Insulating Beneath a Bay Window Floor

There's sometimes a fine line between insulating and renovation. Today's task was equal measures of both. The north side of the house features a bay window in the dining room, a humble adornment that breaks up the monotonous lines of a four-square house design. The bay window adds about 36 square feet of floor area with no conditioned space beneath it:


This infrared image, taken from the interior during our March 2009 energy audit, clearly shows heat loss experienced in this area. The cold area, where the floor intersects the baseboard, is in blue. Note also the heat signature of the air behind these fairly sheer curtains!


An exterior shot shows the 1"x5", tongue-in-groove planking that clad the underside of the 2"x9" floor joists:


I knew that 30 plus years ago, some fiberglass insulation batts had been stuffed into joist cavities from the inside. But as I suspected, there was no attempt to air seal those cavities, certainly because of limited access. So with hard-hat, dust mask, and crowbar, I yanked off the 1"x5" external cladding, yielding full access to the joist cavity for the first time in 94 years. The insulation batts were poorly-fitted. They were also soot-stained, which is perfect evidence of air infiltration/exfiltration-- an action which effectively renders the insulation useless. But now at least, the joist bays were fully exposed. The gaps over the sill are wide open, leading straight into the basement utility room:


My insulation solution involved 2"-thick polyisocyanurate foam boards rated at R-12. These were cut for friction-fit, then sealed with expandable foam. Horizontal slabs (about 32"x16") were applied directly to the underside of the floor. Smaller slabs (7"x16") were placed vertically in the bays, directly over the sills. Per the insulation manufacturer's instructions, the reflective foil side faces the exterior:


I'm not done yet. The bays really should use a second layer of insulation boards, giving me a total of 4" thickness and a cumulative R24 insulation value. However, I ran out of time today. For now, there's some 1/2-inch, B-grade, water-sealed plywood screwed into place. After the area is fully insulated, I will re-clad the under-surface with a no-frills trim that is suitable for this little-used utility room entrance.

This image also reveals the archeology of siding applied to this house over the years. The outermost layer is aluminum siding, which I'll guess was applied in the 1970s. The aluminum siding came with its own sub-surface of expanded-cell polystrene panels-- the same stuff that's used for styofoam cups. Beneath that is a layer of asbestos (!) shingles. Then comes the original finish layer of cedar shingles. There were applied directly to the structural cladding of 1"x6" planks fitted horizontally across the studs. As you may know, that was THE way to construct wooden-frame houses before plywood with waterproof adhesive became available in the 1930s:


This task led to the discovery of some unexpected treasures. A previous owner, probably during the Great Depression of the 1930s, used these joist bays to hide away Christmas presents for the kids. The frugal homeowner apparently reused boxes from year to year. Over time, some were forgotten. These empty boxes, tokens of old-house charm, fell out when I removed the bay window's old under-surface cladding:


If you're curious, the card reads "To Helen, from Uncle Elmer and Josie."

[Update, August 3, 2009] Now it's done. A mildew-resistant latex paint was applied to the new plywood cladding: