Thank you for the opportunity to benefit from the GBA community’s experience. The following design approach using two air barriers is influenced by Martin Holladay’s Q&A comment in his 2010 GBA article “One Air Barrier or Two” wherein he stated “In theory, if you have one good air barrier, you have stopped all air movement.  In most cases the second air barrier is unnecessary. The exception would be a wall without exterior foam insulation that is insulated between the studs with fiberglass batts.”

The subject residential rehab is in climate zone 5A, built in the early 2000s with 2×6 studs sheathed with OSB, mechanically-attached Tyvek and 4″ brick veneer.  It was procured in the gutted-to-the-exterior-sheathing condition; all stud bays are empty and the ventilated attic is exposed.  On windy days, air leakage can be felt in some of the empty stud bays.  Submitted for your comments is a proposed air-sealing and insulation plan intended to be implemented entirely from the interior.

Spray foam and cellulose are intentionally not specified.  Preferred is a DIY approach using high density R-21 fiberglass batts, easily cut and fit tight against obstacles and odd, irregular stud bays.  The goal is pretty-good house (not passive house) performance.  It is realized that during the coldest winter months, the OSB temperature may be below the dew point, but sorption at the OSB due to interior air leakage should be greatly limited by the interior air barrier.

Exterior Air Barrier:  Roll on a latex-based fluid-applied air barrier (Visconn, 26 wet perms per 30-mil thickness) onto the OSB to air seal its porosity, leaks behind/through the Tyvek and leaks at the many protruding nails that missed the framing members.  Mock-ups have confirmed the ability of Visconn and Visconn Fibre to seal 1/8″ and 1/4″ gaps, respectively.  Apply Visconn to the 5-1/2″ faces of the studs and plates and to their connections to the OSB.  Apply Vanna tape (475 Bldg) at all side-by-side connections between multiple plates/studs/headers etc.  Backer rod and caulk window units to rough framing.  Install foaming shells (BoxShells) around electrical boxes.  Install ceiling drywall and seal to wall plates.  Run blower door test; address leakage.  Insulate between the studs with high density unfaced R-21 fiberglass batts.  Unfaced wall batts are specified to eliminate R-value loss at voids due to inset stapling and to permit drywall adhesive.  Blow attic insulation.

Interior Air Barrier:  Glue, screw and air seal drywall using drywall gaskets (Denarco, Constantine, MI) on plates, end/partition wall studs and window framing.  Prime drywall with vapor retarder primer (0.6 perms, Sherwin Williams).  Consistent with the acceptable performance of a simpler pretty-good house design approach, Membrain/Intello/SVRs were intentionally not specified.  Seal electrical boxes by injecting canned foam into the BoxShells.  Blower door test; Aeroseal as last resort.

Calculation indicates a relatively insignificant 4% decrease in the total wet perm rating of the OSB (1 perm)/Tyvek (50 perms) when the Visconn (26 perms) is included.  Questions:

·         Has the effect of the Visconn on the overall perm rating of the exterior side of the wall been properly interpreted?

·         Is the air sealing and insulating plan consistent with a pretty-good house design approach in this climate zone?

·         Other comments on the proposed design approach are appreciated.

Thank you for your comments.