Show Me Some L OVE Advanced Framing/Optimum Value Engineering (OVE)
Paris A. Tabor P.E.
The topic of green building is everywhere. Mostly, architects have led the discussion with some inviting the project team to collaborate with them.
Structural engineers can also lead the green conversation, contributing significantly to their projects. This month's article introduces advanced framing, a method to promote green building.
Advanced framing (developed in the 1970s by NAHB and HUD), sometimes called optimum value framing (OVE), refers to a variety of techniques designed to cut back on the amount of lumber used by reducing structural redundancies and waste generated in the construction of a wood-framed house as well as improving energy efficiency and acoustical performance.
Combining the following principles into the building design would be a big step towards achieving an efficient assembly and ultimately achieving OVE:
- 24'' o/c framing: Beginning with the principle of "direct alignment" or "in-line framing," the roof trusses (including girder trusses) will have to align with the second floor wall studs, which will align with the first floor wall studs.
- Exterior walls: 2x4 versus 2x6: Using 2x6 in lieu of 2x4 assembly will permit higher R-value insulation to be installed within the wall cavity.
- Modular layout: Architectural design (i.e., face of sheathing dimensions) as well as establishing roof overhang depth along the roof pitch/plane based upon a 24'' module.
- Window/door header jack studs/trimmers/cripples eliminated: Use fasteners (nails or hangers) in lieu of door/window trimmer at openings. In addition, eliminate cripple stud trimmers at windowsill.
- Window/door openings layout: Position window/door openings to align with 24'' modules, i.e., door/window king stud align with 24'' stud layout. In addition, utilize window manufacturers that have 22.5'', 46.5'', 70.5'', etc., rough-opening requirements.
- Single wall top plate - exterior and bearing walls: Based upon the "direct alignment" principle, top plates will not experience mid-span loading, thereby reducing assembly requirements (all top chord breaks will require metal strap installation).
- Single wall top plate - interior nonbearing walls: Any nonbearing partitions can be built with a single top plate.
- Correct-size headers: Sizing all roof load-bearing headers for actual anticipated load and eliminate all headers at interior nonbearing walls and use 2x stick-framed headers.
- Floor rim joist header: In lieu of door/window header installation to support the assembly above, utilize the continuous floor rim joist capacity. At higher loading conditions, it will require double rim joist - floor joist hangers may be required over "header" length.
- Framing practices: Use ladder blocking or 2x6 backing nailer in lieu of "Ts," "open corner" framing, drywall clips at interior corners and top plate (eliminating backing). Utilize "blueboard" at the exterior nonshear wall lengths rather than OSB/plywood sheathing when doing "full-building wraps."
With the above practices in place, you should achieve many advantages, including:
Energy Efficiency
With the wider stud spacing, heat loss (conductivity or thermal bridging) through the studs is reduced, and a greater percentage of insulation can be installed within the wall cavity.
With floor rim headers, wall insulation may be placed directly over door/window headers thereby increasing the overall wall assembly R-value.
Potential annual heating and cooling cost savings of up to 5%.
2x4 versus 2x6
Using 2x6 in lieu of 2x4 wall assembly will permit higher R-value insulation to be installed within the wall cavity.
2x6 and 24'' o/c versus 2x4 and 16'' will equate to similar lumber unit project requirements.
Lumber Material and Waste Reduction
Average material cost savings of $1,000 for a 2,400-square-foot house.
Labor Cost Savings
Fewer studs to carry, fewer to install, fewer to cut, fewer to nail and less waste to manage/haul away.
Anticipated labor cost savings of between 3% to 5%.
Optimum value framing (OVE) requires advanced planning and detailing to anticipate all aspects of the construction assembly; consequently, framers unfamiliar with the techniques may need additional training or consultation to become accustomed to a different way to lay out and construct advance framing projects.
Some industry professionals have adopted OVE in a piecemeal manner rather than incorporating all of it at once, as they were more comfortable doing so. Some builders have implemented this piece by piece over many homes, with plans to incorporate more, rather than piling on all the changes in at one time.
Whether you decide to implement part or all of it at once, it is a step forward in the right ... green ... direction.
May 2008 Builder Architect Edition Issue