Net Zero Construction
1. Design: During the design process extreme care is taken to orient the building in a way that maximizes all the advantages the sun has to offer. South facing windows tend to be the largest. Window shades are used to block unwanted summer solar gain, yet allow solar gain in the winter. Wall, roof, or lawn areas are dedicated to PV solar collectors. Energy models are used to determine heat loss and BTU’s needed for heating systems. As the design process proceeds it will yield a cohesive finish design that is both aesthetically appealing and functional in flow and layout. This will allow us to create spaces that both cool and heat themselves.
2. Air sealing: Our buildings are sealed to the point that if inverted and tossed in the ocean the house would float. Any air leaks, as small as a pin hole, are identified and sealed before insulation begins. This is achieved with Blower Door testing, thermal imaging cameras, smoke stick, and the back of our hands to feel for leaks. This is probably the hardest part of the process, but the rest of the systems will not work properly if this step is skipped or done haphazardly. The new “MUBEC standard code” requirements have a target of 3 ACH (air changes per hour). We take our homes to 1 ACH or below (our record being .3). So in a code compliant house you will have to heat the entire air volume of the house 3 times every hour. In our net zero houses you heat the air inside 1 time (or less) every hour. It is important to note that even though the 3 ACH benchmark is a basic MUBEC code requirement, homes are still being built with much worse ratings. Most builders and code officials alike (with the exception of a few) have not been verifying their ACH with blower door testing.
3. Thermal Bridging: Many people know already Wood and steel are very good conductors of Electricity, they are also good conductors of Thermal energy. Meaning if you have a wall stud that is in contact with the cold outside and the other face of it is touching the warm area inside, you are losing heat to the outside and gaining cold on the inside. This effect can be seen on the exterior walls or roof of a standard building during a cold morning after frost has accumulated on the building. Often you can see the outline of the wall studs or rafters clearly as these areas have melted the frost first because of the heat loss through the rafter/stud. There are a couple of different ways to reduce thermal transfer of energy through the structural components of a house. One widely accepted method is Stress Skin Panels, or foam insulation sandwiched between 2 layers of plywood, then hung on the house’s frame. This method poses serious problems in the water sealing aspect; typically, the foam is petroleum based, and has structural ramifications also. The method we prefer is the double stud wall method, composes of two exterior walls separated by a 4″ space between them. Both walls and the space are then filled with Dry Dense pack insulation made from recycled paper. The thermal bringing effect is eliminated.
4. Insulation: The double wall system once filled with cellulose gives us r-45 walls and r-70+ roofs. This one is simple: the more insulation the less energy needed to keep the building warm. Triple glazed windows are often also used to mitigate some of the heat loss through them. The windows are still however the weak point in the envelope, with the best ones on the market at only r-5, so careful design is needed to minimize their effective energy losses.
5. Heating/cooling systems: These houses need very little supplemental heat, in fact if you take it a little further and go to passive house levels they need none. In passive houses the heat from the appliances, sun, and occupants are enough to keep the buildings at 67 deg. without an additional heat source. The associated costs and accompanying size and design limitations to get from net zero to passive house however can be a deal breaker, so we believe“net Zero” is the standard that every new house can be easily built to. Since net zero supplemental heating needs are so small, simple electric baseboard and air source electric heat pumps are used. The heat pumps also have A/C built in, and are 3 times more efficient than simple electric heat in KW used /BTU produced as they simply collect and condense heat from the outside environment, rather than create heat. Ground source heat pumps can also be used (Geothermal), but in our climate, they have been proven to be less effective for heating than the air source when installation costs are factored. Our homes hot water is produced with electricity only by a combination of on demand tank less electric water heaters and heat pump hot water heaters. So, as you can see there is no need for oil, wood, coal, or propane, all of which have health and environmental impacts when burned, and high costs. All you must do is look to the sun for your energy needs.
6. Ventilation: Since our houses are so tight they do not “breath” on their own. We use air to air exchange systems to control air quality. These systems draw fresh cold air from the outside and suck stale hot air from the inside. The two are passed by each other in a heat exchanger and the heat is extracted from the stale air and passed to the fresh air before it enters the building. These systems can recover 95%+ of the heat energy. So, you get fresh air without losing much heat. Some additional features you can get are HEPA filters, dehumidifiers, and humidifiers, which will make your indoor air cleaner than outdoor air. In most standard homes the air quality inside the home is 7 times worse than outdoor air, not so in net zero homes, as indoor air is actually cleaner than outside air. All that with very little heat loss! We also use passive air ventilation systems where practical in some models.
7. Power Generation. Two components are needed for this phase, Photo voltaic solar panels and a Net Metering grid tied power system. We primarily use PV electric panels, as they have gotten very efficient and much cheaper in the last few years. The Wheaton house has 20 panels to generate all the energy needed to heat and power the house for the year. These solar systems have dashboards so you can see your daily use and generation rates, on your smart phone or computer.
With Net Meters, gone are the days of massive costly battery banks taking up an entire room in the house. The Electric supplier installs a split meter outside the building instead of the single model meter found on most homes. The bottom half of the meter acts the same as a standard meter would, any power you use from the grid is accounted for on this meter. The top half of the split meter tracks any excess power sent to the grid that the house didn’t immediately use upon generation. At the end of the month your bill is the “NET” difference between the two meters. In the summer when the heating needs are minimal and the sun is high in the sky for a long period, the home makes surplus energy which is sent to the grid for your neighbors to use. In the winter when the sun is lower in the sky for a shorter time-period and the house is using energy for heating, the additional power needed is pulled from the grid. These homes are designed and built so at the end of the billing year the “Net” difference between the two meters is “zero” $.