Building and designing affordable zero energy homes involves start with smart design!
Start with Smart Design
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. 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.
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.
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.
Think small and design spaces for uses of the client rather than for resale value. Even a small family can live comfortably in a well-designed 1,500-1,800 square foot home with well thought out functionality, storage, and traffic flow. Reducing the home size will save energy and pay for all the energy improvements in a zero energy home.
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.
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 “vault 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.
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.
Design a solar shading strategy that allows 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 sides and on 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 moveable shading, such as awnings, sun screens or vegetation. This will allow greater heat gain during spring and less heat gain during fall.
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-40 for walls, R-60 for ceilings and R-38 for floors. In warm climates typical R-values are R-19 for walls, R-30 for ceilings, and R-19 for floors. Optimal R-values and U-values for your specific climate zone should be determined using energy modeling.
Clearly specify measures for avoiding thermal bridging on the plans. This includes using advanced framing techniques for the wall, floor, and ceiling systems as well as exterior foam sheathing, staggered-stud, and double-stud framing.
Specify that wall insulation is 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, 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.
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 air-tight drywall approach, ZIP System SIGA membrane and tape 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.
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.
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.
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 and a supply of waste heat. 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. 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.
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.