Minimizing Heat Loss through Tapered Insulation

Jonathan Ochshorn

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tapered insulation image courtesy of Dow Building Solutions

Tapered panels built up over flat panels create a four-way slope with internal drainage. Image courtesy Dow Building Solutions.


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 four-way 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 percent for R-value ratios between 1 and 10.

Link to CSI article is here.

Online tapered insulation R-value calculator is here.

Original, more detailed, paper published in ASHRAE Transactions (Volume 117, Part 1) is here.