Just because a wall has R-19 insulation in it does not mean it's an R-19 wall. Using the R-value of the insulation between the studs (the "cavity R-value") as an overall wall R-value is similar to using the center-of-glass value for a window--it ignores the effect of framing.

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) recommends using a "framing factor" when calculating the overall wall R-value for standard wood frame walls. The framing factor is the percentage of the gross wall area that is made up of framing material. The R-value through the cavity insulation and the framing is calculated separately and then combined using the framing factor to find overall wall R-value.

Determining the correct framing factor to use, however, is not easy, because a typical wood wall has more than just studs in it. Other framing elements include:

• Top and bottom plates, with the top usually double
• Jack studs under window sills and above headers
• Cripples and trimmers at the sides of doors and windows
• Solid wood headers
• Fireblocking
• Three- and four-stud corners

Many of the details of residential wood wall framing are typically left up to the framers in the field. Choice of the headers, fireblocking, how many extra studs are used to locate door and window frames, and the number of studs used in corners are therefore not shown on the plans, but can have a big effect on a wall's framing factor. Because framers are more concerned with structural than energy issues, they tend to err in the direction of using more wood. (No one ever failed a framing inspection for having too much wood in a wall.)

Until 1993 the ASHRAE Handbook of Fundamentals recommended a framing factor of 15% for walls with studs 16 inches on center (OC) and 12% for those with studs 24 inches OC. These values have been generally accepted throughout the building community and have been incorporated or referenced by many codes including California's Title 24 and ASHRAE Standards 90.1 and 90.2. However, these factors are based on a very simple wall and account for just the studs and top and bottom plate. This ignores the effect of headers, corners, and wood around doors and windows. In 1993, ASHRAE revised their recommendation upward to 25% for studs 16 inches OC and 22% for studs 24 inches OC.

According to Bill Stezpeck, a member of the ASHRAE technical committee responsible for the new recommendations, the revised numbers were based on a review of a small sample of actual building plans. The committee considered these new values to be conservative estimates until further research could be done.

In 1993, Davis Energy Group (DEG) investigated the effect of framing, water and waste piping, and wiring on the overall wall R-value of six new residential wood-framed houses in the Sacramento, California, area. The area of framing and gross wall were measured at each site before drywall or sheathing was applied, and the effect of piping and wiring on insulation installation was estimated by direct observation. DEG found net wall framing factors averaging 32% (all houses were framed 16 inches OC), and estimated a 3% reduction in overall wall R-value due to piping and wiring.

Dariush Arasteh of the Lawrence Berkeley Laboratory proposes development of a basic index for wall systems that can be used when comparing the relative performance of different wall systems. It would most likely use a representative wall section which includes construction details such as windows, doors, and corners with an area proportional to their occurrence in actual construction.

Andre Desjarlais and Jan Kosny of Oak Ridge National Laboratory are examining the influence of architectural details such as wall/roof, wall/ floor, corner, and window and door perimeter on overall wall thermal performance. They estimate that only 29%-63% of overall wall heat loss is through clear wall area (areas of a framed wall that are not influenced by other features such as window and door framing, corners, and so on).

With the increased interest in steel framing, its effect on energy efficiency is under scrutiny (see "Studs of Steel," HE July/Aug '94, p.9). While an increase in assumed framing factor from 15% to 25% can reduce the calculated overall R-value of a wood-frame wall by more than 10%, the effect on steel is even more dramatic (see Figure 1). Because of the much higher conductivity of steel studs the same increase in assumed framing factor in a steel wall results in almost a 30% reduction in calculated overall R-value.

Figure 1.Effect of framing factor on R-value.

The thermal performance of conventional residential wall construction can be improved using such methods as:

• Replacing solid wood headers with insulated headers. These can be purchased pre-fabricated or built on site.
• Using two-stud corners in conjunction with drywall clips instead of adding studs just to provide drywall nailing.
• Adding insulating sheathing to provide better use of the insulation R-value, as it is not compromised by the thermal shorts of the framing.

In addition, new wood-framed wall construction methods can provide significantly improved R-values. These include:

• Stressed skin panels: These are 4-6 inch thick panels of polystyrene or polyurethane foam sandwiched on both sides by oriented strand board or plywood. The only thermal break in the insulation is where the four-foot-wide panels are joined and at window and door openings.
• Optimal value engineering: This is a design technique developed by the National Association of Home Builders that minimizes the amount of framing needed. It uses 24 inch OC studs, two-stud corners, a single top plate, and windows with widths in multiples of two feet--all openings are lined up with framing elements on at least one side.
• Engineered wall framing: This is a system developed by Davis Energy Group for the Pacific Gas and Electric Company's (PG&E) ACT2 project. It consists of narrower (1.25 inch thick) studs at 24 inches OC with rigid insulation panels between the studs. The 11.875 x 1.25 inch headers are placed on the inside of the studs above a 1.25-inch air space. This allows for full insulation thickness behind the header and provides an insulating air space behind the drywall in which to run wiring and plumbing without compromising the insulation. All the framing elements are made of a laminated strand lumber called TimberStrand, manufactured by Trus Joist MacMillan, which has high dimensional stability (it doesn't warp or shrink). This allows for more precise alignment of the framing elements and a better fit with the insulation.