Sometimes it is hard to separate out what is doing what in a wall, when some materials provide dual functions, and designers have options in how to approach the problem. There are three things we are trying to stop at the wall- moisture, from precipitation; vapor, the water that is in air, which varies by temperature and humidity; and air, AKA drafts. Vapor moving across the building envelope is driven by temperature differential, not wind. Air moving across the building envelope is driven by pressure differential across the building envelope.
To compare housewrap with tape to tar paper/15# felt is to compare apples and oranges. Both provide a vapor permeable moisture barrier, but the reason the housewrap is taped is so it forms an air barrier. The tar paper is not forming an air barrier.
If you use 15# felt, then the air barrier is usually formed on the inner side of the wall , by the poly and the drywall. In an old house, the air barrier , imperfect, yes, but still the air barrier is the plaster. The same durability argument might be made for the tape sealing the joints of the poly, however as it is on the inner face of the wall it is not subject to the temperature variations that the taped housewrap is on the outside of the wall.
My distrust of the tape's durability leads me to attempt to use the drywall as the air barrier, caulking junctions to other materials, using gaskets and using spray foam insulation to make the air barrier continuous across the floor joist cavity.
Both housewrap and 15# felt are vapor permeable , so do not form a vapor barrier. The poly acts as the vapor barrier, without poly in place the vapor barrier is the paint on the wall or the vapour is not stopped , just slowed down.
It is common practice here to use two layers of the 15# felt because as was said, it ain't what it used to be. It is placed on the building lapped shingle style to get two layers, as opposed to a gift wrapped twice method. This two layer felt method is what I prefer. I like tyvek letter envelopes, though.
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Maura Gatensby Intl. Assoc. AIA
Maura Gatensby Architect
Vancouver BC
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Original Message:
Sent: 03-08-2012 12:14
From: Philip Burdick
Subject: Detailing for Durability: Your Questions Answered
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Philip Burdick AIA
Philip D. Burdick, Architect
Springfield MA
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I was unable to attend the webinar, is there a possibility it can be re-broadcasr?
Here is a good link for information on house wrap
http://inspectapedia.com/BestPractices/Sheathing_Wrap.htm
This article states that 15 lb. felt isn't what it used to be. Look it over.
My 1909 house has a thin black asphaltic coated paper under the red cedar shingles. The west facing exposure was replaced a little over 20 years ago with red cedar shingles over Tyvek. So far no problems, although I can't vouch for performance as a air barrier. The sheathing is (full) 3/4" T&G boards. Insulation (full 4" studs) is blown in cellulose (mostly). The north elevation still has the original red cedar shingles form 1909 still onver the original building paper and still in very good shape. The south wall is looking to need new shingle siding. The east and portions of the south wall is a mixture new construction (red cedar over Tyvek) and original shingle work in varying condition.
The roof (replaced about the same time as the western wall shingles) was stripped to the 7/8" x 6' +/- spaced decking (to allow the original wood shingles to breath from the underside) which was then infilled with 1x2 & 1x3 stock and roofed with asphalt shingles over 30lb felt. Asphalt shingles are starting to show signs of wear but no leaks.
The hole in the system is at the eaves where only the large crown moulding separates the outside from the space behind the knee walls of the finished attic. Plenty of ventilation there as well as occasional squirrels, raccoons, etc. I've been considering backing that from the inside with a combination of rigid and spray foam in the manner of a "hot" roof as I have never seen any evidence of condensation on the inside of the exerior sheathing despite having steam heat and relying on oil based primer over the existing wood lath and plaster walls to serve as the interior vapor barrier. I used a conventional vapor barrier when renovation a bathroom and will do so on the exterior wall of the next bathroom that needs to be gutted, perhaps this summer.
I'b be interested in hearing other building wrap stories, This far I would agree that felt still works.
Original Message:
Sent: 03-07-2012 07:53
From: Rand Soellner
Subject: Detailing for Durability: Your Questions Answered
Hello Stephen and Paul,
On behalf of the AIA HKC, CRAN and other AIA forums, Thank You for the excellent online-real-time Webinar Monday. I enjoyed it, as I am sure, others did as well. I am a long time fan of Paul Fisette's and have quoted previous vapor barrier studies he has done on my website, to act as learning aides for clients, contractors and other architects and engineers. This is not a particularly sexy area of architecture, but one that is fraught with peril for those ignoring wise counsel. I noted Paul's apparent interest in the Huber zip system and restate my concerns about the long-term adhesion of the glues in the tape: can an architect and homeowner really rely on the tape, the most critical point in the system, to remain adhered to the coated building panels forever? How long will the adhesion last? 10 years? 20? Then what happens? I fear that the widespread use of this product may be creating an future tsunami of potential failures, similar to the EIFS debacle, by use of a system with inherent weaknesses that at first seems to be a clever way to save a few hundred bucks, but then comes back in a decade or so with a huge due bill for repairs. In other words, I am concerned that the tape will eventually give way, then any moisture under the surfaces of the walls and roof can then leak through those joints and into the houses that they are supposed to protect. I would appreciate being made aware of any accelerated aging tests on this matter that proves my fears unjustified, if that is the case. For me, I think I'll stick with good old 15# asphaltic felt, which is, as Paul says (and as he has on his own house), a smart vapor/water barrier that has a Perm rating that changes according to the humidity, miraculously becoming what we need it to be during each season. I have always had excellent performance with "tar paper" over wall sheathing and based on Paul's excellent studies, I think I'll probably stick with that. No concerns about tape falling loose. Thanks again for your interesting program; I look forward to more!
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Rand Soellner AIA
Architect/Owner/Principal
Rand Soellner Architect
Cashiers NC
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Original Message:
Sent: 03-06-2012 19:13
From: Stephen Schreiber
Subject: Detailing for Durability: Your Questions Answered
I am posting this mesage on behalf of Detailing for Durability presenter, Paul Fisette.
Hello everyone. Wow this is a lot of questions and they are uniformly excellent. I simply don't have the time to provide long thoughtful answers to these great questions. However, I did try to provide some guidance on each and every question. I hope this is useful. - Paul Fisette
Your questions and Paul's answers can now be found with his presentation in the HKC Resource Library:
http://network.aia.org/AIA/Resources/ViewDocument/?DocumentKey=7820c63b-3dff-4b76-9ca2-277f5a451479
The recording of the webinar will be available at the same URL in the coming weeks. Thank you to the 500+ attendees from yesterday's webinar.
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Stephen Schreiber FAIA
Director and Professor
University of Massachusetts Amherst, Department of Art, Architecture & Art History
Amherst MA
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