We have been analyzing energy efficiency for buildings using BEopt, a software program developed by the Department of Energy’s Renewable Energy Lab (NREL). BEopt is free, and it uses one of the most sophisticated energy analysis engines available: EnergyPlus, also developed by NREL. Our experience on recent small projects gives us an indication of the minimum requirements for designing and building net-zero in Maine, particularly in our region, eastern Maine.
Net-zero buildings are buildings that produce as much energy as they consume. What does this mean? First, because all actively used buildings consume energy, a net-zero building must produce energy. Second, on-site energy generation is pretty expensive, so it pays to minimize energy consumption. Third, energy use varies by necessity. Some energy use goes to human comfort: heating and cooling. For many of us, overly hot and overly cold buildings are not worth it. Refrigerators are more than just nice; they are an essential part of modern life. Buildings should maintain some level of thermal comfort and cold food storage. Some energy goes to niceties: lights, washing machine, dishwasher. We can live with less artificial lights, and we could probably do dishes by hand every now and then. Finally, some energy consumption is pretty optional. Think televisions.
By some estimates, a third of energy consumption is optional. We can conceivably cut our energy use by one-third through lifestyle changes. For instance, right now I am writing this blog post on a laptop at night. The lights in the room are on. I can hear a bathroom fan that was left on. The TV is sleeping with Netflix on. I could certainly write this blog with the lights off, and I could turn other lights off, and turn that fan off. Reducing energy use through voluntary measures can tip the scales in favor of net zero. BEopt can model these “optional” energy uses, but aggressively reducing energy consumption by relying on frugal lifestyles is cheating.
What about energy consumption for necessities, like thermal comfort? This is an important area of inquiry, and we spend quite a bit of time modeling the building envelope in order to find the right mix of windows, walls, roofs and foundation conditions to produce an energy efficient building. Especially in our eastern Maine climate, keeping buildings warm in the winter is a major user of energy. It should come as no surprise that the number one way to achieve net zero buildings is to design and build a well-insulated building.
In order to achieve net-zero in Maine, energy-efficient building envelopes in our climate zone have three characteristics. First, they are well-insulated against conductive heat loss. This means they have lots of insulation. Second, they are well-protected against convective heat loss. This means they do not leak air. They have an effective air barrier. Third, they are optimized for radiative heat gains. This means windows have been optimized for collecting solar energy and reducing night-time radiation loss. The phrase “thermally optimized envelope” is perhaps more accurately descriptive of a net-zero building envelope than the phrase “well-insulated”.
What we have found is that code-required levels of insulation are not adequate to achieve net-zero in Maine with any reasonable amount of on-site energy generation. Current code-required insulation levels are R-49 for roofs and R-27 for walls. BEopt suggests net-zero design becomes possible with R-80 roofs and R-40 walls. Similar bumps are necessary for below-slab insulation and frost wall insulation.
We have also found that code-required window U-values need to be bumped up. U-.32 windows should really be replaced with U-.20 or better windows. Just as important is the window to opaque wall ratio (WWR) on the four cardinal sides of the building. South walls seem to find their sweet spot with 25 percent WWR. East and west sides can be around 10 percent. North sides should be at zero, or five percent maximum.
This brings us to acceptable air infiltration rates. It is commonly acknowledged in the energy modeling and design and construction industries that air infiltration effects are a bear to predict. Nevertheless EnergyPlus models suggest even traditionally small amounts of air infiltration can cause significant energy loss. Suffice it to say that designers and builders should do everything they can to minimize air infiltration. We currently recommend the use of fluid-applied air barriers for just this reason.
Given an aggressive, thermally optimized envelope and reasonably frugal user energy loads, how many solar panels might be necessary to produce a small net-zero building?
Recent modeling suggests a 2,000-square foot home or small office building might be able to achieve net-zero in Maine with 6KwH of solar panels. 6KwH is 6,000 watts per hour. 20 solar panels producing 300 watts each can achieve this amount. Those panels will occupy most of a south-facing roof plane. A reasonable cost for a 6KwH installation is $18,000, or $3.00 per watt. The simple payback for this system, assuming an electricity cost of 14 cents per kilowatt, is probably around ten years. Those panels should last twenty years.
Does this mean net-zero is possible in Eastern Maine for the small business or home? It certainly does.
Interested in discussing how we can help you realize net-zero in Maine? Contact Mike Sealander, AIA at 207.266.5822 or email me.