Figure 1

Part of any credible procedure for estimating building heat loss involves determining the total R-values for building panels (e.g., walls, ceilings or floors) that separate heated from unheated space. This is usually done by looking up the R-values of the materials that make up the exposed panel.

Once the R-value of each material is found, a simplistic approach is to add all the R-values together to get the total R-value of the surface. Included in this total are the R-values of all the materials, as well as the inside and outside air films. The latter are the thermal resistances of two thin layers of air that cling to the inside and outside of the panel.

Although it's easy to just add R-values together, the result tends to overrate the actual R-value of a wall that contains framing members such as studs and joists. This happens because the thermal resistance of the wood studs or joists is often significantly lower than that of the insulation in the adjacent cavities. The wooden framing members form a path through which heat can move faster than through the surrounding insulation.

Figure 1 shows a computer simulation of the effects of the wood stud in an otherwise sandwiched construction of drywall, fiberglass insulation and exterior plywood. Notice how the isotherms (lines of constant temperature) are “warped” near the stud.

This indicates that heat is not traveling straight out through the wall as it does at the halfway point between studs (e.g., near the edges of the construction shown). Instead, the better-conducting stud forms a “bridge” across which heat is finding a partial detour around the higher- resistant materials.

The increased rate of heat transfer through the studs causes the inside surface temperature of the panel to be lower at the stud or joist locations. This is indicated by the inward curvature of the isotherm near the inside wall surface.

Figure 2

This condition is worsened by steel studs or purlins that span through the thickness of the wall. A classic example is a truck garage with metal framing and interior cladding. In a cold climate, it doesn't take long before streaks from condensing diesel exhaust appear on light colored interior metal surfaces.

On a smaller level, some of you have also seen how nail heads or screw heads can discolor the inside surface of an exposed wall or ceiling that's poorly insulated. Like a stud, the metal nail or screw forms a high conductance path, and thus lowers the temperature of the inside surface, in some cases enough to cause condensation of water vapor in the surrounding air.

Notice how the isotherm near the outside of the stud is also curved outward. This indicates a higher outside surface temperature near the stud. This higher temperature is what makes framing areas “glow” in an infrared thermogram of the wall as shown in Figure 2.

Making Adjustments

Formula 1 can be used to find the effective total R-value of the panel:

Where:
Reffective = effective total R-value of the panel (degrees F
• ft2
• hr./Btu)
  pf = percentage of panel occupied by framing (decimal percent)
  Rf= R-value of a panel section through the framing (degrees F
• ft2
• hr./Btu)
  Ri = R-value of a panel section through the insulation cavities (degrees F
• ft2
• hr./Btu)

Figure 3

Typical wood-framed residential and light commercial construction have from 10 percent to 20 percent of the wall area as solid framing across the insulation cavity. The lower end of this range is appropriate for 24 inches on-center framing, with insulated headers over windows and doors. The upper end is typical for 16 inches on-center framing with solid headers, and complex wall shapes that often necessitate extra studs.

Here's an example of how to use Formula 1 using the wall construction shown in Figure 3.

The R-values of the individual building materials (including the inside and outside air films) have been tallied both between the framing and at the framing. These values are then substituted into Formula 1, assuming 15 percent of the wall is solid framing:

Figure 4