Typical of a Cape, the second-floor interiors have knee walls on either side; the walls slope inward from the top of the knee walls following the 9:12-pitch roof lines to a narrow, flat ceiling. On the left, a closet has been built in front of the knee wall. The homeowners use the space behind the knee walls for storage, which they access via uninsulated hatches from either the second-floor rooms or the back wall of the closets.
Kraft-faced fiberglass batts fill the rafter bays. Poly vapor barrier is installed in some locations and omitted in others. Much of the floor area of knee wall storage is above the outside (over a covered entry and a screened-in porch). While there is insulation between the floor joists, there is no thermal break. Cold from conduction and infiltration makes it hard to keep this space warm.
Here, IR imaging shows the wind washing of the fiberglass batts where the insulation baffles were butted end to end and were not air-sealed.
Here, the author investigates the unconditioned space behind the bonus-room knee walls. The bays of the vertical 2x4 web members of the attic trusses (which act as the knee wall) were infilled with 6-inch-thick kraft-faced R-21 batts, while the bays of the 2x10 bottom truss chords were infilled with 9-inch-thick kraft-faced R-30 batts. The ceiling insulation had substantial gaps and voids with some evidence of rodent nesting. The fiberglass batt installation was Grade II-level at best with large areas of Grade III work. (For information about insulation assembly grading, see Allison Bailes’ Energy Vanguard blog, “How to Grade the Installation Quality of Insulation” and JLC “Q&A: Installing Faced Batts.”)
IR imaging shows the space behind the knee wall as warm. This space should be cold, but poor air-sealing allows heat drive out of the home’s interior mainly through unwanted ceiling leaks, which caused ice damming issues along eaves and valleys on the garage wing.
Under the stick-framed portion of the garage wing’s roof, roughly 24 inches of flue pipe had been left exposed. The flue (connected to a wood-burning stove on the first floor) contributed to the warming of the unconditioned space behind the connecting “hall” and adjoining bonus room knee wall.
IR imaging of the chimney enclosure on the second floor (below the exposed flue pipe in the unconditioned space) shows a thermal bypass—‘a chimney within a chimney’ effect.
An interior ceiling leak, located at the juncture between the main house and the garage wing, was misattributed to ice damming.
The LVL beam supporting the covered entry was not thermally broken or insulated. On frigid days, warm interior air leaking into unconditioned space behind the bonus room’s knee walls would condense on the cold surface of the LVL beam a few feet into the interior from where it penetrated the exterior wall. The resulting condensation would drip through gaps between the framing to the drywall ceiling below.
The existing LVL beam (dark brown) below an attic truss. The LVL helps support the main house roof rafters.
Looking up at the ceiling in the conditioned space of the garage wing, IR imaging on the left shows the demarcation line between the warm bonus room floor (in foreground) and the unconditioned space behind the knee wall (the cooled ceiling in background showing up as dark purple and blue). Air leaks at a can light, along top plates, and the vicinity of the interior condensation leak are also shown.
The recessed lights in the garage wing were not air-sealed. Penetrations such as bath fans, plumbing stacks, and electrical rough-ins were also not air-sealed.