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Heat loss through tapered roof insulation is generally computed using an R-value based on the average thickness of the tapered section. However, this misrepresents the actual performance of the tapered insulation, which is always less efficient than an equal volume of untapered insulation. For commonly encountered slopes formed with one-way and fourway tapers, the true efficiency of tapered insulation (compared with an equal volume of untapered insulation) depends only on the ratio of high- and low-point R-values, and ranges from 100% to about 70% for R-value ratios between 1 and 10. The impact of curved heat flow trajectories on the efficiency of the tapered forms is a function of taper angle or slope, and becomes significant only at slopes much steeper than those found in typical tapered roof insulation applications. Equations are derived, and tables are presented, for the efficiency of tapered insulation considering volumetric forms typically encountered in practice. Examples illustrate how these tools can be used to accurately calculate heat loss through a roof assembly with tapered insulation.

Citation: ASHRAE Transactions, Volume 117, Part 1, Las Vegas, NV