Near-Zero-Energy Design Approach
Our design approach is to create a super-insulated building shell that makes use of passive solar gain to lower space-heating demands, allowing the cost and complexity of the mechanical systems to be minimized. The metric used to determine the target level of energy performance for a building is the German Passive House standard, which represents a 90% improvement on the home’s space-heating loads from code-compliant construction. The significant financial savings resulting from minimizing a building’s heating systems can be reinvested in the building-shell improvements, including walls at R50, foundation at R70, roof at R80, and triple-glazed R8 windows and doors. The combination of these improvements, in conjunction with heat recovery ventilation, results in a building with a very small energy demand for space heating. Furthermore, due to the minimized heat load, a solar electric system and a separate evacuated-tube solar thermal system can cover the building’s space heating and hot water needs, resulting in a grid-tied, net-zero-energy building.
Our version of high-performance and cost-effective construction relies on a super-insulated slab-on-grade, hybrid structural insulated panel (SIP) walls, a truss roof, triple-glazed German windows, and ventilation with heat recovery.
We build atop a slab, and not a full-height basement for several reasons. Most importantly, a slab-on-grade foundation is the most cost-effective way of providing both high levels of insulation and thermal mass on the interior. To ensure that the energy from passive solar gain is modulated and stored, and does not result in overheating, it is critical to have a large thermal mass area exposed to the solar gains. In many cases a slab-on-grade requires little excavation work and can accommodate challenging site conditions, such as ledge.
Our wall section is made up of a 2 x 4 load-bearing stud wall insulated with blown-in dense-pack cellulose with 8-inch-thick SIPs (a sandwich of oriented strand board and expanded polystyrene insulation) wrapping the exterior. The R-value of this assembly is about R50, with virtually no thermal bridging. Additionally, this wall is easy to air seal properly. This approach also enables all of the mechanical, electrical, and plumbing systems to be run through the 2 x 64 wall conventionally, as opposed to creating chases in the SIPs. Finally, the SIPs are factory cut so there is less site waste and reduced labor for installation.
We typically like to use some sort of truss for our roof construction. The benefit of raised-heel trusses is that they are a very cost-effective structural system and are easy to install. We blow 24 inches of cellulose into the truss cavity. In addition to the insulation, we install a layer of ZIP sheathing on the underside of the trusses and tape the seams. This layer provides an air barrier above the conditioned living space below. We also find that the air barrier is effective in controlling moisture transfer through the roof assembly, but we install eave and gable end vents as well to reduce the potential of condensation on the underside of the roof deck.
The beauty of this type of design and construction is that with the correct orientation, the house needs very little space heating or cooling to be comfortable year-round. The super insulation combined with the thermal mass and solar gain results in temperatures between 62 and 75 degrees in winter with no heating input. As a result the main mechanical system for the house is not the heating system, but the ventilation system, which ensures a supply of tempered fresh air throughout the house. The only heating system for these homes is a few feet of electric resistance baseboard, which warms the house from 62 to 70 degrees for about $200/year. Because there will be little or no air leaking, a ventilation system is required to ensure indoor air quality and to control moisture levels inside the building. With such a tight building shell, the ability to continually flush the building with fresh air through the ventilation system is improved because there are no leaks to disrupt or short-circuit the designed air flows. Through the heat recovery ventilation, fresh air is circulated into the bedrooms and living spaces, and exhaust from where moisture and odors are created most—in the bathrooms and kitchen.
Don’t Sacrifice Aesthetics
Even when working within the constraints of a tight budget, we still acknowledge the value of creating inviting and attractive spaces. When funds are limited we focus the available resources on materials that are durable and that will age gracefully over time. In particular, we allocate larger portions of the budget to finishes and materials that will endure the most use, such as flooring, countertops, and bathroom fixtures and materials.
The intent is to keep the interior clean and simple to maintain focus on the surrounding landscape, but to also introduce a few beautiful, natural accents in the interior.
That idea is reflected on the exterior as well. We typically select natural cedar shingles for the walls that will weather to a silver grey and evoke the New England vernacular. Other siding options are also available, such as fiber cement siding, wood clapboards, or steel. Instead of asphalt shingles on the roof, we install a painted standing seam metal roof that is both durable and long-lasting.
Excerpt from an article written by Matthew O’Malia for Fine Homebuilding, January 2013