Zero Energy Home Design
By Joe Emerson and Jason Offut
“The most powerful instrument of change on the planet is the stroke of a designer’s pen.” Edward Mazria, Architecture 2030
An affordable Zero Energy Home begins with an integrated Zero Energy Home (ZEH) design. It is best to develop a ZEH design in conjunction with a project team, consisting of the owner, the builder, the energy consultant, the landscaper, and the designer. This approach is key to developing a home design that meets the owner’s needs, integrates the construction methods necessary for reaching the Zero Energy goal, and takes into account how the home interacts with the site and it’s surroundings – both local and regional. Getting to the level of insulation, air tightness, energy efficiency, and solar exposure needed to create a cost-effective Zero Energy Home requires that a wide variety of small issues be effectively addressed in the design phase, including exploring the most cost effective options and ideas for reaching Net Zero – and the site is the best place to begin.
Understanding the site, with its assets and limitations, is essential to creating a successful Zero Energy Home design. Topography, weather patterns, temperature, winds, solar exposure, shading, heating/cooling degree-days, and many other site factors need to be reviewed prior to putting pen to paper. Once the complexities of the site are understood, the Zero Energy home design can complement the site.
For the most cost-effective ZEH design, the selection of the site and the siting of the home must take climate, wind, sun, shade and topography into account. It is important to choose a site with a sufficiently wide east-west lot line to allow for the placement of the home on the site so that there is adequate south facing roof for solar collectors and adequate south facing windows and doors for passive solar gain. The site should be free of immoveable obstructions, such as trees, neighboring homes, and land forms that could interfere with adequate solar access. Also, the site should not be excessively exposed to prevailing winds or have topography that would unnecessarily increase the costs of a Zero Energy Home.
A thorough solar evaluation of the site, using a handheld shade analysis tool, such as a Solmetric SunEye is recommended. The Suneye measures shade and solar access for the home and offers digital and graphical displays of the amount of solar energy available at the site. Many solar companies will visit the site and offer this solar analysis at no cost to the homeowner or builder. The solar analysis is important for designing passive solar, solar photovoltaic (PV) and solar thermal (hot water) systems. It impacts the site selection, the location and orientation of the home and roof, the roof area needed for solar, and the window area needed for passive solar. This data may also indicate which trees need to be cleared or trimmed to increase solar exposure. Taking full advantage of the solar potential for each site is one of the lowest cost strategies for achieving a successful Zero Net Energy Home Design.
After the site analysis has been concluded, the next step is for the project team to assist with identifying and defining the project parameters and specifications and for the designer or architect to complete the Basis of Design (B.O.D.). This document acts as the “road map” for the project. The B.O.D. identifies key project elements such as homeowners’ requirements, preferences and vision; building type, scope, and key design details; the goals, strategies, and specifications for reaching Zero Net Energy; and the sustainable and renewable resources to be included.
Design elements specified in a Zero Energy Home B.O.D. may include: raised heel trusses; double walls with offset studs or other energy conserving wall assembly; simple, appealing lines without sprawl; placement and sizing of windows; strategies for reducing heat loss through conduction; plans for ducts and wiring to be inside the conditioned space; and other features. At different stages during the design process, the energy consultant should conduct energy modeling using the energy parameters being discussed to contribute approximate energy usage data to the B.O.D. The project team contributes to the B.O.D.throughout the design process.
When contemplating the design and the construction of an affordable Zero Energy Home, size and shape do matter. Smaller homes use less energy for space heating and cooling. Limiting the size of the home will have a direct impact on overall energy required on site, and should help reduce costs. The savings from building a home about 10% smaller may fully pay for the additional cost of a Zero Energy Home without sacrificing quality or livability. Smaller homes can be designed to look and feel larger while also having ample storage space; and for many people they can be more convenient and livable than larger homes.
