Hello Kevin, You pose an excellent question regarding the longevity of not only felt but other materials that we rely on but cannot see, once covered with finishes. I do not know the answer to your question. Perhaps manufacturers of these materials would be willing to share the results of any accelerated aging tests with the AIA CRAN.
As for roofing substrate protection, it has been my experience that this is one location where asphaltic felts, even 30# do not perform very well, for a variety of reasons, the most significant, in my opinion is the fact that after only a few days, in the presence of even medium winds, the felt tears, often significantly, and is not replace, but rather simply nailed back, in a tattered mess, just prior to receiving the actual roof shingles or other finish roofing. I like to regard the substrate membrane as the "real roof" that actually keeps out the leaks that do come through the shingles and other outer materials, as well as condensation. The best answer for roofing substrate protection in my opinion is something like a 60 mil peel and stick uncured polybutylene like Grace Ice and Water Shield or EcoStar Glacier Guard. Products like these seal around the thousands of nail and screw penetrations through it that comes from the shingle/roofing installation, providing a much more watertight assembly than mere asphaltic felt. The main job of the roof is to keep out actual water pouring down over it, which is why I believe this step is necessary in this part of the building envelope. Also, the underside of the roof sheathing along with its insulation can be ventilated from underneath with soffit vents and ridge vents, to dissipate any condensation.
Going back to the issue of asphaltic felt on walls, I would welcome any input regarding longevity from manufacturers! And here's a bit of my own input regarding the Zip system we all have been seeing much of during the last decade: if you can find some of the older warranties for that system, when it first began, the manufacturer of the OSB panels specifically did Not guarantee the tape (I know; I read it), which is my opinion is the critical issue of that system. I think this warranty may have changed since then, to include this, however, with Kevin raising the issue about substrate longevity, I question how long the adhesives in such tapes will last and I believe we could be seeing a huge round of substrate vapor barrier failures (both in taped seam roofs and taped seam walls). I think any of us who specify such taped seam system would do well to ask for accelerated aged tests on the tape's adhesive properties 10 years, 20 years and 30 years+ from now. For my liability, and the benefit of my clients, I would rather have an aging continuous layer of asphaltic felt than possibly failing tapes all over the structure.
Thanks again for your interesting question, Kevin. Any manufacturers out there care to respond?
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Rand Soellner AIA
Architect/Owner/Principal
Rand Soellner Architect
Cashiers NC
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Original Message:
Sent: 03-19-2012 15:47
From: Kevin Harris
Subject: Detailing for Durability
This message has been cross posted to the following Discussion Forums: Custom Residential Architects Network and Housing Knowledge Community .
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Rand:
I appreciate the research on 15# felt verses Tyvek - type houssewraps. One concern I have is the durability of current 15# felts. Many of the renovations we have performed were built +/- 50 years ago and were built using a 15# felt. When the wall cladding (brick veneer or wood siding) opened, the felt is frequently very brittle. The houses and their exterior cladding seem to outlive the life of the felt hidden inside. This is also the case with older tile and slate roofs where the underlayment becomes the weak point for water entry.
Has the formula for currently available felts been improved such that it extends the life of the underlayment?
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Kevin Harris FAIA
FAIA
Kevin Harris Architect, LLC
Baton Rouge LA
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