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<DIV>I am proposing a wall assembly of siding nailed to 2x3 furring strips over
6" continuous eps over 7/16" sheathing over 2x framing left hollow. John
Straub expressed concern about the water barrier. Coming from John Straub,
I am concerned. My questions regarding water protection is contained
in message below. </DIV>
<DIV> </DIV>
<DIV>Though multiple vapor barriers may pose a threat of trapping moisture
through imperfections, I do not see the same threat with multiple air barriers,
all allowing vapor to pass if any air barrier is flawed. For the most
part, I like thick foam as an air barrier or a vapor barrier because it is
warmer on the warm side, reducing the threat of condensation. </DIV>
<DIV> </DIV>
<DIV>Eli </DIV>
<DIV> </DIV>
<DIV>-----Original Message----- </DIV>
<DIV>From: Eli Talking </DIV>
<DIV>Sent: Saturday, February 09, 2013 5:38 PM </DIV>
<DIV>To: jfstraube@uwaterloo.ca </DIV>
<DIV>Subject: Re: [Greenbuilding] Managing Sheathing as an air barrier. </DIV>
<DIV> </DIV>
<DIV>John</DIV>
<DIV> </DIV>
<DIV>In your message you ask, <FONT color=#9e7c7c>"The water barrier is more
important than air </FONT></DIV>
<DIV><FONT color=#9e7c7c>barrier. What will it be?"</FONT></DIV>
<DIV> </DIV>
<DIV>My comments:</DIV>
<DIV>When you say water, do you mean liquid water or vapor? I am relying
on the </DIV>
<DIV>space created on 2x furring strips to allow liquid to drain and vapor to
</DIV>
<DIV>vent out the top by being open to soffit vent as well as every seam between
</DIV>
<DIV>horizontal siding. I expect the siding to absorb the wind driven
pressure </DIV>
<DIV>allowing that would I hope eliminate the possibility of liquid reaching the
</DIV>
<DIV>foam surface. If it does, it should still drain or dry, lacking an
</DIV>
<DIV>additional driving pressure to penetrate EPS. I know that EPS is
considered </DIV>
<DIV>an air barrier, but not a vapor barrier. This would allow for some
drying I </DIV>
<DIV>suppose. In my mixed humid climate of Virginia, conventional wisdom
is that </DIV>
<DIV>we should avoid a vapor barrier to allow drying in both directions. </DIV>
<DIV>However, building wrap is considered the bulk liquid protection where
siding </DIV>
<DIV>is commonly installed directly over without a drain dry space I am </DIV>
<DIV>proposing.</DIV>
<DIV> </DIV>
<DIV>The roof liquid barrier is the metal roofing. The space below allows
</DIV>
<DIV>healthy ventilation to keep things dry. </DIV>
<DIV> </DIV>
<DIV>Do you think I need an additional layer for water? I could easily add
a layer of building wrap for a liquid water barrier. However, I do not
understand where the water threat would be coming from. </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV>Thanks</DIV>
<DIV>Eli</DIV>
<DIV> </DIV>
<DIV>-----Original Message----- </DIV>
<DIV>From: John Straube</DIV>
<DIV>Sent: Sunday, January 27, 2013 6:41 PM</DIV>
<DIV>To: Eli Talking</DIV>
<DIV>Subject: Re: [Greenbuilding] Managing Sheathing as an air barrier.</DIV>
<DIV> </DIV>
<DIV><FONT color=#9e7c7c>The water barrier is more important than air
barrier. What will it be?</FONT></DIV>
<DIV>For an AB, I would just tape the OSB, and test the house before the foam
</DIV>
<DIV>goes on to catch the installation errors.</DIV>
<DIV>Tape sandwiched between foam and OSB will see very little mechanical load.
</DIV>
<DIV>Protected by all that EPS, it will never see wide temperature ranges.
Both </DIV>
<DIV>means that modern tapes should last >100 years.</DIV>
<DIV>We have used butyl-based sealing tapes from Dupont and Dow, and SBS bitumen
</DIV>
<DIV>strips from Henry-Bakor, and Grace. Acrylic tapes (from 3M, Dow,
Dupont, </DIV>
<DIV>etc) will stick really well, but I think the thicker ones will be better
for </DIV>
<DIV>this application.</DIV>
<DIV>Install all tapes, including around windows and doors, then air test.
This </DIV>
<DIV>is one way, and a good way, but not the only way.</DIV>
<DIV> </DIV>
<DIV>On 13-01-26 11:09 AM, Eli Talking wrote:</DIV>
<DIV>> As described in previous messages, I am planning on installing 12” eps
</DIV>
<DIV>> foam boards on top of 7/16”osb roof sheathing with 2x4 furring strips
at 2’oc </DIV>
<DIV>> for nailing corrugated roofing to. The exterior walls are
similar with </DIV>
<DIV>> 2x3 furring strips aligned with studs over 6”eps over 7/16 osb.</DIV>
<DIV>> I am planning the osb and eps rows to be perpendicular so that only at
the </DIV>
<DIV>> intersections will there be a continuous potential leak. I want
to manage </DIV>
<DIV>> both the eps and the osb to be a continuous air barrier. Only
one is </DIV>
<DIV>> required to achieve performance. However, the vulnerability to
execution </DIV>
<DIV>> flaws, even when you are trying, makes me want to manage both layers
to </DIV>
<DIV>> provide a redundant continuous air barrier.</DIV>
<DIV>> The osb would be installed conventionally with plywood clips between
</DIV>
<DIV>> rafter top chords. This creates a continuous gap the thickness
of the </DIV>
<DIV>> metal clip. Can someone recommend a tape that will not give up
in time? </DIV>
<DIV>> Would sealing it with acoustic caulk be practical, maybe from the top
</DIV>
<DIV>> side?</DIV>
<DIV>> The foam will be air sealed at the seams with acoustic caulk. As
a third </DIV>
<DIV>> air barrier, I might consider tape at the foam seams on top. I
am going </DIV>
<DIV>> to put a dab of caulk on the bottom of the furring strip where the
pilot </DIV>
<DIV>> holes show where the screw will penetrate the full thickness to seal
that </DIV>
<DIV>> potential leak. Another potential air barrier is to cover all
the foam </DIV>
<DIV>> with a plastic or tight building wrap before installing the furring
</DIV>
<DIV>> strips.</DIV>
<DIV>> If I am able to achieve to my satisfaction the air barrier in osb and
eps </DIV>
<DIV>> layers, I should not need the membrane layers. Since the foam is 6”
and </DIV>
<DIV>> 12” thick, the nails from installing finished materials
penetrating .5” </DIV>
<DIV>> or so into foam should not threaten the air barrier if it does not
fall at </DIV>
<DIV>> a seam.</DIV>
<DIV>> You comments, as always will be appreciated.</DIV>
<DIV>> Eli</DIV>
<DIV>> www.conservationarchitect.net <<A
href="http://www.conservationarchitect.net">http://www.conservationarchitect.net</A>></DIV>
<DIV>></DIV>
<DIV>></DIV>
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<DIV>></DIV>
<DIV> </DIV>
<DIV>-- </DIV>
<DIV>Prof. John Straube, Ph.D., P.Eng.</DIV>
<DIV>Faculty of Engineering</DIV>
<DIV>Dept of Civil Engineering / School of Architecture</DIV>
<DIV> </DIV>
<DIV>www.buildingscience.com</DIV></DIV></DIV></BODY></HTML>