Design - Vapor Barriers and Air Barriers (Cont'd)

The previous post tried to make sense of vapor barriers.  This post tries to do the same for air barriers.  However, air barriers are hard to address without further discussion of vapor barriers since the two are so intertwined. 

My primary source of information for the last post was an online paper by Joseph Lstiburek on vapor retarders; my authority here is another Lstiburek paper, "Understanding Air Barriers".   Some of what appears below paraphrases his work.

What Is An Air Barrier? 
An air barrier controls airflow between conditioned space and unconditioned space. It is located in the assemblies that separate the building interior from the outside environment but can be in assemblies that separate living space from potentially dangerous space such as a garage.  The barrier can exist on the exterior of wall/ceiling assemblies, on the interior, or both.  In cold climates in the absence of air conditioning, they tend to be located interiorly to control exfiltration of moisture-laden air. In warmer climates and any climate with air conditioned space, they are located exteriorly where they not only control infiltration of exterior air, they also prevent air penetration into the insulation materials within the wall or ceiling (wind washing). Very often air barriers are both exterior and interior as will be the case with our project.


Merits of Exterior and Interior Air Barriers
The advantage of exterior air barriers, such as house wrap, is ease of installation and automatic control of wind washing.  The disadvantage of an exterior system is that it is on the wrong side of the wall to stop exfiltration of moisture-laden interior air that degrades cavity components.  

The advantage of interior air barriers, such as latex painted drywall, is they stop exfiltration of moisture-laden air. Their disadvantage is that they are on the wrong side of the wall to stop wind washing or infiltration of insulation-compromising moisture from the exterior.

However, installing both exterior and interior air barriers can compensate for the disadvantages of each.  Interior barriers were covered in a recent post. The take-away was that, in all but the coldest climates, drywall, carefully detailed and painted with latex paint, suffices as an interior vapor barrier.  Lstiburek also recommends drywall as an air barrier and uses the nearby illustrations to demonstrate the detailing that is necessary for it to be successful.  We plan to follow his recommendations.

Cathedral Ceilings
Tentatively, most of our ceilings will have tongue and groove pine boards instead of drywall because the exposed wood enhances our country decor and the 3/4" boards will be better able to support 16" of rice hull insulation. The boards will serve as a Class III semipermeable vapor retarder (same recent post ) to help protect the insulation from exfiltration of moist interior air. The boards will serve as an air barrier but not well enough but what the definitive air barrier will have to be at the top of the ceiling trusses.  Accordingly, I am in the process of covering the tops of the roof trusses with 1/2" plywood then sealing the cracks between sheets with flashing tape (bottom photo). Above the plywood will be ventilated space in the form of the "mini-attic". The design and construction of the cathedral ceilings appear in the post immediately preceding this post for the north-facing roof and in two posts hence for the south facing roof.


Blower Door Testing
Verification of air barrier efficacy is easily done with one or the other of two "blower door" tests that create an air pressure differential between the interior and the exterior of a building.  For small buildings, the fan is embedded in a temporary exterior door; for larger buildings, it is installed in the building's air handling system.  The amount of air leakage in the envelope of the building can be quantified and, with the help of a smoke gun, can be pinpointed for remediation
.

Choosing An Exterior Air Barrier
All cladding materials admit varying degrees of rainwater to underlying structures. Some, like brick veneers, fibercement siding. stucco and even wood siding, are worse than others, like vinyl or metal siding, in that they absorb and hold water that the sun can then drive through the wall (solar vapor drive).

While the primary selling point for exterior barriers (such as the ubiquitous "Tyvec") is to reduce air infiltration, their vapor barrier specs turn out to be the thoughtful basis for choosing between them.  The choices range from something like 30# felt paper as an attempt to keep water out entirely, thereby favoring wet-prevention mechanisms, or something like Class IV permeable house-wrap to keep most of it out then let that which does penetrate the opportunity to exit back out through the barrier, thereby favoring drying mechanisms.

The exterior walls and the floor of the mini-attic (last post) are being sheathed with plywood, rather than OSB.  (The rationale for using plywood is explained by Joe Listburek at Building Science Corp).  At the time of this writing, I was still undecided whether to use foil-backed OSB instead of plain OSB for the second layer of sheathing that forms the roof of the mini-attic and provides the decking for the metal roof.  The advantage would be that its radiant barrier would provide a cooler roof. Its disadvantage would be that the decking would be vapor impermeable on the inside so that, for proper drying, the underlayment for the metal roofing would have to be vapor permeable to allow drying outwards.  Our dilemma is that the slope of some of the roofs may not be steep enough for a vapor permeable underlayment for the metal roof, in which case, we may have no choice but to use a vapor impermeable underlayment and no foil backing so the roof could dry inwards.  It will take more research before making a final commitment but I am inclined to forego the foil backing and rely instead on a metal roof color that has a high solar reflectance.

With metal roof or metal wall cladding, we can expect some moisture to make its way into the underlying assemblies,

Plywood sheathing sealed with flashing tape; holes
that were created by first unsuccessful attempt at a
 temporary covering using a tarp and sheet plastic
 have been closed with spackling

especially during horizontal rains. So, assuming that any house wrap or roof underlayment is an adequate air barrier, we need to choose wraps that minimize water penetration but allow outward drying of the underlying wood when necessary.

With regard to the wall assembly, there will be five layers:

• Metal cladding
• Wrap
• 1/2" plywood sheathing
• 15" wall trusses filled with rice hulls
• 1/2" drywall detailed per Lstiburek and sealed with latex paint

With regard to the roof-ceiling assembly, there will be seven layers:

• Standing seam metal roof in a color that has a high solar reflectance
• Air/vapor barrier (type to be determined after more research)
• 1/2" OSB without foil backing
• 3 1/2" air space (mini-attic)
• 1/2" plywood nailed to the tops of the rafters sealed with ZIP tape (air barrier)
• 18" tall trusses filled with rice hulls
• 3/4" tongue and groove pine ceiling or 5/8" drywall ceiling

In an ideal world, the best choice for our metal roofing and siding would be a vapor permeable wrap instead of a vapor impermeable wrap, thereby favoring drying mechanisms over wet-prevention mechanisms.  The barrier that we use for the walls will definitely follow this approach.  The barrier that we use for the roof will be dictated by the roof pitch, has yet to be determined and will be discussed in future posts on roofing.