The Envelope Corner: When Can You Average Roof Insulation?

Thursday, April 5, 2018

The 2015 Energy Conservation Construction Code, adopted and modified by New York State in 2016, increased requirements for the convective thermal insulation above roof decks by 50 percent for commercial buildings in Climate Zones 5 and 6  over the code that was in place just a few years ago. It makes more sense than ever to understand the Code requirements, so that money isn’t wasted, and all the benefits of being code compliant are gained.

 One important question that frequently comes up:  Can you average roof insulation values across a roof? This is an especially important question when tapered insulation is involved, so the thicknesses will increase significantly from the low point (usually the roof drains) to achieve adequate drainage. The answer depends on which compliance path you follow.

  • If you are using the Prescriptive R-value compliance path, then the answer is NO - the minimum R-values listed in Table C402.1.3 area are minimums, even at the edge of roof drains. Actually there is verbiage allowing averaging if the thicknesses vary by no more than one inch.  However, that’s not very useful, in practical terms.
  • The prescriptive U-factor compliance path DOES allow insulation averaging. This was recently clarified in a memo from the Department of State, since the wording in the Energy Code is rather unclear. However, it must be done correctly. If the average U-factor for the roof - calculated by area-weighted average – is less than the prescribed maximum U-factor in Table C402.1.4, then you’re compliant. And the U-factors can include the contribution of materials in the assembly other than insulation, including interior and exterior air films. 

Neither the Building Envelope Component Tradeoff method (COMcheck and REScheck), nor the Performance method (energy modeling) make sense  to use for most re-roofing projects, since other existing building elements would need to be considered.

We have found that many architects are not clear on how to show compliance for various configurations of roof insulation. There are formulas, tools, and charts that can be useful. Contact us if we can help.

- James A. D'Aloisio, P.E., SECB, LEED AP

Energy Code Training: 

Come join us at Klepper, Hahn & Hyatt on April 12th for Jim D’Aloisio’s seminar on “Conquering the Commercial Energy Code.” This is an in-depth course with continuing education credits. Learn more at https://urbangreencouncil.org/content/events/conquering-energy-code-architects-and-engineers-commercial-51

 Jim will also present this seminar in Albany on May 3rd and May 24th. Please see the schedule at https://urbangreencouncil.org/content/projects/energy-code-training-architects-and-engineers

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