State-of-the-Art Windows for A New Home
Two layers of glass plus two plastic films — along with three low-e coatings and krypton gas fill — make these Alpen windows top performers
Having written about windows and emerging window technologies for longer than I care to admit (since before low-e coatings even existed), I must say that it’s incredibly fun to be building a house and having an opportunity to try out some of the leading-edge stuff I’ve been writing about.
In my effort to create a “demonstration home,” we are actually installing two very different types of windows in the 1812 farmhouse rebuild that’s underway. On the north and west facades we’re installing state-of-the-art, fiberglass-framed casement and awning windows from Alpen High Performance Products. These windows, which we ordered from Pinnacle Window Solutions in Maine, are the subject of this blog.
On the south and east facades (which you see from the road) we’re doing something very different that I’ll describe in a future blog.
Fiberglass frames are durable
Traditionally, residential windows have been wood-framed. I love the look of wood, and if properly maintained, wood windows can last a long time: the twelve-over-twelve windows in the late-1700s house we currently live in are still hanging on after more than 200 years. But there are drawbacks to wood, including decay and the need for regular maintenance.
Besides wood, the primary materials used for window frames today are vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). (a misleading abbreviation for polyvinyl chloride or PVC), aluminum, steel, and fiberglass. Due to the very high conductivity, aluminum and steel are less common today in residential windows. Due to its low cost, vinyl has increased dramatically in popularity, finally surpassing wood as the leading window frame material a few years ago.
A lot of wood windows try to achieve the best of both worlds with vinyl or aluminum claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. on the exterior (for durability) and exposed wood on the interior. I think this is a nice compromise between appearance and durability and I recommend cladding for most wood windows.
The Alpen windows we installed are fiberglass-framed. Fiberglass is much stronger than vinyl, it has a much lower coefficient of thermal expansion (i.e., it doesn’t expand and contract as much when warmed by the sun and cooled at night), and it has hollow cavities that can be insulated with polyurethane insulation.
Our window glazings are 1 3/8 inch thick — much thicker than standard insulated glass (typically 7/8 inch or 1 inch). With the polyurethane insulation, these frames provide an insulating value of about R-4.3 (U-0.23), as calculated using industry-standard methods. Being fiberglass, they are highly durable and should not require maintenance — though fiberglass does take a coat of paint much better than vinyl, should we ever choose to paint them.
Outrageously high-performance glazing
While standard windows today are double-glazed (two layers of glass separated by an air space), our Alpen windows are quad-glazed — meaning there are four layers of glazing. The inner and outer glazings are 1/8-inch glass, while the two inner glazings are suspended polyester films.
On three of these layers of glazing there are low-emissivityAmount of heat radiation emitted from a particular body or material. Emissivity is expressed in a fraction or ratio, with the lowest values indicating low emissivity and the highest indicating the high emissivity of flat black surfaces. (low-e) coatings. The outer pane of glass is made by Cardinal Glass Industries and includes a high-solar LoE-180 coating on the inner surface of that pane (the #2 surface in window-industry parlance). This low-e coating is appropriate in northern climates because it lets a lot of solar gain through and it’s clearer to look through.
The suspended polyester films both have Heat Mirror 88 coatings (on the #4 and #6 surfaces). Heat Mirror, made by Southwall Technologies, was actually the first type of low-e coating to be commercialized back in 1981. Heat Mirror coatings are available in various forms (HM88, HM77, SC75, HM66), with the number indicating the transmittance through the glazing; HM88 allows the most solar gain.
Another important strategy for reducing heat loss through windows is to substitute a low-conductivity gas for air in the air space. ArgonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows. is commonly used as a gas fill, and for windows the size of ours replacing air with argon would boost the insulating performance by about 28%. For our windows, though, Alpen used a mix of 90% kryptonA colorless, odorless inert gas, often used with argon in fluorescent lighting and sometimes used as gas fill in high-performance glazing. and 10% air. This results in a 40% improvement over argon and a 79% improvement over air!
The specifications tell the story
So what do all these bells and whistles provide in terms of energy performance? I was astounded when my friend at Alpen, Robert Clarke, sent me the following performance numbers.
Performance for the glazing only (calculated using Window 6.0):
- Center-of-glass R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. : R-12.2 (U-0.082)
- Solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.: 0.44
- Visible transmittance (Tvis): 62%
- Light-to-solar gain ratio (Tvis/SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.): 1.4
- Passive performance coefficient (SHGC/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. ): 5.3
- Winter interior glass surface temp. (assuming 0°F outdoor, 70°F indoor, 12 mph wind): 65°F
- Acoustic control (STC): 34
- UV blockage (380 nm): 100.0%
The National Fenestration Rating Council (NFRC) has developed methodologies for testing and reporting unit or full-frame window performance. Our window configuration has not gone through that NFRC testing, but estimated full-frame values are as follows:
- R-value: R-8.3 (U-0.12)
- Solar heat gain coefficient: 0.39
- Visible transmittance: 51%
An R-12 window (R-8 unit value) is hard to believe. This insulates as well as a 2x4 wall insulated with fiberglass, yet also brings in significant solar gain and daylight, while providing clear views to the outdoors. I look forward to reporting on the performance and durability of these windows over time.
Alpen High Performance Products is a leader in window technology
We have installed these exceptional windows partly as a research experiment. Since our house will not meet the Passivhaus standard (a rating system that originated in Germany for super-low-energy homes), I’m not sure we would have been able to justify such high-performance windows if Alpen’s Robert Clarke hadn’t wanted me to have them and provided them at a great price.
I’ve known Robert and his company (Alpen) for many years. He and Alpen have been the leaders with high-performance windows in the U.S. since way back in the mid-1970s, consistently way ahead of the curve in introducing new technologies.
Several years ago Clarke sold Alpen Windows to Serious Materials, a venture-capital-funded company that sought to change the world with innovative products and materials. But Serious Materials may have spread itself too thin, and there were some quality-control problems with their windows.
Last year, Robert and a partner were successful in buying back Alpen from Serious Materials. I’m hopeful that the company can regain its stature at the top of the window-performance pack — and give the European Passive HouseA 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. windows a run for their money.
It is thrilling to have installed in our home in Dummerston what may be among the highest-performance windows in the country.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.
- Alex Wilson
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