 and R-13 are common — with the lower-end being represented by basic double-glazed windows utilizing argon gas and no low-emissivity coatings; and the higher-end by vacuum insulated units and some cold-weather-oriented quadruple-glazed or quintuple-glazed options.
Sound-proofing is a benefit of double-glazed and triple-glazed windows, which accompanies and adds to the thermal insulation capacity features. While all windows featuring multiple panes will provide some sound-proofing capacity, designs featuring higher numbers of panes and/or asymmetric thickness in different panes perform the best. And, as noted previously, the potent greenhouse gas sulfur hexafluoride functions particularly well in this capacity. The spacers used will of course greatly affect sound transfer as well.
Potential Problems — Cracking, Condensation, & Lifespan
Potential problems accompanying the use of double-glazed or triple-glazed windows include: cracking due to high temperature differentials between the panes (primarily with regard only to vacuum insulated windows); and condensation problems and loss of effectiveness to the breaking of the seal (and saturation of desiccant).
Condensation is only a problem, it should be realized, after the perimeter seal has failed (and the desiccant has already become saturated) — once this has occurred, then thermal performance will have already been impacted for some time (due to the loss of the vacuum or of the argon/krypton). That being the case, condensation is the least of the owner’s problems if high thermal performance is needed (as in a passive solar house).
The primary cause of such seal-failures is the same as it is for the formation of cracks: the presence of large differences in temperature between the inner and outer glass panes, and thus the spacer adhesives as well. Think of a glass bottle being moved rapidly from extreme heat to cold to get an idea of what happens. Partial-shading of single panes can also lead to cracking — as is true of conventional glass windows as well.
These problems can be prevented to some degree by utilizing thicker glass panes in the windows — which comes with associated higher costs and weights. Also, limiting the differences in temperature between panes by tripling-up, quadrupling-up or quintupling up the number of panes, can be an effective way to limit the temperature differentials between adjacent panes.
With regard to longevity, most double-glazed and triple-glazed windows simply aren’t going to maintain their high thermal performance for as long as the windows themselves remain functional as windows. While the manufacture of highly durable multiple-pane glazed windows is of course possible, in practice, keeping costs affordable seems to require that warranties and projected lifespans be limited to ~10–25 years.
Speaking generally, lifespans will vary to a great degree depending upon placement, typical indoor/outdoor temperature differences, shading or sunlight exposure, and local climate and weather conditions.
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