Site Selection
The ideal site for a zero energy home would have unobstructed sun, flat topography, and little exposure to the weather. It would also be located near access to services, shopping, and mass transit. While few sites will be perfect, being selective about the site will certainly pay off in lowered costs and better living. Solar access is especially important. A solar energy contractor can perform a site analysis to be sure that sufficient sunlight is available on the wall or roof surfaces and suggest the optimal orientation and size of the solar surfaces. As is often the case, a site may have less than perfect solar access. Even with less than optimal solar access, or no solar access, you can still employ all the strategies discussed here to make the home as close to net zero as possible.
Building Orientation
Orient the building to take greatest advantage of seasonal sun angles for both passive heating and cooling and for maximum solar energy production. Depending on climate, this could involve maximizing passive solar heat gain in cold climates or natural shading in warm climates. For solar panels, a direct southern roof orientation is preferable. However, when southern orientation is not possible, consult with your solar installer to determine the optimal orientation for optimizing solar gain for your local climate conditions.
Climate
Consider how the local climate affects design. One size does NOT fit all climates. Insulation levels, air tightness, moisture control strategies, daylighting opportunities, and many other design elements must reflect climate zones and local conditions. Special attention should be paid to design needs in warmer climates.
Shape
During the conceptual design phase, consider using fewer, simpler shapes, rather than many smaller shapes with lots of architectural complexity. Simpler building masses will be easier and less expensive to build, air seal, and insulate in the field.
Size
Think small and design spaces for uses of the homebuyer rather than for resale value. Many smaller families can live comfortably in a well-designed 1,500-1,800 square foot home with well thought out functionality, storage, and traffic flow. The building itself may cost hundreds of dollars per square foot, so shaving a few square feet from the total can save thousands of dollars in construction costs. With careful planning, reducing the home size will save energy and pay for all the energy improvements needed for the home to achieve zero net energy.
Thermal Boundary
Clearly define the thermal boundary on design plans. That means deciding what is inside and what is outside the conditioned space. For example: vented attics and crawl spaces are outside the conditioned space. If a ducted heating and cooling system is selected, it is especially important to allow space within the conditioned envelope to hold all the elements of the system.
Ceilings
Use only one type of ceiling throughout the house: either flat or cathedral. Whenever ceiling heights change, there will be a wall separating the room with the high ceiling from an unheated space, usually an attic. This “high wall” can be very tricky to air seal and insulate. The insulation level of that wall should equal other exterior walls, and it will need to be covered with a rigid material to enclose the insulation. If more than one ceiling height is present, develop clear details for air sealing, insulation, and rigid backing.
Roof Overhangs
Design a solar shading strategy that allows the sun to heat the building when needed and avoid overheating when not needed. One strategy is to design and build fixed roof overhangs, especially on the south and west sides when they are exposed to direct afternoon sun. Calculate and specify the southern roof overhang to maximize winter sun exposure and minimize heat from the summer sun. These fixed overhangs must be a compromise between similar sun angles in spring and fall when the heating or cooling requirements are much different. An alternative would be to consider a shorter fixed overhang of 12 to 18 inches along with exterior shading, such as awnings, sun screens or vegetation. This will allow greater heat gain during spring and less heat gain during fall.
R-Values
Specify R-values on the plans for wall, ceiling, and floors and U-values for windows and doors. In cold climates, typical R-values are R-30 to R-40 for walls, R-60 for ceilings and R-38 for floors. In warm climates typical R-values are R-19 for walls, R-38 for ceilings, and R-19 for floors. Optimal R-values and U-values for your specific climate zone should be determined using energy modeling.
Thermal Bridging
Clearly specify measures for avoiding thermal bridging on the plans. These may include using advanced framing techniques for the wall, floor, and ceiling systems as well as exterior foam sheathing, staggered-stud, and double-stud framing.
Enclosing Insulation
Specify that wall insulation be fully enclosed with rigid sheets of OSB, Thermoply, or similar materials and never design walls where it is difficult to properly cover insulation. Pay particular attention to soffits, attics, bathtub surrounds, HVAC chases, and fireplace enclosures. If you’re drawing double-stud walls, be sure to include details for enclosing the framing cavity, including a plywood cap, across the parallel top plates and plywood bucks inside window and door openings. A good guide to basic insulation and air sealing is the ENERGY STAR Raters Field Checklist.
Air Sealing Goal
Specify the air tightness standard to be achieved on plans. This is generally expressed in air changes per hour at 50 Pascals (ACH50). The threshold needed to reach net zero energy should be 2.0 ACH50 or less.
Air Barrier Systems
Identify the type of air barrier system to be used. Will it be an air-tight drywall approach, ZIP System SIGA membrane and tape, Aerobarrier or something else? List air sealing materials and techniques on design plans.
Blower Door Directed Air Sealing
Specify that blower door directed air sealing be conducted, after ceiling drywall has been installed and before insulation is installed, in order to locate unexpected air leaks and to effectively seal them. This process is especially useful if you are pushing your air leakage goal lower. Once you’ve mastered the techniques and can reliably reach your target, an end-of-construction blower door test should be sufficient.
Heating and Cooling Equipment
Locate all heating and cooling equipment, along with their pipes, ducts and refrigerant lines. Locate the hot water system and specify its efficiency rating. Draw these on the plans and specify the need for sealing any penetrations. For ducted systems, strive to bring the air handler and all ducts inside the thermal boundary. Consider ductless mini-split heat pumps as they are highly energy efficient.
Ventilation
Draw mechanical ventilation equipment and ductwork on the design plans and locate equipment and ducts within the conditioned envelope of the building where feasible. Remember that heat recovery ventilators need a condensate drain. Specify all equipment efficiency ratings on the plans. Follow these six steps to proper heat recovery ventilation design and equipment selection.
Water Heater
Decide on the type of water heater to be used and the best location. Electric resistance water heaters should be centrally located inside the conditioned space in heating-dominated climates and outside the conditioned space in cooling-dominated climates. In heating-dominated climates, heat pump water heaters should be located outside the conditioned space in areas with about 1,000 cubic feet of volume. A garage or basement is often good, because these locations are not heated directly, but stay relatively warm all year long. Heat pump water heaters also offer ducting options that can allow them to be located inside the home. If gas-fired water heaters are used in an air-tight home, they must be sealed combustion models.
Solar Energy System
Based on an accurate energy model, determine the optimal size of the photovoltaic system to meet the energy needs of the home. Check that there is adequate roof area with the proper tilt and orientation to supply sufficient energy to reach the zero energy threshold. Make sure that chimneys, plumbing vents, and other roof protrusions are located outside of the roof area planned for solar panels.
Appliances
Specify energy-efficient appliances and their ratings that were selected during energy modeling. The Energy Star Products page is a good resource for choosing efficient appliances. And be sure to consider electric heat pump clothes dryers and induction stove tops because of their high level of energy efficiency.
Engage the Team
Early in the design process, create a project team including all the relevant building trades, including framers, insulators, plumbers, electricians, and solar contractors. The team should identify the most cost-effective energy efficiency measures in the design and the most cost-effective sequence for implementing these measures. Ask the team to review the design and incorporate their feedback.
