Batt Insulation in Kensington: Half-Story Knee Walls, Stripped Rooms and Attic Floors

"Kensington" names two places with different rulebooks. The Town of Kensington covers about half a square mile and roughly 2,100 residents; the 20895 postal area around it holds close to 19,000. Census figures put the median house in both at 1958 or 1959, but the mix differs. Across 20895 about 78 percent of homes are detached and the 1950s are the biggest decade. Inside the Town only about half the units are houses, and about a third sit in buildings of 50 or more units. A batt needs framing we can see and reach: a half-story with knee-wall doors, a room stripped for a remodel, an attic floor under a tired layer of old fiberglass. Finished plaster and drywall are a different job.

Six steps from the attic hatch to the drywall crew

1. Walk the framing. We measure bay width (16 or 24 inches on center), bay depth and the attic floor, and look for wiring before anything is cut.
2. Seal the gaps. Top plates and wire holes get sealed, and a bath fan hose that ends in the old fill is run to a roof cap. ENERGY STAR says to seal before insulating.
3. Set baffles at the eaves, so a 1-inch air space stays open under the roof sheathing.
4. Pick thickness by the bay. A 2x4 bay is 3.5 inches deep and takes R-13 or the denser R-15; a 2x6 bay takes R-21 at 5.5 inches. On an attic floor we lay unfaced R-30, about 8.25 to 10 inches thick, crosswise over the old layer. A batt pushed into a shallower bay loses the R-value you paid for.
5. Work around heat and age. A recessed can that is not IC-rated gets a box that keeps insulation 3 inches clear, and knob-and-tube wiring is never covered, so in the oldest houses an electrician looks before we finish that pocket.
6. Photograph, then clean. Every bay is photographed before drywall goes up, and the offcuts leave with us.

Nothing requires a set R-value in a house that is already built. ENERGY STAR's suggestions, R-49 for an attic with 3 to 4 inches of fill and R-60 for a bare one, are goals, and the energy code governs permitted new work and additions.

Two batt scopes built around Kensington houses

Half-story bedroom in a Queen Anne house in the Kensington Historic District, near Carroll Place

The top floor is being finished as a bedroom, which leaves two knee-wall pockets and 380 square feet of attic floor behind them. The pockets are separate lines at $600 to $1,800 apiece, so $1,200 to $3,600 for the pair, and floor batts at $1.20 to $2.00 per square foot add $456 to $760. The job lands at $1,656 to $4,360. Expect a low crawl behind the knee-wall doors and bays that vary in width, so batts are cut one at a time. Baffles go in at the eaves, and any knob-and-tube we meet is left for an electrician. It is interior work, so no historic area permit is involved, and it takes a day.

Sunroom rebuilt to the studs on a 1950s rambler in South Kensington, plus the attic floor over the rest of the house

The sunroom's two exterior walls are open 2x4 framing and the drywall hangers are waiting. R-15 batts are cut to each bay and split around the new wire. A single-room job sits in our $800 to $2,500 band, and a room this small belongs near the bottom. The main attic floor is about 1,050 square feet with roughly 5 inches of old fill; unfaced R-30 laid crosswise, working toward R-49, costs $1,260 to $2,100 at $1.20 to $2.00 per square foot. Together that is $2,060 to $4,600. The hatch is about 22 by 30 inches, so every bundle goes through it, and the bath fan hose is re-run to a roof cap. Plan two days: walls first, attic second.