ASHRAE - American Society of Heating@ Refrigerating and Air-Conditioning Engineers@ Inc.
Scope
ABSTRACT 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.