The wood elements that comprise conventional framing (studs, plates, posts, headers, and blocking for openings) fill an average of 27% or more of the wall cavity when framed 16” on center (According to the report prepared in 2002 by Enermodal Engineering for California Energy Commission). While the wood members provide strength, they also displace the insulation and thermally bridge the wall, undermining the rated R-value of a wall.
When considering the total environmental impact of constructing and operating a building, most of the environmental impact comes from the on-going operations of the building. (Ochsendorf & Norford, 2010) Design inefficiencies can have a big impact on the annual operating costs, both environmentally and financially over all the years of the building’s operation. One of the design weaknesses in the traditional Western stud-based building is the significant thermal inefficiency that results from the studs themselves.
Two aspects of stud-based designs can significantly degrade the anticipated thermal performance of a stud-built wall insulated with rolls of typical fiberglass batting – (1) thermal bridging and (2) incomplete packing of the batting. Thermal bridging results because wood studs are nearly three times as thermally conducting as the typical insulation used in the wall cavity. Load bearing studs are generally placed a maximum of 16” apart. Additional studs or posts are then added at every intersection of two walls and on both sides of every window or door opening. Beams are also added as headers over doors and windows. The result is that thermally conducting wood, not insulation, comprises on average 27% of a standard framed wall (According to the report prepared in 2002 by Enermodal Engineering for the California Energy Commission). While we think of a wall with R-19 insulation performing per R-19 specifications, unfortunately, the presence of the wood based thermal bridges substantially reduces the effective R-value.
Other solutions are sometimes attempted to avoid the thermal bridging problem, including the placement of a continuous layer of insulation outside the structural wall. The improvement in the insulation performance by adding additional insulation is offset by having a less rigid surface close to the exterior surface, where surface rigidity is needed for siding, masonry, or fascia. In some designs, when exterior slabs of insulating foam are used on the exterior, the wall cavity is left without insulation to dampen the sound coming from the plumbing and drain lines.
Another aspect of the BamCore Wall System that will help improve the insulation of a wall results from the ability to complete a solid blow-in fill of the entire wall cavity. Typically, the batting of fiberglass or other insulation will not completely and continuously pack the wall void between studs, leaving many small, and sometimes large, uninsulated voids along every wood framing element. When insulation is blown into the BamCore Wall System, the insulation forms a more continuous body to completely fill the wall void.
The figure below shows an 8’ wide BamCore Prime Wall section with a window rough opening and rough electrical already installed. Plexiglas is used on one side of the wall to allow visual inspection of the insulation fill. As can be seen, the entire wall void is nearly uniformly packed with the blown insulation.
One of the many cost-advantages of the BamCore Prime Wall System is that the wall thickness can be set anywhere between a 2×4 to a 2×12 equivalent for nearly the same cost. Making the wall thicker allows for more blown-in insulation thereby improving the building’s thermal envelope as depicted in the graph below.
As explained above a traditionally framed wall will never achieve the full R-value of the insulation used in the voids due to the thermal bridging. The graph immediately below illustrates the nominal and adjusted R-values of a range of wall thicknesses for both a wood framed wall with a framing factor of 27% and a BamCore Prime Wall with a framing factor of 2.46%.