Originally Published: January 15, 2016 6 a.m.
The common perception is that windows are energy black holes. However, some believe that today's high-solar-gain triple-glazed windows can gather more radiant heat than they lose, and perform better than an insulated wall that can only lose heat.
Such windows achieve this feat by admitting more useful solar heat gains during the heating season than energy lost through conduction, convection and infrared radiation, therefore acting as a heat generator. The low-e coating used allows external solar heat to pass through the window (unlike the regular low e coatings that prevent solar heat passing) while reflecting the inside infrared heat back into the room. This in turn reduces the overall heat transfer.
Most triple-glazed windows provide a superior U-factor of between 0.14 and 0.24 and an SHGC of between 0.2 and 0.47, but in doing so reduce VT levels down to below 0.5 - and in trying to meet these high standards pay little attention to cost-effectiveness. These investments in very expensive windows are probably a waste of money for most applications and certainly not required in Prescott.
Let's take a simple example: We have a basic home with a living area of 2,025 square feet and wall heights of 9 feet. This gives us a wall area of 1,620 square feet. Energy Star recommends that no more than 15 percent of the wall area be glazing, which in this case is 243 square feet.
Now if we look at the average pricing for the different glazing designs, double pane configurations cost between $30 and $35 per square foot, triple pane between $40 and $50 per square foot and high-performance or super-triple-pane windows discussed here as high as $90 per square foot.
Now if we look at the cost for the example above, the average double pane solution would cost $7,897 and the average triple pane solution $10,935, the high-performance solution would cost $21,870. This shows that the triple pane glazing cost is 38 percent higher than the double glazing and the high-performance cost is 100 percent higher than triple glazing and 177 percent higher than double glazing.
Now let's look at the impact on insulation levels. The 2015 building codes require a minimum wall insulation level of R19 (U = 0.0526) and Energy Star requires a maximum window area of 15 percent; therefore we have 85 percent of the envelope at R19 and 15 percent at the window R value. The best high-performance windows support a U value of 0.16 (R 6). Now R total = 1/ ((0.85*0.0526) + (0.15*0.166)) = R14; down from R19.
Now let's use the 2015 maximum level for windows U=0.35. R total now becomes R10. Therefore, the high performance solution provides a 40 percent increase in R value for a 177 percent increase in cost.
The reason a triple-pane window upgrade fares poorly, from an economic perspective, is that the heating load in an energy-efficient home is small compared to standard houses. As an example, reducing the U value of windows from U= 0.33 to U=0.2 showed that a heating system reduced its heating cost by 3 percent per year.
If we look at the Energy Star estimates, an average home's conditioned space is 1,700 square feet and the average cost of home energy in 2009 was $2,200. If we add the average annual energy increase of 4 percent through 2015 it will be $2,728 today. Now the heating and cooling part of the total energy cost is 46 percent, or $1,254 and 3 percent of that is $37. Now looking at the cost difference between double glazed and triple glazed pricing shows the $3,038 payback period would take 82 years and the difference between double glazing and the high-performance version 377 years.
An alternative to high-performance windows in cold climates is to install affordable fairly standard, double-glazed windows with low-e coatings and then an energy-efficient storm window in winter. Storm windows are the most economical way to improve existing single or double-glazed windows. The storm window would have to have a durable (hard-coat) low-e coating, since it wouldn't be protected in a sealed air space, as most low-e coatings are.
Ultimately, the optimum choice of window and glazing systems will depend on many factors. The number of glazing choices can be complex, and you could be easily talked into choosing windows that sound ideal, but that don't make sense for your climate, your building or even for the specific wall where you install them, and window replacement isn't the best place to start energy upgrades.
When planning energy improvements to an existing house, replacing windows should show up toward the bottom of the list as it almost always makes sense to improve an existing home's air tightness and insulation first, and unfortunately, energy tax credits for window upgrades expired at the end of 2014, so you now have to bear the whole cost.
For more information, contact Paul Scrivens at www.greenhomeenergyadvisors.com.