Tuesday January 29, 2013

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 under way. 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 are the subject of this column.

On the south and east facades (which you see from the road) we’re doing something very different that I’ll describe next week.

Fiberglass frames

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 12-over-12 windows in the late-1700s house we live in currently 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 vinyl (a misleading abbreviation for polyvinyl chloride or PVC), aluminum, steel, and fiberglass.


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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 cladding 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" thick -- much thicker than standard insulated glass (either 7/8" or 1"). 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" glass, while the two inner glazings are suspended polyester films.

On three of these layers of glazing there are low-emissivity (low-e) films. The outer pane of glass is made by Cardinal and includes a high-solar LoE-180 coating on the inner surface of that pane. 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. Heat Mirror, made by Southwall Company, was actually the first type of low-e coating to be commercialized back in the 1970s or early ‘80s. 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. Argon 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 percent. For our windows, though, Alpen used a mix of 90 percent krypton and 10 percent argon. This results in a 40 percent improvement over argon and a 79 percent improvement over air!

Energy performance

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-value: 12.2 (U=0.082)

-- Solar heat gain coefficient: 0.44

-- Visible transmittance, Tvis: 62 percent

-- Light-to-solar gain ratio (Tvis/SHGC): 1.4

-- Passive performance coefficient (SHGC/U-factor): 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 percent

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: 8.3 (U=0.12)

-- Solar heat gain coefficient: 0.39

-- Visible transmittance: 51 percent

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 HPP a leader in
window technology

We have installed these exceptional windows partly as a research experiment. Since our house will not be up to Passive House standards (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 such 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 House windows a run for their money.

It is thrilling to have installed in our home in Dummerston what may be the highest-performance windows in the country, if not in the world.

Alex Wilson is the founder of BuildingGreen, Inc. and the Resilient Design Institute (www.resilientdesign.org), both based in Brattleboro. Send comments or suggestions for future columns to alex@buildinggreen.com.