Finally figured out why my Cape Cod upstairs was a literal oven

I went trough this exact same headache in my 1945 Cape last summer. I tried stuffing pink fiberglass against the rafters originally, but without a ridge vent or any soffit intake, it just trapped the moisture and heat against the roof deck. I eventually pulled it all down and focused on the living space boundary just like you did. One thing that realy helped me was using R-23 Rockwool for those vertical knee walls. Since it’s denser, it doesn't sag over time like the fiberglass batts do, expecially when you’re trying to secure them behind those tiny access doors. I also used a few cans of Great Stuff Pro to seal the junction where the floor joists run under the knee wall. Even with the floor insulated, I found that wind was still blowing right under the floorboards from the unconditioned eaves. It’s amazing how much better the AC holds the temperature now. My electric bill dropped about $40 a month after I stopped trying to fight the roof deck and just focused on the rooms themselves. It’s alot of crawling around in tight spaces, but it is the only way these old houses stay comfortable.

I actualy went the opposite direction with my 1952 Cape because I use those knee wall spaces for storage and didn't want my holiday bins melting in the summer. Instead of the floor, I hired a crew to spray three inches of closed-cell foam directly against the roof deck. Since it acts as its own air and vapor barrier, I didn't need the soffit vents that your fiberglass setup required. It cost more than DIY batts, but it effectively brought the whole attic into the house's conditioned space. My energy bills dropped by about 25% because my HVAC ductwork runs through those side crawl spaces. If you keep the insulation on the floor, those ducts are still sitting in a 120-degree oven all July, wich makes the A/C work twice as hard. It relly depends on if you need that extra storage space to stay cool or not.

Have you ruled out the stack effect pulling air from your basement up trough the interior walls? Even with the floor insulated, these balloon-framed Capes often act like a chimney. I'd be a little cautious about just moving the boundary to the floor. By cutting off that heat transfer, you might have created a new moisture trap in the attic rafters during the winter. Have you checked the humidity levels up there since you pulled the fiberglass? Sometimes the immediate comfort can mask a long-term rot issue if the air sealing isn't 100% perfect between the floors.

I did the same thing with the floor and knee walls, but the big win for me was the access doors. Most people forget them, but those little plywood panels are basially holes in your wall. I cut pieces of 2-inch rigid foam board and glued them right to the back of the doors, then added heavy-duty weatherstripping around the frame.

Before I did that, I coud feel a draft hitting my ankles every time the wind blew. Now that those doors are sealed tight, the bedroom actualy stays the same temp as the rest of the house. It was a cheap fix, but it made a massive difference in how the room feels.

I'm just starting to look at my attic and I'm honestly so confused by all this. Everyone mentions the knee walls, but what happens to the space behind them if you insulate the floor? Does that mean I can't use it for storage anymore becuase it'll get too hot? Also, I saw someone mention using that Great Stuff foam for the floor joists—is that hard to use for a total beginner? I’m worried I’ll just make a huge mess and not actualy seal the air leaks. How do you even tell if you've done enought?