The structural insulated wall panel systems (SIPs) have many advantages when creating a well-insulated and air-sealed building shell that is cost effective to produce. The panels we chose to use on the prototype are 6-inch thick urethane panels, that are 4 feet wide by 24 feet long, factory manufactured and pre-cut by Winterpanel in Vermont. The most significant advantage of the SIPs is that they provide an uninterrupted thermal barrier for the shell that is also a durable and easily sealed air barrier.

The Passive House requirements for a building’s shell are very specific. To qualify for Passive House Certification, a building should not have any thermal bridges in the foundation or building shell, and the building, when complete, must meet strict air sealing requirements verified by a blower door test. The SIPs system has allowed the prototype’s construction to conform to these Passive House requirements, while still allowing for simple and cost effective construction.

The second advantage of the SIPs system has to do with the size of the panels and the ability to have them factory pre-cut to fit each building design. These benefits are maximized by utilizing our computer designs in the actual production of the construction components. By utilizing advanced three-dimensional computer models, we are able to coordinate all the expensive building shell components (including windows, SIPs and structural frame), and then incorporate the computer’s accuracy in the actual construction process. By putting emphasis on planning and leveraging that in the construction, we can improve the speed, accuracy, and quality of the site work.

One frequently asked question regarding SIPs panels is the environmental impact of the foam insulation used in the panels. The three standard foam types used in SIPs panels are urethane foam, and Expanded Poly Styrene (EPS). We choose to use urethane foam for the prototype’s panels because it has a higher R-value per inch, resulting in a thinner panel and a higher total R-value per unit cost. The criticism of this foam type is that the R-value decreases over time. The total aged R-value of the urethane, however, is still higher than the comparable EPS R-value per unit cost. While both foam options are considered green products according to the USGBC, it has been suggested that the EPS production is more environmentally friendly of the two foams. G•O Logic has used both foam types and considers their application on a case-by-case basis.

With the grade beam footings complete, the next step in the construction process is the erection of the hybrid timber frame.

The concept behind the structural timber frame system was to create a structural system that is simple, quick to install, and easy to replicate. The structural concept also required a separation of the load bearing structural system from the thermal shell of the building, which enables greater flexibility in configuring the facades as well as reducing the potential for thermal bridges in the shell.

The thermal envelope of the building is created using SIPs (structural insulated panels) that are as large as 24 feet long by 4 feet wide and factory cut to fit the structural frame and façade. The benefit of the size of these structural panels is that they only need to be attached to the structural frame at the floors and roof, resulting in a reduced quantity of framing materials and fasteners. The use of SIPs in conjunction with the hybrid timber frame also results in a building shell that is easy to air seal and has virtually no thermal bridges.

The timbers that we choose for the structural frame at the exposed locations are locally harvested white pine, with some standard dimensional framing where the structure is not exposed. To reduce the cost of the frame, we worked closely with our structural engineer Albert Putnam to simplify the basic design and the structural connections. The details for the frame connections have been reduced to the most basic form, including butt joints, hidden metal straps, and hidden lag screws at the connections. We reduced the number of framing members to save costs, incorporating the minimum members to create a stable structure. The timber frame also relies on the SIPs panels for its lateral resistance, thereby avoiding expensive let-in bracing in the actual timber frame.

The roof structure of the building was created with a scissor truss spanning the north to south walls. The scissor truss was chosen because it is cost effective to build and install. The scissor truss also allows for the easy installation of a thick layer of blown-in insulation in the web area of the truss, easily accommodating an average of 24 inches of cellulose. The scissor truss also allows for spatial flexibility on the 2nd floor with the opportunity to create an insulated attic area, loft space of vaulted ceiling.