[Greenbuilding] And the debate rages on to add an air barrier or not to air barrier!

Thu Jan 31 18:07:36 CST 2008

Hi Joshua:

Its not really the face that is penetrated with fasteners 
that provides the barrier.  Based on the construction, I'm 
assuming you are working on a commercial structure rather 
than a residential one.  In residential construction, 
rather than the exterior gypsum sheathing (for a fire 
rated listed assembly?) you would typically see an 
insulated panel such as celotex, usually with some kind of 
shear value, or celotex with oriented strand board at the 
corners for shear.  With this construction, the residence 
is wrapped in tyvek to prevent infiltration.  The 
infiltration occurs primarily at the joints, not the face, 
so all the fasteners on the face have minimal effect on 
infiltration, its where the tyvek covers the sheathing 
joints, where the floor to floor joints are.  Usually they 
don't fasten right on the joints, so there is a perceived 
barrier.   Now with residentail construction, Group R 
occupany usually doesn't require active ventilation.  The 
10% glass 5% vent per room meets the code.  When I built 
my house outside of Detroit, I opted to go without the 
wrap to prevent poison house syndrome.  If you are working 
commercially, there is probably an air handler with a 
certain percentage of outside air introduced by the system 
set to comply with occupancy.

So the real benefit would be to prevent infiltration in a 
structure where active infiltration is designed in anyway. 
 If you are taping the joints, that would have the same 
effect, actually better, than if the tyvek spanned the 
joints?

Bill Bloom
Former Detroiter
Now Sunny New Mexico

On Thu, 31 Jan 2008 18:28:35 -0500
  "Joshua Lloyd" <JLloyd at markwoodarch.com> wrote:
> John,
> I have to pick my battles.  It is what the "old guys" 
>wants.  However
> the batts are manufactured only 50 miles from the site. 
> So a minimal
> transportation footprint.  But I do have 1/2 lb open 
>cell spray applied
> foam in the specs as an alternate.  So if we can just 
>quantify the extra
> cost, I believe it will be a go.
> 
> And I agree with you about the Tyvek.  I think the same 
>thing about
> Asphalt shingles.  Especially after renovating my home 
>and doing most of
> the work myself.  We call it weather resistant, yet we 
>riddle it with
> holes and hope the overlap is enough.
> 
> 
> 
> -----Original Message-----
>From: John Messerschmidt [mailto:john at ducecc.com] 
> Sent: Thursday, January 31, 2008 6:11 PM
> To: Joshua Lloyd; Greenbuilding at listserv.repp.org
> Subject: RE: [Greenbuilding] And the debate rages on to 
>add an air
> barrier or not to air barrier!
> 
> Joshua,
> I never got how Tyvek or the rigid insulation could be a 
>vapor (or
> weather) barrier if you have to nail through it to 
>attach the siding.
> But besides that, why do you want to use fiberglass 
>batt?  Yuk.
> 
> 
> John
> 
> 
> 
> 
> -----Original Message-----
>From: greenbuilding-bounces at listserv.repp.org
> [mailto:greenbuilding-bounces at listserv.repp.org] On 
>Behalf Of Joshua
> Lloyd
> Sent: Thursday, January 31, 2008 6:09 PM
> To: Greenbuilding at listserv.repp.org
> Subject: [Greenbuilding] And the debate rages on to add 
>an air barrier
> or not to air barrier!
> 
> Ok, I am hoping that I can get some educated help on 
>this topic for we
> are going around in circles with this issue in our 
>office.  We are
> working on a project here in Central Ohio (Climate Zone 
>5) and we are
> trying to increase the performance of our exterior wall 
>so we currently
> have the following assembly:
> 
> Brick Veneer
> 1" air space
> 1" Foil faced Polyisocyanurate
> 1/2" Exterior Gypsum Board
> 6" Metal stud w/ Unfaced Fiberglass batts 5/8" Gypsum 
>Board Latex Paint
> 
> Now the question is;  Do we need to add a weather 
>resistant barrier (ie.
> Tyvek) between the foil face rigid board insulation and 
>the exterior
> gyp. board if we are taping and sealing all the joints 
>of the rigid
> board insulation?  It is my understanding that the 
>weather resistant
> barrier at this point is solely used as a drainage plane 
>if water was to
> penetrate through the rigid board.  But if we are taping 
>and sealing, I
> don't see the need for the drainage plane because the 
>rigid insulation
> will be acting as that drainage plane.
> 
> What are your thoughts on this?
> 
> 
> Joshua Lloyd
> Assoc. AIA, LEED AP
> Phillip Markwood Architects, Inc.
> 240 North Fifth Street
> Suite 140
> Columbus, Ohio 43215
> Ph: 614.461.8300
>Fax: 614.461.8302
> email: jlloyd at markwoodarch.com 
><blocked::mailto:jlloyd at markwoodarch.com>
> 
> 
> 
> 
> 
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