Mother Earth Living Blogs > Wiser Living

Wiser Living

Finding a natural solution

Passive House Design: The Importance of High-Performance Windows

Mark A. Miller is a practicing architect/builder/developer living in Chicago who designs projects around the country. His studio, Mark A. Miller Architects + Builders, designs and builds high-performing, energy-efficient homes that speak to the soul. Mark recently co-founded the Passive House Alliance Chicago and is lecturing about the Passive House standard throughout the Midwest. You can learn more about his unique approach to designing thoughtful homes at his websites: Zen + Architecture and Passive House Midwest . 

I wanted to highlight some of the techniques that factor into achieving the Passive House standard. The Passive House standard is a performance benchmark for buildings. If met, the structure will achieve significant energy efficiency and fall in line with the Architecture 2030 challenge to have all buildings move to a net zero status (meaning the building does not require power outside its site to operate), thus reducing the large contribution buildings make to CO2 output and global warming.  The initiative is the brainchild of Edward Mazria, an architect who wrote one of the bibles on passive solar design called The Passive Solar Energy Book. If you don’t have this on your shelf, I highly recommend it. The illustrations are just wonderful and the information is as relevant as ever.

Matson living room windows 
South-facing windows help passive solar homes maximize solar heat gain during the day. Photo By Scott Shigley. 

Now I mentioned this earlier, to keep folks from getting confused on terms here. Passive solar design attempts to maximize free solar heat gain through a large amount of south facing glazing.  It uses large thermal mass, such as a concrete slab or trombe wall, to store this heat during the day so that it is slowly released into the home over the night. These homes experience temperature swings throughout the day. In comparison, a home built to the Passive House standard attempts to have very stable temperatures 24 hours a day. Passive Houses are less focused on maximum solar gain, thus south facing windows are very controlled and calculated into the yearly performance of the structure. Thermal mass is an option, but Passive Houses do not rely solely on this solar gain for the majority of their comfort.

So, since we are talking about Edward Mazria and the benefits of passive solar design, we might as well start our Passive House techniques with high-performance windows. These are an important component of the system, as windows notoriously are the weak link for any structure regarding its energy-efficiency. Why is this? 

We strive to increase the insulative values in our building envelope. High performing walls have previously been in the R-20 to R-30 range (a measure of the capacity of a material, such as insulation, to impede heat flow, with increasing values indicating a greater capacity). To give you a reference base, a typical 2x4 stud wall insulated with fiberglass batts is around R-13. With the Passive House standard, walls are now thickening to achieve ratings more in the R-40 to R-60 range.   

Let’s compare this to windows, which have thin glass and typically uninsulated frames.  Window’s insulative performance is given typically in “U-values." If we do the math and convert the U-values into R-values, we find our typical American-made window lies in the R-1 to R-3 range. This is a drop in insulative performance from our walls by 80 to 90 percent, which is dramatic. We all like windows—they bring in the benefits of natural light and connect us with the views and sounds of nature and our neighborhood, which is very important—but the common double hung window is no longer used in a Passive House project because it is too leaky. The sash slides in the frame, which makes it tough to completely seal the sash to the frame.  Instead, casement windows and awning windows are favored. The same applies to patio sliding doors. French type doors are preferred as they have a better air-tightness than the other doors.

Optiwin windows 
Optiwin produces handcrafted, high-performance windows that meet Passive House certification requirements. Photo Courtesy Optiwin.

So, the Passive House standard focuses on using the highest performing windows on the market.  These include triple glazing; use of low-E window glazing, a microscopically thin, metallic window coating that reduces the amount of heat radiating through the surface; insulated glass spacers; and insulated window frames and several gaskets to make the window as air-tight as possible when closed. These windows now achieve R-values anywhere from R-5 up to R-14. This is an exponential jump in energy performance.   

Unfortunately, most of these windows are only made in Germany (Optiwin windows) or Austria right now, apart from a couple of companies in Canada and only one that I am aware of in the U.S., which is Serious Materials. With the interest in high-performance building increasing, we are all hoping that we will see more options available in the U.S. market and that prices will come down.  

One thing that has always bothered me is why we feel that once windows are installed, that’s it! Why can’t we have some material that covers the window to make it more energy-efficient at night, when we don’t need to look through it? Remember shutters? They actually had a purpose decades and centuries ago. When weather was inclement, one could close shutters to add a layer of protection to the windows. Shutters stopped the sun's heat from coming in during a hot New Orleans summer, and they kept the wrath of a winter storm from getting to the windows. It was probably a combination of the modern movements to steel and glass buildings and the low cost of energy that brought us to this point. I personally would like to see some research and development go into this concept.

I can’t stress enough the importance of proper window design in high-performance home building. Because these elements are the weakest link in our building envelope, great care must be taken in properly selecting and using windows in the right locations for the optimum energy-saving performance. 

Read more thoughts from Mark Miller on Passive House design!