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

Tue Mar 5 12:44:13 CST 2013

All vapor barriers are NOT air barriers. Perhaps you are thinking materials not walls and roofs.
A common vapour barrier in Canada is polyethylene sheet.  A great vapour barrier.  Overlap the joints and staple it to the wall, and it is NOT an air barrier. We regularly investigate real life building problems caused by people who point at a material on a drawing and say "but we did install an air barrier, there it is!".  Of course it leaks because they did not seal it up and so it was not an air barrier.
There is no required perm rating for an air barrier system, it depends.
Water vapour is not part of an air molecule. There are no air molecules.  There are molecules of nitrogen oxygen, argon, etc mixed to make air.

Please can all of us in the conversation please try reading some of the resources at
http://www.buildingscience.com/resources/vapor_barrier_code_changes
before going on.

I will send another primer I wrote 15 years ago in a separate email.

On 2013-03-05, at 12:07 PM, Eli Talking <elitalking at rockbridge.net>
 wrote:

> 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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Prof. John F Straube, P.Eng.
www.BuildingScience.com