[Greenbuilding] Difference between an Air Barrier Strategy vs. AB materials (was Re: Water Barrier)

[Eli Talking]

All vapor barriers are air barriers.  However, not all air barriers are
vapor barriers.  The perm rate for vapor barriers is much smaller than
required for air barrier. I looked for that exact number, but was not able
to put my eye on it.   I think of it like a screen where an air molecule
that can contains some vapor is much larger than a vapor molecule, allowing
vapor to pass if there is vapor pressure (humidity difference) from one side
to another.  However, if the air barrier is affective, the air will not
carry vapor into the assembly.  The air barriers I am discussing are the non
vapor barriers such as osb, plywood, building wrap.  Some of those tapes and
acoustical sealants may be vapor barriers.  However, if they are used to
seal air barrier that are not vapor barriers such as osb or building wrap,
allowing vapor to go around the seal, the assembly is not a vapor barrier.
Though our aspirations are to be as perfect as we can, I am still looking to
understand the threat of flawed multiple air barriers
that do not qualify as a vapor barrier.   Perhaps the main threat is that
air will pass to a part of the assembly that is cold enough to condense it's
contained vapor.  However, with no affective vapor barrier, the vapor should
be able to dry to the warm side of the assembly, easily passing through the
air barriers.  As a liquid the water would stay trapped.  The eps foam I am
proposing to use is not a vapor barrier.  xps foam is a vapor barrier.  The
trick to using it safely is to install a thick enough layer so the warm side
stays warm enough to be above dew point.

I am in search of a cost affective
high standard.  Because eps is dramatically less expensive/ unit R and can
be an affective air barrier, it is a good product to use.  I do believe that
cost is somewhat reflective of embodied energy required to create.

Because of this discussion, I am adding an additional air barrier of 
building wrap on the outside of eps foam to separate the foam from potential 
liquid water that would be absorbed.

Eli

-----Original Message----- 
From: RT
Sent: Monday, March 04, 2013 11:18 PM
To: Green Building
Subject: [Greenbuilding] Difference between an Air Barrier Strategy vs. AB
materials (was Re: Water Barrier)

andOn Mon, 04 Mar 2013 16:34:15 -0500, Eli Talking
<elitalking at rockbridge.net> wrote:

> Does any body see risk for multiple air (non vapor) barriers?  I
> understand the risk of trapping liquid between multiple vapor barriers.
> However, my thinking is that just an air barriers will allow drying to
> the warm side even if one has leaks in the barrier.  Because so much of
> the performance for tightness hinges on the execution of the air  barrier,
> I tend to want several layers of air barrier, such as sealing  osb seams,
> sealing foam seams, taping foam seams, et.  Chances are all  of those may
> have some execution flaws.

I think that it might be useful to go back to basics.

An "air barrier" is a strategy.

If the strategy is effective, then the building is air-tight.

There are any number of materials that can be utilised in the creation of
an effective air barrier --gypsum board, plywood or aspenite, plastic sheet
membranes (ie Tyvek, poly), sheet metal, glass, caulking, gaskets, spray
foam etc.

Most typical buildings will some of the above in numerous layers
through a particular component cross section, as Eli proposes.

It is the detailing of the discontinuities between the materials used that
determines whether or not the strategy is effective.

But I think that a simple example would let Eli answer his question by
himself.

Take 3 or more plastic bags that each have pinholes or tears in them in
different spots and then them inside of each other so that you'd get the
"multiple air barrier (material)s" with non-aligned leaks" situation that
Eli proposes.

Then fill the multi-layered bag with water and tie it shut.

I think you know the rest.

-- 
=== * ===
Rob Tom AOD257
Kanata, Ontario, Canada

< A r c h i L o g i c  at  Y a h o o  dot  c a  >
(manually winnow the chaff from my edress if you hit "reply")

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