A building is many things, but it is always a device to provide environmental control. We want to live, work and play- at least some of the time- in a space that is thermally comfortable. Buildings control environments by expending energy (active systems) such as fuels and electricity; being selectively permeable to the outside to take advantage of the sun or favorable outdoor thermal conditions (passive systems); and by acting as a barrier to heat transfer between the outside and the inside.
In Climate Zone 6B, where my projects generally are located, it’s the cold that we fight to keep outside. A basic building feature for combatting the cold is the thermally high performance envelope. Generally, a high-performance envelope in a cold climate has an exceptional R-value, which blocks heat transfer even though the difference between the outside and inside temperatures is significant. Anything that compromises this resistance to heat transfer is a detriment. Thermal bridges, weak links at changes in assembly, and holes in assemblies all fit this category. Windows, almost by definition, are holes in opaque wall assemblies. They are detriments to net zero buildings.
As an example, we are designing a building right now where the opaque wall R-value is around 40. This wall has double studs at 24 inches with cellulose insulation and a three-inch gap between studs. Outboard of the double studs is three inches of continuous insulation. R-40 is a good R-value, and is necessary for the building to reach net-zero with a modest (5kW) photovoltaic array. Windows in that envelope might be R-5 or R-6. Along with the thermal bridging that always occurs at the window to wall interface, windows are an detriment to net-zero.
That windows are a detriment to net-zero buildings becomes immediately obvious by performing energy model simulations. Of particular use here is BEopt, the user interface by the National Renewable Energy Laboratory (NREL) that sits on top EnergyPlus, one of the premier modeling engines available. BEopt can simulate and compare large numbers of variables that affect building performance. What we find is that any building with significant amounts of window is less energy efficient than buildings with less, or no, window area.
What is a designer to do? After all, we love windows just as all people do. Designing buildings without windows is not sane.
It turns out that when looking at all the myriad permutations of building envelope and windows, there are islands of sanity where windows do not significantly impact the energy performance of buildings. In other words, it is possible to design a building with certain types of windows in certain areas of the envelope without significantly compromising energy performance. It should come as no surprise that south-facing high-performance glazing is relatively benign in its effect on building performance. Our simulations indicate up to 30 percent of south walls can be glazed with windows before significant performance degradation occurs. Even small amounts of glazing on non-south sides produces measurable performance degradation.
What type of south-facing window works? High R-value, high solar heat gain windows work. These include triple pane, non-metal, coated glass windows. R-values for these windows should be at or greater than R-5. Solar heat gain co-efficients (SHGC) should be around .6. In fact, few windows in production today are better than R-5 and .6 SHGC. The take-away is that the intuition of the passive solar movement from 40 years ago was largely correct, but it is today’s high performance window technology that makes passive solar design work.
It is a fact of nature that buildings in cold climates require a lot of energy to maintain human comfort unless they have very high R-values and few compromises to that R-value, including glazed areas. Energy modeling shows how windows can be strategically incorporated into envelope design. It is only through the iterative process of mathematically testing designs through simulation that we will find the sweet spot where our buildings are energy neutral and enjoyable places to be.