This article was originally published in the March/April 1995 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online March/April 1995
Is an R-19 Wall
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:
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.
Improving Wall R-value
The thermal performance of conventional residential wall construction can be improved using such methods as:
In addition, new wood-framed wall construction methods can provide significantly improved R-values. These include:
Leo Rainer is a senior engineer
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