Shape is also important. The shape of the home should be kept simple and in scale to the user and the site. Rather than a sprawling design, a building with a low exterior surface to volume ratio, with as few corners as possible, and with simple clean lines will save energy and construction costs. It is useful to think of the home as an insulated six-sided box with the insulated floor, four walls, and ceiling making up the box. Each additional corner or cantilever increases the amount of framing required and makes air sealing and insulating more challenging.
The excellent book, The Not So Big House, describes how a small home can be designed to look large, spacious and comfortable. Features, such as extra storage spaces within otherwise poorly utilized spaces in a home, as well as, the addition of extra storage space or a flex room in the garage area outside of the more costly conditioned space, can contribute to making a smaller home function as well as a larger home.
Zero Energy Homes should be designed to use the sun’s energy as much as possible, for such things as: generating electricity, heating hot water, and utilizing passive solar space heating. After a site analysis and an understanding of what solar resources are available on site, passive solar design concepts should be incorporated next. If the home can be located and appropriately exposed to the sun for the site and local climate, energy savings are possible from the start. These savings can be achieved by enhancing solar gain in the winter and by reducing heat loads in the summer. In climates where heating is more important than cooling, the building should have its long side facing the sun, providing more opportunity for the sun’s warmth to heat the home. In climates where cooling is more important, the long axis of the home could be perpendicular to the sun.
Depending on its specific climate and location, the area of southern facing windows in the home should be determined to optimize passive heating of the home. Similarly, the home’s overhangs, the window header heights, window sill heights, and door heights should be coordinated and designed to optimize the use of the solar energy available for the specific site to optimize the heating and cooling needed for the local climate. The width and height of roof overhangs will depend on site specific solar exposure and climate and are important factors in determining the amount of passive solar heating possible. They should be designed to avoid excessive heat gain in the summer and optimize solar heat gain in the winter.The width and height of overhangs will depend on site specific solar exposure and climate, and they must be coordinated.
Design for Added Insulation
Think of the home as a six-sided box in which all six sides need to have the most cost-effective insulation possible. The R-values on each side of the box, as determined by energy modeling, must be sufficient to reach the zero net energy goal. Thicker walls, deeper floor assembly and raised heel trusses for the roof may need to be included in the design to accommodate sufficient insulation depth depending on the insulation materials and methods selected. If dense-pack fiberglass or cellulose insulation is used, 8″ to 12″ thick, double walls with off-set studs may need to be included in the design, depending on the local climate, to provide adequate space in the walls for the insulation. The R-value for each side of the six sides should be detailed on the plans. Moisture related issues should be considered in the design of highly insulated building assemblies such as these. Moisture related challenges are best prevented by designing assemblies that are both breathable and airtight.
Design with a Continuous Air Barrier
The house should be designed with a continuous air barrier. All the cracks, holes, and exterior envelope penetrations of the home’s six-sided box must be systematically sealed. All penetrations, such as for electrical, ERV/HRV venting, gas, and water should be minimized and included in the design. For example, recessed can lighting can be eliminated or the cans can be recessed in soffits. Similarly ERV/HRV vents could replace bathroom venting systems, and possibly kitchen venting systems, further reducing penetrations. The air barrier could be drawn on the building plans and carefully detailed, indicating which contractor is responsible for each air sealing detail and what material they should use. This information could later be included in the scope of work for each subcontractor.
One option to consider is to move the thermal boundary from the ceiling to the roof by insulating the rafters (a.k.a. a “hot roof”) by applying spray foam to the underside of the roof sheathing, creating an exterior air barrier. Drywall on the interior face of the roof rafters would serve as the interior air barrier. This alignment of the thermal boundary with the roof can simplify air sealing details and create interior spaces ventilation ducts and other mechanical equipment. The cost effectiveness of this approach and the environmental issues of spray foam should be carefully evaluated
Using a Thermal ByPass checklist during the design phase will help ensure that all aspects of designing a continuous airtight thermal barrier are considered. An in-depth Thermal ByPass Checklist is available from Energy Star and another is available from Green Building Advisor. The thermal barrier should be explicitly detailed on the home design.
