The principal of sustainable building based on efficiency. The concepts of efficiency are paramount to the understanding the practice of building and the application of choosing framing types based on the most desirable outcomes. Regardless of the external environment the practice of building revolves around the acceptability of the interior environment of the building to the occupants and objects therein contained.
The pursuit of constructing energy efficient buildings requires the builder, owner or designer to weigh the options available to them to create a structure that is comfortable, energy efficient, economical to build and acceptable for the environment in which the structure is placed. Energy usage for the heating and cooling the building is the greatest energy cost for a single family home. The fabrication of exterior walls provides an opportunity for lowering energy cost. According to Dan Chiras, “Heating and cooling the interior of our homes consumes the largest portion of residential energy–about 44 percent” (Chiras, 2008).
The framing of a wall is most often the simplest type where the minimum insulation allowed by present building codes is utilized. This type of wall structure is of 2 X 6” wood construction with R-22 batt type insulation. Where a building is intended to be of greater operating efficiency there are several methods of fabricating the walls where the R-value of the wall far surpasses the code minimum. Building to a higher insulation level creates savings on heating and cooling cost over the life of the building can offset greater cost of fabrication when compared to the minimum code method.
These types of homes are termed Super-insulated homes. Advanced frameing methods for super-insulated homes are growing in number, resulting from new methods being developed and technology aiding in lowering cost. One method which has been under development is The SIPs (structural insulated panel) system. The development of Structural Insulated Panels (SIPs) began over 70 years ago at the United States Forest Products Laboratory in Wisconsin (B Ledford 2010). SIPs is a method where wall panels made with a foam insulation core are fabricated in a regional factory.
OSB (oriented strand board) or other material is applied on both faces creating a sandwich. Oriented Strand Board adhered to the foam becomes a structural unit as the sandwich creates a type of I-beam which is stronger than traditional wood framing, as shown in a UK study, than standard framing practice “Walls constructed of SIPs provide superior racking resistance to a comparable traditional stud wall designed to BS 5268 or EC5” (Kermani, Hairstans, 2006. p. 1811).
It has been a long standing conjecture that this method should save labour costs on site as the time required to erect the system should be less than a traditional 2” X 6” framed wall. “Compared to stick framing, SIPs are faster to erect in the field and also provide more strength to resist most loads; they are better with axial and transverse loads. ” (B. Ledford, 2010. p 2). In contrast, anecdotally one author/ architect has noted that in his experience SIPs cost 5-10% more than traditional framing “In my experience, using SIPs usually costs slightly more than stick framing, adding about 5 to 15 percent to the total cost of the home” (D.
Wright. p 57). The panels are delivered pre-sized in large sections and erected as a complete section of wall without the need for additional steps for sheathing, insulation and vapor barrier. There is however little direct comparisons which have been made in ease and speed of construction. It is rare that two similar enough structures have been studied side by each. The most notable of direct comparison is an informal study by Habitat for Humanity accomplished through building two structures similar to each other.
One of these structures was traditionally framed and the other was framed with SIPs. “Volunteers interviewed after framing the SIP home believed that SIPs reduced construction effort significantly, averaging about one-half the effort of conventional wood-frame construction. ” (Mullens, Arif, 2006 p 788). Other notable restrictions from framing in the SIPs method are the proximity of the SIPs factory to site. As the distance from the factory grows, the cost of shipping increases making other methods more desirable.
Where SIPs is not advantageous for the reason of shipping costs, modifications to traditional methods can be utilized. The “Larsen truss” is one such system whereby readily available materials are used to expand the insulated space in a wall. In this system the walls are fabricated in the traditional method of stick framing where the wall is build and sheeted to carry the loads of the structure and roof. Applied to the exterior of this framing is a truss which is build from 2 – 2X2 pine studs joined by gussets of Oriented Strand Board or plywood.
