Dive summary:

• Zeitgeist Design, architect Florian Speier's shop in Boulder, Colo., takes commissions only for homes that can win Passive House certification, so Speier knows a lot about what building products contribute to the goal.
• One thing that helped Zeitgeist's first passive house – a renovated home overlooking San Francisco Bay in Belmont, Calif. – get across the finish line was excellently insulated Zola windows made in Europe.
• The experience prompted Speier to set up a separate company to be Zola's U.S. representative for its insulated glass units, and now he wears a second hat as a consultant to other designers and to builders on high-end windows and building insulation.

 From the article:

The double-glazed IGUs installed in Belmont have 80 percent visual transmittance and an R-value of 5. The architect established a buying cooperative to obtain them. ...