Re = the equivalent resistance of the layer containing the insulation and studs

= Rinsulation x Fc


Fc = the correction factor from Table 9.2

The use of the above multipliers in wall U-factor calculation is demonstrated in Figure 9.6.

Notice that only one path is calculated through the assembly, rather than two. The insulation layer is simply corrected by the ASHRAE multiplier and the calculation is complete. Also notice that it is only the metal stud/insulation layer that is corrected, not the entire assembly. This does not mean that only this layer is affected by the metal studs, but rather that this is the approach ASHRAE Standard 90.1 intends for the factors to give results consistent with tested performance.

The above example shows that the presence of the metal stud has increased the wall heat loss by 75%! The impact is even more severe for R-19 walls. The importance of accounting for the impact of metal studs in envelope U-factor calculations cannot be overstated.

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