Why high-performing windows?

We are excited to see that the Root River House is progressing so well, and that the window and door installation is complete. The windows they’ve installed dramatically outperform traditional U.S. windows, and John and Nancy have asked me to describe why. Can we really not buy a good window here in the States? Why is it worthwhile to ship them around the world?

I founded Zola Windows when I was an architect practicing in the U.S., frustrated over window offerings on the American market. Originally Zola was a buying cooperative of my architecture clients, established to purchase better windows at a lower price. But demand grew exponentially as people saw our products and realized what a beautiful and high performing product they can get.

The first advantage of European windows is the glass. We source glass from large solar cell glass manufacturing plants, which means it is extremely clear and low in iron. Second, low-e coatings* developed for the European market have always had an emphasis on being very color neutral—Germans would never accept the color shift in windows we’re used to in the American market and have pushed the industry. Third, codes in Europe have consistently pushed for higher performance, so the industry invested and developed. Codes even require manufacturers to ensure the argon gas does not leak out a rate greater than 1% per year. The result is clearer, color neutral, higher performing glass.

But glass alone is not the secret to European windows. The success of standardized, precision Tilt &Turn hardware started in the 1950s, and the system has developed since. Today this hardware is extremely reliable and capable of carrying large window sashes, and it’s easy to service and replace.


Finally, window frames must insulate well. In the 1950s, European windows started with 58mm (2-9/32 inch) deep wood frames with an insulating value of about R2.5. Depth was steadily increased as glazing packages became more substantial and energy efficiency became more important. Today, 88mm (3-15/32 inch) deep frames with additional insulation and thermal breaks are common. Multi-layer mortise joints make it possible. Most workers go through three-year apprenticeships to become window craftsmen, which ensures quality and consistency of the product.

Passive House has played a vital role in the last decade of window development. Tens of thousands of Passive Houses built in Europe have proven there is a market for the highest quality windows. Multiple Passive House workgroups and extensive support for manufacturers have helped spread knowledge and interest in furthering window technology.

Europe has always had fairly stringent energy codes. Passive House has been able to vastly exceed code performance by helping building professionals understand that a good building is not just the sum of quality parts, but that assembly as a whole and the interaction of the parts are a critical link. Thermal bridging and the effects of unevenly performing components were severely underestimated prior to Passive House.

A typical example I like to bring up is that if you build an R-50 wall with 15% glazing area using an R-3 Energy Star window, overall wall performance drops to about R-15.

If you want to solve this problem by turning your wall into an R-100 wall, but use the same window, overall performance is still only R17. You need to stop the leak, not make the wall better.

Going back to the R-50 wall, but using a Passive House style R-8 window instead, the whole wall performance will rise to R-37.

Windows, and especially window frames, will for the foreseeable future be the weak point in high performance, Passive House construction. They are the leak in your building. This is why investing in the best windows you can afford typically results in the greatest overall benefits.

On a final note, window installation detailing also plays a large role in performance of the installed window. Many customers wonder why we don’t supply our windows with nailing flanges for a quick and easy install.

The answer is simple. To get the best performance out of a window it must be over-insulated and installed roughly in the middle of the wall. A nailing flange prohibits this. In short, annual energy consumption savings between a standard, outside flush install (like a nailing flange would require) vs. a perfect install can be over 30% in a Passive House. That is 30% energy saved, every year for the rest of the life of your house. And all that at nearly zero additional material cost. The only difference is method of installation.

Learn more on benefits of this type of installation, including two articles recently published in Energy Design Update, on our website.


Florian Speier is founder and vice president of Zola European Windows.

* Low-emissivity (Low-E) coatings are a very thin metallic coating on 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. Source: GreenBuildingAdvisor




Lanesboro, Minnesota
Climate Zone 6 (cold/moist)
Latitude: 43° 44' 18'' N
Longitude: 91° 54' 48'' W

House Size

Net Treated Floor Area: 1,514 SF
Gross Square Footage (House only): 2,210 SF

Building Envelope

Roof: R-99
Wall: R-61
Ground: R-53

Windows & Doors

Glazing: U-0.10 BTU / hour / sq. ft.
Solar Heat Gain Coefficient (SHGC): 0.48”
Frame: U-0.19 BTU / hour / sq. ft.

Modeled Performance

Specific Primary Energy Demand (Source Energy Demand): 12.1 kBTU / sq. ft. / year

Specific Space Heat Demand: 7.0 kBTU/sq. ft. / year

Peak Heating Load: 7,047 BTU / hour

Space Cooling Demand: 0.44 kBTU / sq. ft. / year

Peak Cooling Load: 3,625 BTU / hour

Pressure Test Goal: Whole House Air Changes Per Hour (ACH) = 0.4 ACH 50


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