January 2010
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Unintended consequences | Tech Today

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Unintended consequences

Concern about moisture accumulation in reflective roof assemblies

by Mark S. Graham
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NRCA's Technical Services Section has received a limited number of reports of moisture accumulation in relatively new, highly reflective roof assemblies. Although the specific circumstances vary somewhat, a commonality is that the roof assemblies consist of nonadhered roof coverings usually over a single layer of high R-value insulation. The reported problems have occurred in relatively hot, dry climates, such as the Southwest, as well as cool, damp climates, such as northern portions of the Midwest.

There are likely causes of such moisture accumulation, and NRCA has some recommendations for minimizing or eliminating this phenomenon.

Self-drying roofs

Experience and limited research have shown that nonadhered membrane roof systems with highly reflective surfaces are more susceptible to moisture accumulation than roof systems without highly reflective surfaces. This phenomenon is most pronounced in roof systems with single layers of rigid board insulation, which results in "thermal shorts" at board joints and mechanical fasteners that penetrate through a roof...



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Comments (1) Login to post a comment or rating

Posted by pierrobill on 1/6/2010, User rating 
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Yes! In colder climates, all roof systems would perform better when both a vapor retarder and a 2 layer system is installed. This is for both low slope and steep slope, and especially true on steep slope with shingles. Thank you

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