Are High-Performance Windows Worth Their High Cost?
Simple payback may not be enough to justify the investment in triple-glazed windows, even in a cold climate
Randy George is in the final planning stages for a new house he will be building this summer in Vermont, and from the sound of it he won't have much trouble staying warm through those long winters.
In addition to R-45 walls, an R-65 roof and R-20 slab, the house will have air infiltration rates lower than one air change per hour at 50 pascals of depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home.. Although not quite meeting the PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. standard, that's extremely tight construction.
George wants his windows to have a U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. of no more than 0.2 (equal to R-5), but his contractor tells him he doubts that windows meeting that criterion will be worth the expense. "All along I've felt that one of the high-performance Canadian windows would be critical to the performance of our house," George writes in his Q&A post at GreenBuildingAdvisor.
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But after meeting with the contractor, George went back to the heat-load calculator he'd been using for another look. While his original calculations showed that he'd need a heating system capable of producing 20,000 Btuh, reducing the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of his windows from 5 to 3 would only boost that to 22,500 Btuh. Projected heating costs would go up only $150 a year. Even after juggling the numbers slightly — boosting the required Btu and adjusting the cost of propane upwards — the difference in heating costs would be about $200 a year.
"I'm assuming that the cost difference between these windows would be several thousand," George writes. "This is making me question the wisdom of the good windows. Can anyone tell me what I'm missing in all of this, or is there a little too much hype surrounding the 'good' windows?"
Payback is a more complex than it looks
GBAGreenBuildingAdvisor.com senior editor Martin Holladay doesn't find much to challenge in George's number-crunching, but he points to three other factors that are worth considering.
First, although current energy prices make the more expensive windows hard to justify, there's no guarantee that fuel prices will remain that low.
Second, a house with triple-glazed windows will ride out a wintertime power outage better than a house with double-glazed windows because it won't cool off as quickly.
And third, Holladay writes, "Triple-glazed windows provide comfort advantages that can't be quantified. When it's below zero outdoors, you can sit in a chair beside a triple-glazed window and not feel cold. That's worth something."
Kevin O'Meara speaks to that last point in recalling a Thanksgiving dinner he sat through at a friend's house. The dining table was next to a big picture window. "Even with the thick drapes pulled it was miserable to be sitting next to the window," O'Meara said. "My feet were freezing!"
Another way of looking at the numbers comes from Eric Tollefson, who points out that if George plans to borrow money to build his house, then net cash flow trumps payback. "By that I mean you can calculate that at 5% for a 30-year mortgage, borrowing $1,000 costs roughly $65 per year. (Insert figures appropriate to your financing situation here)," Tollefson says. "So, if you spend $3,000 to upgrade the windows, that will add about $195 per year to the mortgage. But, if that upgrade saves $200 or more a year in energy costs, it will be cash-flow-positive from Day One. You could also say that it has a 15-year payback, but that really only applies if you're paying out-of-pocket."
Getting the heat gain you need
In addition to the window's U-factor, which rates the window's resistance to heat flow, another important attribute is the window's solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1., or SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1., which describes the heating effect of sunlight striking the window. (For more on that and other window performance characteristics listed on the manufacturer's label, visit the National Fenestration Rating Council .)
John Klingel surmises that inexpensive windows won't give George the right SHGC. (In northern climates, windows with a high SHGC are beneficial.) "Cheapos will not likely have what you need," Klingel says. "My guess is [that you need] a value of about 0.4 or higher, at least on the south side. I'll be using 0.58. I suspect that proper SHGC windows will do a lot toward paying for themselves."
The relationship between the SHGC and the window's U-factor also has Matthew Amann somewhat puzzled. He writes that buying triple-glazed windows from his preferred manufacturer would give him a lower U-factor (a good thing) but also lower SHGC (not so good). Plus, it would add $6,000, or 30%, to the cost of his windows.
"Most standard low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. glass is going to lower the SHGC along with U-factor when triple glazed, and I don't think I can honestly sell someone on clear glass and get fading callbacks, so the dilemma continues. Let's tickle two birds with one feather and get to some specifics on window glass and glazing on this thread..."
To that end, Holladay points to a useful chart comparing window Energy Ratings published by Thermotech Fiberglass, a well-regarded Canadian window manufacturer. The higher the ER rating, the better the window for southern Canada and northern New England, he adds.
Does the contractor have an ax to grind?
Keith Gustafson introduces another possibility: is the contractor in George's situation suggesting the cheaper windows because it's to his advantage? "Not to unnecessarily trash a profession," Gustafson writes, "but contractors make money by using slightly cheaper materials than asked for. It is just business. It is your job to make sure that you get what you want."
The suggestion rubs Aaron Vander Meulen the wrong way. "I'm sorry you feel this way," he says. "... Actually, we make more money, due to discounts, by using higher cost materials; 10% off $1,000 in poplar trim is more than 10% of $500 in MDF. And if you're getting billed for poplar, but getting MDF, then that's fraud.
"Windows are even easier to see if you're getting what you want by looking at the sticker right on the window," he adds. "If your contractor hurries up and takes those off immediately, be afraid. Get a reputable contractor. One you can TRUST. By the way, we probably won't be the cheapest."
"First, I apologize," Gustafson replies. "Second: Fer cryin' out loud, how polite must one be? I quite specifically did not indict all contractors. It must be mentioned, that for one reason or another, if a contractor suggests either a more expensive or less expensive product, their motives are fair game. If you guys don't think that a lot of contractors out there would push an inferior product because the margin is 2 percent better, well, you have lived a charmed life... I hope that every one of your customers is your friend for life; that would be awesome. Life is not always that perfect."
Not worth the cost
Raff Winks also went through extensive mental gymnastics in evaluating various types of windows for a 500-sq. ft. glass curtain wall — and he came to the conclusion that triple-pane units weren't worth the extra cost.
"After researching/costing and comparing double vs. triple performance data, I felt that the additional cost of triple-pane windows was not worth the marginal performance gains," Winks says. "The glass industry has a lot of catching up to do, but with only three or four major glass suppliers, it might be a while before we see affordable, high-performance windows.
"IMO, the current market offerings are falling short of the type of performance we should be installing in our high-performance homes."
Our expert's opinion
Here's what GBA technical director Peter Yost had to say:
Good points all around from GBAers on this Q&A. Some additional points to consider:
Payback: I have always had trouble with payback calculations, whether simple or net value. Each relies on some assumption about the relationship between current and future prices of energy, a relationship that is inherently unstable even for short periods of time, much less decades. Count on energy prices going up and up, and always purchase as much performance as you possibly can.
Low-e: Not all low-e coatings are the same when it comes to associated solar heat gain coefficients (SHGC). Pyrolitic or hard-coat low-e coatings typically have much higher SHGC, so it’s worth shopping around for higher SHGC for cold climates.
Window attachments: Another option is to explore high-performance window attachments to boost the heat loss reduction of windows at night in the winter, in this case. BuildingGreen is currently working with Lawrence Berkeley National Laboratories on ways to fairly and more accurately assess and compare the thermal performance of window attachments, like shades, quilts, and low-e storm windows. Although I can’t report specifically on the results yet, I can tell you that some window attachments are performing very well, in terms of air tightness and added overall thermal resistance. The results of the first year of this project will be available within the next month or so, on the Web — stay tuned.
I can tell you that if the attachment you select has good or excellent thermal performance and gives you other features you would need for your windows anyway — privacy, dayligthing, glare control, and solar gain control in the summer — there is some economy in multifunction window attachments.
- Thermotech Fiberglass
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