Currently, most wall R-value calculation procedures are based on experience with conventional wood frame construction, and they do not factor in all the effects of additional structural members at windows, doors, and exterior wall corners. Thus they tend to overestimate the actual field thermal performance of the whole-wall system.5
Clear-wall R-value (Rcw) accounts for the exterior wall area that contains only insulation and the necessary framing materials for a clear section, with no windows, doors, corners, or connections between other envelope elements, such as roofs and foundations.
Center-of-cavity R-value (Rcc) is the R-value estimation at the point in the wall that contains the most insulation. This uses a 0 percent framing factor and does not account for any of the thermal short circuits that exist through the framing.
Whole-wall R-value (Rww) is an R-value estimation for the whole opaque wall, including the thermal performance of both the clear-wall area and typical interface details such as all typical envelope interface details [e. g., wall-wall (corners), wall-roof, wall-floor, wall-door, and wall – window connections]. The whole-wall R-value is a better criterion than the clear-wall R-value, and much better than the center-of-cavity R-value methods used to compare most types of wall systems. The value includes the effect of the wall interface details used to connect the wall to other walls, windows, doors, ceilings, and foundations. Taking into account the interface details can have an impact on as much as 50 percent of the overall wall area. For some conventional wall systems, the whole – wall R-value is as much as 40 percent less than the clear-wall value.5
With the increasing use of alternatives to dimensional lumber-based systems (such as metal-frame and masonry systems) for residential construction, this procedure highlights the importance of using interface details that minimize thermal shorts. Local heat loss through some wall interface details may be double that estimated by simplified design calculation procedures that focus only on the clear wall.
The effect of extensive thermal shorts on performance is not reflected accurately in commonly used simplified energy calculations that are the current bases for consumer wall thermal comparisons. Consequently, the marketplace does not currently account for the thermal shorts that exist in building walls. Computer software for modeling is available, but is beyond the scope of this text.