Confusion over dew point calculation methods

by Erik Missio | September 11, 2017 3:49 pm

After the publication of the article, “Illustrating the Dew Point: An Effective Way to Determine its Value”[1] by Anthony Katona, CDT, in the July 2017 issue of The Construction Specifier, we received a letter from CSI member Darv Dombach, RA, CSI, CDT, LEED AP.

It read:

I believe the author of this article has largely drawn on an article entitled “Calculating Temperature Gradients” by Joan P. Crowe, which was published in Professional Roofing in February 2005. Consequently, he has referenced several outdated standards, including one that affects the whole premise of the article.

His third paragraph states, “According to NRCA, dew point calculations are necessary whenever the outside average January temperature is below 4 C.” This is a reference to the fifth edition, which was last updated in 2006.

The current 2015 edition of The NRCA Roofing Manual: Architectural Metal Flashing, Condensation and Air Leakage Control, and Reroofing states on page 199: “Currently, there are not consensus or widely accepted guidelines for determining whether a specific low-slope roof assembly should include a vapor retarder in its design.”

The same chapter explains that vapor retarder considerations are much more complex than a simple dew point calculation.

For clarification, we reached out to the article’s author. He offers this response:

Thank you for bringing your concerns to my attention.

This article was written in good faith and was not intended to be the last word on dew point temperatures, vapor retarders, or psychometric charts—or invent mathematical equations. My primary purpose was to demonstrate one way to graphically illustrate the dew point temperature of a very specific roofing assembly by using step-by-step mathematical calculations on the above-deck insulated, low-slope built-up roof (BUR).

The information contained in my article was acquired and gathered using multiple current resources and references, including data taken from other reference materials and more than 30 years of professional industry experience. However, the letter-writer is correct—my article references the fifth edition of the NRCA Roofing Manual, which I now realize is outdated.

Moreover, while the calculations used to determine the dew point temperature may appear somewhat similar to those used in the Crowe article, they are not the same. For example, my article uses current-day long-term thermal resistance (LTTR) values for various thicknesses of polyisocyanurate (polyiso) rigid insulation boards—these values were then used as the coefficients in the mathematical calculations, so to that degree, there may be some parallels. However, by breaking the procedure down into eight individual steps using totally different coefficients to arrive at the dew point temperature, I maintain the method does in fact make it easier to understand the basic principles of one way to determine the temperatures within the specific guidelines presented.

As the article recommends, it is important for readers to have any findings be confirmed using the methods advocated by NRCA. If performing the step-by-step computations determines the calculations suggest a vapor retarder, then other entities mentioned in the piece—the U.S. Army Corp of Engineers (USACE) Cold Regions Research and Engineering Laboratory (CRREL) and the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE)—could provide further support.

Again, thank you for highlighting your concerns, and I apologize for any lack of clarity.

Source URL: https://www.constructionspecifier.com/confusion-over-dew-point-calculation-methods/

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