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
It Begins with the Design
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-owner, builder, energy consultant, the landscaper, and designer. 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 for reaching Net Zero and verifying them through Energy Modeling.
For the most cost-effective ZEH design, the selection of the site and the siting of the home must consider climate, weather patterns, wind, sun exposure, shade, temperature, heating/cooling degree-days and topography. 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 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. The site should not be excessively exposed to prevailing winds or have other topography features that might 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 can 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, location and orientation of the home and roof, roof design, and roof and window area needed for solar PV and 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.
The Basis of 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 and preferences; building type, scope, and key design details; specific 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 or unnecessary corners; placement and sizing of windows; placing and sizing of south window overhangs, strategies for reducing heat loss through conduction; plans for ducts and wiring to be inside the conditioned space; and other features that will make the home easy to insulate and easy to make air tight. 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..
Size and Shape Matter
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, helping to reduce costs. The savings from building a 10% smaller home can help pay for the additional cost of a Zero Energy Home without sacrificing quality or livability. Smaller homes can be designed to look, feel, and live larger while also having ample storage space, and 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 simple clean lines and minimal “corners” 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 smaller home can be designed to look large, spacious and comfortable. Features, such as extra storage spaces within otherwise poorly utilized spaces in a home, or 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.
Design to Use the Sun
Zero Energy Homes should be designed to use the sun’s energy as much as possible, for: 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. 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 the specific climate and location, the total area of southern facing windows in the home should be determined to optimize passive heating of the home. The home’s overhangs, 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 and 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.
Once passive solar design considerations have been taken into account, the home design should optimize the active solar components, such as the solar hot water and solar photovoltaic systems. Adequate roof area should be planned for the solar panels, solar installation codes, and the associated fire department access codes. Since roof pitch affects the efficiency of the solar PV and hot water panels, it should be optimized within the design constraints of the home. The more effectively passive solar features, air sealing, insulation, and other energy efficiency measures can be designed into the home, the less energy the home will need to receive from the more costly active solar systems, such as large PV systems.
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 goals. 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. Depending on the local climate, the use of dense-pack fiberglass or cellulose insulation may require, 8″ to 12″ thick, double walls with off-set studs 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 and airtight building assemblies such as these. Designing assemblies that are both breathable and airtight prevents moisture related challenges.
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, 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 can be used in place of less efficient, leaky bathroom and kitchen venting systems, further reducing penetrations. The air barrier should 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 for 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 or from Green Building Advisor.. The thermal barrier should be explicitly detailed on the home design.
Keep Ducts Inside
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 conditioned areas such as a basement or crawl space, unvented conditioned attic, open-web floor trusses (especially in a two story home), soffits, dropped ceilings, or 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 very important 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 northern 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 because the frames have a lower u-value than the glass area of the window. Casement and fixed windows are less leaky than sliders or hung windows. Windows and doorways should be located to reduce exposure to prevailing winds, reducing heating load requirements and weather infiltration. If doors are exposed to prevailing winds, they 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 much 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 inches on center. Headers are sized for actual loads so that unnecessary framing members are eliminated, saving cost and resources. Using engineered lumber and recycled lumber production bi-products and allowing for longer spans compared to standard lumber products is recommended.
Heating, Cooling, Ventilation and Hot Water
If heat-pump ductless mini-splits are used for heating and cooling, airflow to all the rooms needs to be considered during the design phase. Utilizing door cuts under the bedroom doors, transoms over some of the doors, or sound insulated air vents in the walls could be part of the design. Preferably, energy efficient ventilation systems (ERVs and HRVs) may be used to optimize airflow in the home. Specifying models of ERV/HRVs that have a circulation mode that actively circulates air from the heat source in the living area to the bedrooms can bring both heat and cool air to all rooms without needing any additional venting, and ensures that the heating requirements of the building code are met. In cold climates, heat-pump mini-splits that provide heat down to minus 18 degrees below zero should be specified, such as the Mitsubishi “Hyper Heat” model. Care should be taken that the heating system is sized properly for the local climate and home dimensions and characteristics. For greater efficiency the HRV/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. The location of the HRV/ERV should be part of the design and should be designed to accommodate the specific model chosen, as size varies considerably.
The hot water tank needs to be centrally placed to bathrooms and kitchen to minimize hot water runs, or consider a half-loop hot water recirculation system. Energy modeling should be conducted to determine which hot water system is most cost effective: Solar Thermal, Heat Pump Hot Water Tanks, or a well-insulated standard electric water heater with electricity supplied with additional solar panels. Once the most cost effective hot water system has been determined, the design needs to include adequate space for the system selected.
Designing For Builders
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. All such building strategies for a Zero Energy Home should be clearly outlined and specified on the design and in accompanying notes. When an architect or designer clearly outlines all the details required to make a Zero Energy Home, he or she makes it possible for any experienced builder and subcontractor to successfully build an affordable Zero Net Energy home. To ensure the efficacy of a Zero Net Energy Home design, the designer should be familiar with all 12 steps of the Zero Energy Home building process.
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