The Usual (and Not-So-Usual) Air-Sealing Suspects

Access Holes

Holes were cut in the gable-end of the garage wing to access the spaces behind the bonus-room knee walls. (Note the raking of the roof’s perimeter of snow; the homeowner had to be diligent about snow raking to help prevent ice damming along the eaves and bottom of valleys.)

Prepping for the Primary Air Barrier

The 2x4 web members of the attic trusses, which act as the knee wall, were infilled with 6-inch-thick kraft-faced R-21 batts. Two inches of the existing fiberglass was removed from the back of the batts to prevent compressing them when the crew installed a layer of 2-inch XPS insulation (the XPS would serve as the primary air barrier between the bonus room and the cold space behind the knee wall). Compressing the batts would affect the insulation’s “lofting,” reducing its performance.

Air-Sealing Bath Fans and Recessed Lights

A crew member replaces a length of 4-inch flex duct run from a bath fan with 4-inch rigid PVC piping. The PVC duct sloped 1/4 inch per foot toward the soffit vent. In the foreground, a recessed light is air-sealed with a site-built OSB cover foamed in place.

Air-Sealing the Bath Fans

Here, new 4-inch rigid PVC is connected to a short length of 4-inch flex duct run (the existing bath fan was located under the bonus-room floor and difficult to access).

Miscellaneous Air-Sealing

On the left, Wigluv tape is used to air-seal the wall-to-floor juncture and an existing junction box is foamed in. (Junction boxes often have multiple holes in them and can be tricky to air-seal.) On the right, a plumbing vent stack and electrical wiring are air-sealed. The 2-inch XPS insulation primary air barrier (dark gray) is cut around dropped ceiling and foamed to the drywall.

Dropped Ceiling

At the junction between the conditioned space and the unheated garage, the ceiling drops down roughly 18 inches from the 10-foot-high garage ceiling, shown in the background. The unheated garage’s ceiling is strapped with a poly vapor barrier; the dropped ceiling doesn’t have strapping or the poly.

Stopping Thermal-Bridging

The interior leaking caused by condensation (left) was remedied with spray foam (right). The exposed, “cold” end of the LVL beam was hit with a froth pack (red arrows).

Propavent Insulation Baffles

The new insulation baffles were taped at their articulated bends to help prevent them from breaking during installation (and over time). Loose-fill cellulose was blown in under the bonus room floor, on top of the ceilings behind the knee walls, and below the tops of the Propavents. See illustration, “Air-Sealing the Garage Wing,” on main page.

Ineffective Thermal Control

Note the existing ineffective thermal control where the garage wing intersects with the main roof. The main roof under the stick-framed section forming the valleys had been left partially sheathed with large areas of exposed fiberglass and Propavent insulation baffles loosely placed.

Fixing a Tricky Area

To cover the exposed batts and to make sure the existing baffles would keep venting to the main roof’s ridge vent, Majvest diffusion-open membrane (blue) was taped with Wigluv high-performance tape on the back of the existing baffles; the Majvest was then taped to the sheathing to create a continuous air-seal. Also, the short section of exposed flue pipe was covered with coilstock and Rockwool (red arrow), while 2-inch XPS insulation (dark gray) was mechanically fastened to the connecting hall’s 2x6 framed wall and foamed along the edges and seams.

Fixing a Tricky Area

Here above the foamed-in LVL beam, the Majvest was taped on the back of the existing baffles and then taped to the existing roof sheathing. Not an ideal assembly, but given the client’s budget and desire to avoid interior demolition, the author wanted to create a continuous air-seal, making sure the baffles would keep venting to the main roof’s ridge vent.

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