[Greenbuilding] Water Barrier

[Eli Talking]

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.  

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. 

Eli 

-----Original Message----- 
From: Eli Talking 
Sent: Saturday, February 09, 2013 5:38 PM 
To: jfstraube at uwaterloo.ca 
Subject: Re: [Greenbuilding] Managing Sheathing as an air barrier. 

John

In your message you ask, "The water barrier is more important than air 
barrier.  What will it be?"

My comments:
When you say water, do you mean liquid water or vapor?  I am relying on the 
space created on 2x furring strips to allow liquid to drain and vapor to 
vent out the top by being open to soffit vent as well as every seam between 
horizontal siding.  I expect the siding to absorb the wind driven pressure 
allowing that would I hope eliminate the possibility of liquid reaching the 
foam surface.  If it does, it should still drain or dry, lacking an 
additional driving pressure to penetrate EPS.  I know that EPS is considered 
an air barrier, but not a vapor barrier.  This would allow for some drying I 
suppose.  In my mixed humid climate of Virginia, conventional wisdom is that 
we should avoid a vapor barrier to allow drying in both directions. 
However, building wrap is considered the bulk liquid protection where siding 
is commonly installed directly over without a drain dry space I am 
proposing.

The roof liquid barrier is the metal roofing.  The space below allows 
healthy ventilation to keep things dry.  

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.  



Thanks
Eli

-----Original Message----- 
From: John Straube
Sent: Sunday, January 27, 2013 6:41 PM
To: Eli Talking
Subject: Re: [Greenbuilding] Managing Sheathing as an air barrier.

The water barrier is more important than air barrier.  What will it be?
For an AB, I would just tape the OSB, and test the house before the foam 
goes on to catch the installation errors.
Tape sandwiched between foam and OSB will see very little mechanical load. 
Protected by all that EPS, it will never see wide temperature ranges.  Both 
means that modern tapes should last >100 years.
We have used butyl-based sealing tapes from Dupont and Dow, and SBS bitumen 
strips from Henry-Bakor, and Grace.  Acrylic tapes (from 3M, Dow, Dupont, 
etc) will stick really well, but I think the thicker ones will be better for 
this application.
Install all tapes, including around windows and doors, then air test.  This 
is one way, and a good way, but not the only way.

On 13-01-26 11:09 AM, Eli Talking wrote:
> As described in previous messages, I am planning on installing 12” eps 
> foam boards on top of 7/16”osb roof sheathing with 2x4 furring strips at 2’oc 
> for nailing corrugated roofing to.  The exterior walls are similar with 
> 2x3 furring strips aligned with studs over 6”eps over 7/16 osb.
> I am planning the osb and eps rows to be perpendicular so that only at the 
> intersections will there be a continuous potential leak.  I want to manage 
> both the eps and the osb to be a continuous air barrier.  Only one is 
> required to achieve performance.  However, the vulnerability to execution 
> flaws, even when you are trying, makes me want to manage both layers to 
> provide a redundant continuous air barrier.
> The osb would be installed conventionally with plywood clips between 
> rafter top chords.  This creates a continuous gap the thickness of the 
> metal clip.  Can someone recommend a tape that will not give up in time? 
> Would sealing it with acoustic caulk be practical, maybe from the top 
> side?
> The foam will be air sealed at the seams with acoustic caulk.  As a third 
> air barrier, I might consider tape at the foam seams on top.  I am going 
> to put a dab of caulk on the bottom of the furring strip where the pilot 
> holes show where the screw will penetrate the full thickness to seal that 
> potential leak.  Another potential air barrier is to cover all the foam 
> with a plastic or tight building wrap before installing the furring 
> strips.
> If I am able to achieve to my satisfaction the air barrier in osb and eps 
> layers, I should not need the membrane layers. Since the foam is 6” and 
>  12” thick, the nails from installing finished materials penetrating .5” 
> or so into foam should not threaten the air barrier if it does not fall at 
> a seam.
> You comments, as always will be appreciated.
> Eli
> www.conservationarchitect.net <http://www.conservationarchitect.net>
>
>
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-- 
Prof. John Straube, Ph.D., P.Eng.
Faculty of Engineering
Dept of Civil Engineering / School of Architecture

www.buildingscience.com
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