Home Energy Magazine Online March/April 1997

Wall R-Values that Tell It Like It Is

by Jeffrey E. Christian and Jan Kosny

Jeffrey E. Christian is the manager of the DOE Building Envelope Systems and Materials Program at the OakRidge National Laboratory, Oak Ridge, Tennessee, and Jan Kosny is a research engineer at the University of Tennessee in Knoxville .

There's a lot more to most walls than meets the eye, and the R-value of a whole wall can be considerably lower than the R-value of the insulation that fills it. At DOE's Buildings Technology Center, scientists have developed a system for measuring whole-wall R-value, and have already tested several types of wall system.

How Wall R-Value Is Usually Calculated

Currently, most wall R-value calculation procedures are based on calculations developed for conventional wood frame construction, and they don't factor in all of 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.

In these common procedures, the user enters a framing factor (ratio of stud area to whole opaque exterior wall area). The framing factor is usually estimated, is seldom verified against actual site construction, and is frequently underestimated (see " Is an R-19 Wall Really R-19? " HE Mar/Apr '95, p. 5). Framing factors range from 15% to 40% of the opaque exterior wall area, yet lower values are commonly used. Unfortunately, the wall's energy efficiency is usually marketed solely by the misleading clear-wall R-value (Rcw).

Clear-wall R-value accounts for the exterior wall area that contains only insulation and necessary framing materials for a clear section. This means a section with no windows, doors, corners, or connections with roofs and foundations. Even worse is the center-of-cavity R-value, an R-value estimation at the point in the wall containing the most insulation. This converts to a 0% framing factor and does not account for any of the thermal short circuits through the framing.

The consequences of poorly selected connections between envelope components are severe. These interface details can affect more than half of the overall opaque wall area (see Figure 1 ). For some conventional wall systems, the whole-wall R-value (Rww) is as much as 40% less than the clear-wall value. Poor interface details may also cause excessive moisture condensation and lead to stains and dust markings on the interior finish, which reveal envelope thermal shorts in an unsightly manner. This moist surface area can encourage the growth of molds and mildews, leading to poor indoor air quality.

Metal-framed walls are particularly vulnerable to thermal shorts. Unfortunately, builders often attempt to solve metal wall problems by making thicker walls and adding more insulation in the cavity between the metal studs. In fact, the thicker walls have an even higher percentage difference between clear-wall and whole-wall R-value.

Measuring Whole-Wall R-values

To compare wall systems more accurately, we have developed a procedure for estimating the Rww for various system types and construction materials (see "Wall R-Value Terms" ). The methodology is based on laboratory measurements and simulations of heat flow in a variety of wood, metal, and masonry systems (see "How We Evaluate Wall Performance" ). The whole-wall R-value includes the thermal performance not only of the clear-wall area, with its insulation and structural elements, but also of typical envelope interface details. These details include wall/wall (corner), wall/roof, wall/floor, wall/door, and wall/window connections.