All duct-work could be designed to be within the conditioned space to optimize the integrity of the air barrier and insulation. According to Green Energy Advisor, ducts can be kept inside by locating them in a conditioned basement, in a conditioned, crawl space, in an unvented conditioned attic, in open-web floor trusses (especially in a two story home), in soffits, in dropped ceilings, or in a chase designed into special roof trusses.
Minimizing Thermal Bridging
During the design phase, thermal bridging should be eliminated as much as possible at foundations, edges, corners, soffits, eaves, connections, decks and penetrations. Door and window installation details and second story floor interfaces are other areas of concern. Double walls with offset studs and with separate top and bottom plates will greatly reduce thermal bridging in the walls, as will a thick layer of exterior foam on the studs. Fewer exterior wall offsets will also reduce the potential for thermal bridging. Decks and porches can be designed so that they are actually separate from the house, completely eliminating thermal bridging. The design should specify for the builder and subcontractor, how thermal bridging can be avoided.
Windows and Doors
The orientation of doors and windows must take climate, wind, sun and shade into account. Since the home is a well-insulated, highly airtight, “six-sided box,” windows and doors are relatively poorly insulated “holes” in that box, and are more expensive than the wall assembly that make up the “box”. Therefore minimizing window area is a highly recommended design strategy for achieving an affordable Zero Net Energy Home. Minimizing window area, while providing plenty of light, optimal passive solar heating to living areas, and adequate ventilation in the summer presents a unique design challenge. An overall window to floor ratio of about 14% and in norther climates a south facing window to floor ratio of 6% are often recommended for a Zero Energy Home, but that ratio will depend on the climate, as well as site and design considerations. Low U-value windows are important (from U 0.14 to U 0.23 as needed to reduce heat loss and maintain optimum comfort), but are less effective as the window area increases. South-facing windows should have a Solar Heat Gain Coefficient (SHGC) of o.5 or greater, in conjunction with climate appropriate overhangs, to allow for passive solar heat gain in the winter months, but not in the summer months. Fewer larger windows are more energy efficient than more smaller windows, because there is a higher glass to frame ratio with larger windows, and the frames are where the leakage occurs. Casement and fixed windows are less leaky than sliders or hung windows. Windows and doorways should be located to reduce exposure to prevailing winds to reduce heating load requirements and weather infiltration. If doors are exposed to prevailing winds, the doors should have three point door latches or be part of an air-lock entryway with double entry doors.
Advanced Framing Techniques
Advanced framing techniques should be included in the design as far as possible, because they use less lumber, allow for more insulation, and reduce opportunities for conduction. Members are sized based on calculated loads and continuous load paths are used from the roof to the foundation with studs 24” on center. Headers are sized for actual loads so that unnecessary framing members are eliminated, saving some costs and resources. Engineered lumber, using recycled lumber production bi-products and allowing for longer spans compared to usual lumber products, is recommended.
Other Design Considerations
If ductless mini-splits or centrally located ductless heaters or stoves are used, air flow to all the rooms needs to be considered during the design phase, and perhaps door cuts under the bedroom doors, transoms over some of the doors, or sound insulated air vents in the walls should be part of the design. Energy efficient ventilation systems (ERVs and HRVs) may also help optimize air flow in the home. The hot water tank needs to be centrally placed to bathrooms and kitchen to minimize hot water runs, or a half-loop water recirculation system should be considered. Similarly the HRV or ERV needs to be centrally located so that it has short runs for supplying air to the living area and bedrooms and for extracting air from the kitchen and bathrooms.
While affordable Zero Energy Homes should be designed using as many standard building techniques as possible, some aspects of building Zero Energy Homes, such as double offset walls, may be new to builders and subcontractors. So all such building strategies should be clearly outlined and specified on the design and in accompanying notes. Clearly outlining all the details required to make a Zero Energy Home makes it possible for any experienced builder and subcontractor to successfully build an affordable Zero Net Energy home.