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

Tue Mar 5 12:44:31 CST 2013

>DEMYSTIFYING THE USE OF VAPOR BARRIERS
>by John Straube
>
>The most common building science question I get from home builders,
>engineers, code officials, and architects is, "Do I need a vapor
>barrier?"  The answer is usually simple, but first one has to know
>more about the question and the specific situation.
>
>To decide how to control vapor diffusion properly, you must have
>information about three different aspects of your specific situation:
>the exterior climate, interior conditions, and the properties and
>arrangements of the wall assembly. Let's consider each.
>
>Exterior Climate
>
>Vapor diffusion moves from areas of more to less.  For a hot, humid
>climate like Miami, Florida, where the vapor outdoors is higher than
>indoors almost all the time, it stands to reason that you should
>place a vapor barrier on the exterior side of the wall assembly.
>Not all codes recognize this yet, but it is a fact.  Similarly, for
>a climate with less moisture outside all the time (e.g., northern
>Alaska), a vapor barrier should usually be placed near the interior.
>For all other situations, we need to know more before we decide.
>
>It must also be remembered that "outside" could also mean the
>conditions created behind rain-wetted, absorbent cladding (like brick,
>cedar shakes, stucco, wood, cement board) exposed to sunshine.  This
>creates a "climate" outside of the wall or roof similar to a sauna,
>which drives moisture inward.  For enclosures with absorbent
>claddings in rainy, temperate climates, this effect can become
>quite important.
>
>Interior Conditions
>
>If you are building an indoor swimming pool, you can be quite
>sure that it will be very humid and warm inside all year long.
>Thus, a vapor barrier on the inside is practically mandatory in all
>but the hottest and most humid climates.  On the other hand, if the
>enclosure is around a deep-freeze storage facility, there will be
>more moisture outside most of the time, and the vapor barrier goes
>on the outside, even in a climate like Pittsburgh, Pennsylvania.
>Houses should typically be maintained at a moderate interior
>humidity level by using ventilation or dehumidification.
>
>Wall Assembly
>
>Obviously, the wall assembly plays a very significant role in
>deciding on your vapor diffusion control needs.  Although designers
>tend to be fixated on the need to label vapor barriers, the fact
>is that many materials in an assembly may control vapor diffusion.
>Although batt insulation (permeable: 20 perms) has practically no
>vapor resistance, 8 inches of concrete is a pretty good barrier
>(impermeable: 0.5 perms) and latex paint on gypsum board is
>semipermeable (about 3 perms).
>
>Thus, a wall with painted gypsum already has some pretty good vapor
>control and would not need an additional layer if used to separate
>a moderate exterior climate (e.g., Boston, Massachusetts) from a
>moderate interior climate (say a house with good ventilation).
>For a colder climate (e.g., Minneapolis, Minnesota), an 8-inch
>structural concrete wall or 6 inches of expanded polystyrene
>insulation (about 0.75 perms) would be sufficient for all but
>very humid interior conditions.
>
>The order in which layers of different permeance materials are
>arranged in an enclosure is also important.  For example, using
>an unventilated low-permeance layer (such as a roofing membrane,
>precast concrete, etc.) on the exterior in a cold climate will
>prevent water vapor from escaping to the exterior (this slows drying
>to the outside).  The permeance of the interior layers must be
>considerably less than the permeance of outer layers (various rules
>place the ratio at 3:1 to as much as 10:1).  Using insulating
>sheathing also changes the behavior drastically.
>
>The rules are reversed for hot climates.  Increasing the temperature
>inboard of the insulated sheathing essentially transports the wall
>to a warmer and more temperate climate zone, thereby also reducing
>the need for low-permeance vapor barriers.  For example, an R-12
>wood-frame house wall with R-7.5 insulated sheathing in Nebraska
>would not require a sheet vapor barrier, but would require a
>normal latex paint layer.
>
>Figure It Out
>
>Given the information I've shared above, it's reasonably easy to
>decide if, where, and what kind of vapor barrier you need.  Keep in
>mind that air barriers are important and necessary components in
>almost all building enclosures in all climates, whereas vapor
>barriers are typically less important components that may or may
>not be needed.
>
>As you decide, remember that you must include the exterior
>climate, interior conditions, the properties of materials
>(e.g., permeance, capacity for wetting) and the arrangements of the
>enclosure assembly.  A useful tool, which describes the process
>in detail, is Chapter 22 of the *Handbook of Fundamentals*, published
>by the American Society of Heating, Refrigerating and Air-Conditioning
>Engineers.  More sophisticated users should investigate these
>aspects using a dynamic computer model, such as WUFI, available
>for free at http://www.ornl.gov/ORNL/BTC/moisture .
>
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