Look at some of the advice being given on home building on other forums:


"Dual panes have very little value in a cooling environment where heat gain is overwhelmingly radiant. Even then, it is not going to make much difference on the bill. And you can always use blinds or curtains. Anything that blocks radiant heat will help. The best energy upgrade you can do (or look for in a home) in Phoenix area is a high efficiency AC. The rest of it is just nibbling around the edges.

Single pane windows are used extensively in commercial/industrial construction where a high degree of radiant reflection is desired - the AZ deserts for example. Thicknesses of 1/2 or more are common. They work better but cost more than dual panes found in residential applications.

As for your R7 windows, there is no such thing. R value is the inverse of U values that are used to rate the conductive transmissivity of window glass. An R value of 7 would correspond to a U value of .14. The very best triple pane, low-e windows you will find for homes with have a U value of about .25 or and R value of 4. Windows, any windows, are lousy insulators.

Typically a single pane, plain glass window might have U factor of 1. A dual pane might be as good as .5 - which would be R2. A low-e could drop that to .35 (low-e is about radiant flux not conduction like R values describe). .35 is roughly R3. Again, windows are not the place to be putting money if you want to reduce energy costs. The benefit cost ratio is much smaller than most other improvements. "


They were recommending SINGLE PANE windows over dual pane windows and were claiming that energy efficient windows are overrated and not necessary in a desert climate.

There are elements of truth- the context makes a difference.

Windows ARE a low-R hole in your thermal boundary, but they're also usually THE DOMINANT solar gain factor on walls that get direct sun, far more than absorption and conduction through walls. In new construction Low-E double panes are worth paying for from a comfort as well as an energy-use point of view. Low-E coatings on the exterior is exactly the sort of radiant flux reduction that the author supports in statements like "Anything that blocks radiant heat will help." Damn-straight it will help! Low SHGC windows can be key to lowering the peak loads and the compressor tonnage needed.

Low E double glazed replacement windows are often a lousy investment in a retrofit situation though (which is maybe the unstated context?), and the money may indeed be better spent replacing an aging AC system with a right-sized high SEER version, in terms of return-on-investment. In a retrofit the size, shading factors, and orientation of the windows aren't likely to change, and the improvements from higher-performance windows may not be worth the money.

But in a new house, if your "...nibbling around the edges..." on several fronts at the design phase it's very easy to make the rationale for something better than plain-glass single panes. Yes, a so-so U0.34 low-E window is only R3, but moderate and low SHGC double-pane windows in the U0.25 range are available. Sure that's R4, (sounds pretty crappy, right), but the direct solar gain can be cut by 60-75%, cutting the direct solar gain from that whole side of the building by more than half.

Even a U0.35 double pane window can have a SHGC as low as 0.35 or lower, less than half that of typical low-E windows designed for heating dominated climates, and isn't (usually) a huge expense. Sure it's only ~ R3, but it blows away more than half the cooling load related to that window due to it's rejection of radiated heat.

But walls & window are usually only about half the total sensible cooling load, and roofs often dominate. But nibble around ALL of those edges and the comfort & energy use pictures get a lot rosier. Many things that are cost-prohibitive in a retrofit are often cost-neutral in new construction.

Designing out solar gain where it's not wanted in the design phase, or at least reducing it by reducing the glazed area (particularly on the west side, which is more difficult to shade with overhangs) is usually a cheaper performance upgrade than going with ever more higher performance windows.

But single pane windows (even thick ones)? "Single pane windows are used extensively in commercial/industrial construction where a high degree of radiant reflection is desired" REALLY? Perhaps for daylighting, with spectrally selective low E coatings on the exterior, in non air-conditioned buildings. I'd love to know the details behind that assertion.

Hold up a piece of Low e coated insulated glass up to the direct rays of the sun, then use a meter to measure the relative amount of heat (infrared) that gets through the glass compared to the amount of heat that gets through a single pane glass. You will quickly arrive at the conclusion that Low E coated glass is FAR superior in blocking heat gain. Possibly efficient enough to justify keeping an older A/C unit for a couple of more years. There is a reason Low E technology is so widespread...it works!