The trusses are then filled with insulation and sheeted again with OSB. This system was designed by John Larsen of Edmonton, Alberta in 1979 (Holladay, 2011. ). Larsen trusses have been built from 8. 5” to 12” in depth (M. Holladay, 2011), adding up to R-44 to the building in addition to the insulation in the structural framing. This would create an overall system wall R-value of up to R-66. Interestingly to note about this system is that the trusses can be added to existing building structure as part of an energy saving renovation. The Larsen truss has since been modified to a truss wall.
The significant difference with this system is that the truss is structural and there is only one layer of sheeting on the wall. The benefit to this modification is that a similar increase in the R-value “nominal R43” according to (Straube and Smegal, 2009. p 41) of the wall system with the addition of saving on material costs over the Larsen Truss system. This modified truss system is only achievable on new construction. Notable efficiencies with both the Wall truss system and the Larsen truss system is the reduced thermal bridging through the use of the OSB gussets.
Thermal Bridging is a result of conductive material that is in contact with the interior and the exterior of the building envelope. Thermal bridging is an important inclusion in the overall R-value of a wall system. Some of the newer forms of framing include double walls which are framed within the footprint of the foundation and on the framed floor in these systems the exterior course of framing carries the load of the structure and the inner framing is used to extend the cavity of the wall to accommodate the additional insulation. “The overall thickness of the double wall of 9. ” appear to be most common” (Straube, Smegal, 2009). The material used in insulating can vary, however it is most likely to be batt or blown in cellulose at R-3. 5 and R-3. 7 respectively. This results in a wall that is at 9. 5” approx. R-33 and R-35 respectively. (R-value with regards to whole wall and floor joist). The double wall system leaves the rim joist exposed to the effects of the cold exterior reintroducing thermal bridging and reduces the overall R-value to closer to R 30 (Straube, Smegal, 2009) for both insulation types.
While it is my assumption the use of an exterior finished to the foundation as in the case of a double wall reduces the complication of exterior finishing over both the Larsen Truss and Truss Wall and therefore reducing the building costs. Another system that addresses thermal bridging is EIF’s (Exterior Insulated Finishing system). EIFs was developed to provide efficiency at a low cost. This well insulated, well sealed building envelope reduces thermal bridging by the application of XPS foam to the exterior to which a “stucco” finish is applied.
In the early years of its application to buildings several, and severe, issues became apparent. Water infiltration, exposed the limitation of the system in different environmental conditions. The most notable is the application of this system in the Vancouver area of B. C. EIFs buildings in the Vancouver environment where rain falls constantly during winter months allowed water to penetrate the exterior and rot the wood framing resulting in water infiltration that leads to mould and the failure of the structure.
The cost of repair as cited by an article in The Toronto Star “Total repair costs came in …between $35,000 and $60,000 per unit” (Toronto Star. 2002). The lessons learned in the analysis of this application can be used to identify the acceptability of the environment for the building style and the completeness of the envelope design. EFIs can be very effective as a framing/insulating structure and provide a well protected and efficient building envelope. The proviso being that the environment is appropriate for the system and that the system must be installed and maintained correctly.
The system is built on standard 2 X 6 stud wall construction. The wall is then sheeted, insulated, air and vapor barriers applied. Where this system differs from the basic wall framing technique a layer of XPS (Expanded Polystyrene Foam) is applied on the exterior sheathing, joints are sealed and a coating of stucco is applied. The foam acts as an additional barrier to air movement as well as providing an additional R-value to the envelope creating an overall wall performance of R-30 (Straube, Smegal, 2009. p 60. The reduction of construction costs of a home is some of the most primary considerations when designing an energy efficient structure. Budget concerns in the planning stage can modify building methods in order to fit the construction budget. With costs of heating a super-insulated home 50-70% lower that of a traditional wood frame house, it is also important to assess the ongoing cost of heating and cooling in choosing options for increasing insulation to take advantage of the long term savings of a super-insulated home.