Spent the weekend sealing my 1930s attic and learned a lot

I had a similar nightmare with my 1925 bungalow last winter. Once I pulled up those old pine boards, I realized the previous owners had just shoved newspaper into the wall headers. I ended up using about six cans of Great Stuff Pond & Stone becasue I liked the black color for visibility, but for the smaller gaps around the electrical boxes, I switched over to a 100% silicone caulk. It's less messy than the foam when you're working in those four-inch clearance spots near the eaves. One thing I learned the hard way was checking the top plates of the interior walls. Even if the floorboards look tight, there’s often a massive gap where the plaster meets the wood. I used some Roxul Mineral Wool scraps to stuff the bigger cavities before foaming over them to save on costs. It definately helps with the drafts, but keep an eye on your attic ventilation now that you’ve sealed it up tight. I had to go back and install a few more soffit vents because my humidity levels spiked once the house stopped "breathing" through the ceiling.

I went thru the same struggle with my 1940s colonial, but I quickly gave up on the individual straw cans of foam. After the second one clogged halfway through, I bought a Great Stuff Pro 14 dispensing gun and the larger 24oz cans. It sounds like an extra expense, but the precision you get for those tiny gaps between the joists is night and day. Plus, you can leave a can on the gun for weeks without it curing shut, wich saved me so much frustration when I had to finish the job over three separate weekends. One thing I ran into that caught me off guard was the gap around my masonry chimney. My inspector warned me not to use standard expanding foam there beacuse of the heat. I ended up packing that 2-inch gap with Roxul Comfortbatt strips and topping it with a bead of 3M Fire Barrier sealant. It took a bit more time than just spraying foam everywhere, but knowing the house is actually up to code gives me a lot more peace of mind when the furnace is running hard in January.

Don't stop at the top plates. If you've got those old 1930s light fixtures, check the electrical boxes. Most guys forget the wire penetrations. I hit mine with 3M Fire Block Sealant becuase it holds up better than the standard stuff if the wiring gets warm. Also, once you finish the air sealing, don't just throw fiberglass back in. Blow in at least 15 inches of Greenfiber cellulose to hit R-60. It’s way better at deadening sound and stopping convective loops than the pink stuff ever was. Did you check the chimney flashing? That's usally the biggest air leak in those old bungalows.