Many of the techniques show above where incorporated into the framing of the ELEMENT. However, in several situations, mainly due to the placement and direction of the exterior openings, these techniques could not be used so that the building could maintain its structural integrity. This brings to light an important subject: the integration of design and thermal performance of a building. As the chart above clearly demonstrates, using advanced framing techniques can greatly increase the thermal performance of the building. A more efficient building envelope will lead to a decrease in the energy load on the building and hence lower the energy consumption of the entire building.
To illustrate this point, an energy analysis program called the Passive House Planning Package (PHPP) was used to retool the framing (and thus the window / door placement) of the house. This scenario was set up to see the net effect of the façade design on the building’s performance. The building’s envelope already had a structural system comprised on 2 x 6 studs placed 24 inches on center. Therefore, any opening in the building’s envelope would now be placed on this existing grid.
The Current Framing Design An Advanced Framing Design
As one can clearly see from the sketch above, the current framing design uses a great number of studs in order to support the interrupted window placement. With the current framing, the wall in the lower left corner (north wall of the living module) is made up of 23.9% wood studs which act as thermal bridges. By removing the “panoramic windows” and replacing them with windows also placed at human eye height, but more importantly, within the designated framing grid, the wall using advanced framing techniques yields a 23% reduction in stud use and an 8.7% increase in the overall R-Value of the wall assembly. Another advanced framing technique not used in the current framing design would be the use of hangers instead of cripples to transfer roof loads around window and door openings. While the overall effect of this method is not as largely significant as window placement, it can still yield close to a 5% increase in insulation area in the building envelope. To fuller illustrate the importance of proper window placement, one can examine the effect of window trim. While windows have an overall negative effect on the buildings thermal performance, they are undoubtedly any important feature of any building. This being said, the window frames have absolutely no positive attributes when it comes to either energy performance or aesthetics. Thus, it is important to minimize the frame area when looking at the overall building envelope (this also makes the windows cheaper to purchase). Looking again to the sketch above, the framing in the upper left corner shows the current framing of the south wall of the living module. By replacing the three small windows with one large window and placing it in the framing grid, the wall experiences a 47.4% reduction in window frame area. As mentioned earlier, troubled spots for air infiltration and exfiltration occur mainly at windows, doors and corners, therefore, it is important to reduce the window framing area and likewise the chance for energy loss.
Corner Framing Detailing, Notice Window / Door Interruptions on Left SideAdvanced framing techniques produce several methods to deal with the effects of thermal bridging. Perhaps the easiest method, at least in terms of overall wall design and detailing, would be to add an extra layer of rigid insulation (commonly expanded polystyrene otherwise referred to as EPS) to the exterior sheathing of the wall assembly. This method is also quite useful in retro-fit projects because nothing has to be done to the wall assembly, the rigid insulation is added to the exterior after which stucco or siding can be installed.
Sketch of Heat Transfer Through Typically Framed Wall and Same Wall with XEPS Added
