WestCon Tribune

October Review

Air Infiltration. The Nemisis of Condensation Control and Underlayment Upgrades That Sometimes Aren’t

Presented by Phil Dregger, Technical Roof Services

At the October Westcon meeting, Phil Dregger, a registered engineer and roof consultant and President of Technical Roof Services, gave a thorough and well organized presentation on air infiltration and condensation.

Following is an excerpt from an article by Mr. Dregger published in Western Roofing  on Underlayment Upgrades which were the basis of his presentation:

Self-adhering modified bitumen membranes are often installed as continuous waterproofing layers below steep roof systems to enhance weather protection. Sometimes, however, these “bullet-proof” roofs develop an unexpected problem, condensation. This article explains why and offers some suggestions on how to enhance weather protection and avoid problems,

This author wishes to specifically acknowledge that self-adhering “ice and water” type waterproofing membranes for steep roofing, in his opinion, are useful in the market place, and has personally specified their installation as part of steep roofing systems like copper, slate, clay/concrete tile, and asphalt shingles. These products provide excellent waterproofing and even possess a sort of “self-sealing” characteristic around fastener penetrations. They are especially useful to protect against ice dam conditions along eaves and to help achieve weather protection at penetrations, transitions and terminations where rigid materials are inherently difficult to make weathertight on a long term basis.

A problem can be created when these products are  installed without thinking through how they might effect the potential for condensation. Although self-adhering modified bitumen sheet type products are waterproof and quite flexible, they are virtually impermeable to moisture vapor transmission. The manufacturer of one of the ”ice and water” type products publishes a “maximum” moisture vapor transmission value of about 0.05 perms. In contrast, “15-lb.asphalt felt” underlayment is listed in the ASHRAE Fundamentals manual as having a moisture vapor transmission rate (permeance) of about 1.0 perms (dry cup method), or about 20 times more vapor permeable than the “ice and water” type product. Materials with perm ratings below about 1.0 or 0.5 perms are usually considered vapor retarders.

It is important to keep in mind that recommendations included in well known manuals such as Copper & Common Sense by Revere Copper Products, Inc., and Slate Roofs  by Vermont Structural Slate Co., Inc., and Architectural Sheet Metal Manual by Sheet Metal and Air Conditioning Contractors National Association, Inc. discuss conventional felt underlayments but are generally silent in regards to “ice and water” type waterproof membranes such as these, except along eaves with potential ice dam conditions.

Shake Reroof. Recently, during an investigation of “leaks” reported the first winter after a home in the California Bay Area was reroofed with concrete tile, this author observed literally thousands of water droplets covering the underside of the roof deck in the attic. The owner reported no previous leaks, condensation or otherwise, from the old wood shake roof installed over spaced sheathing. The roofing contractor reported that he had installed new plywood throughout and had even “upgraded” the underlayment from a conventional #30 felt to a self adhered modified bitumen membrane. And, was therefore convinced that the leaks were not his problem. Although attic venting was about 50% short of the code required minimum, the old wood shake roof installed over spaced sheathing, had been just air and vapor permeable enough to avoid noticeable condensation until now. Unfortunately, installation of the new plywood sheathing and  waterproof membrane greatly increased the condensation producing conditions by virtually eliminating moisture vapor migration and the “supplemental” attic air exchange previously occurring directly through the wood shake roof.

It must be remembered that “ice and water” type underlayment membranes are excellent vapor retarders. And, as vapor retarders, if they are installed on the cold (exterior) side of the ceiling insulation rather than on the warm (interior) side, this can sometimes lead to problem conditions.

Other example projects where this author has observed very noticeable condensation conditions created by or at least contributed to by the installation of self-adhering “ice and water” type waterproofing membranes include several non-vented cathedral ceiling assemblies and dome/barrel shaped roofs.

If you would like a copy of the article in its entirety, please call Rikki at (415) 451-4897. Mr. Dregger has been asked to publish another article which is due out this summer. We hope to make that available to Westcon members as well.