Where exactly does the insulation go in my upstairs eaves?

The standard approach for these 1.5-story Cape Cod layouts is to insulate the **knee walls** (the short bedroom walls) and the floor of that eave space. This keeps the thermal envelope tight around your actual living area. If you go this route, you have to treat the eave like an attic: it needs to breathe. You’ll want to keep the roof deck clear so air can flow from the soffit vents up to the ridge. If you stuff batts against the roof underside without a specialized baffle system, you're begging for ice dams and roof rot because you've trapped heat and moisture against the plywood.

But before you even touch the fiberglass, you have to focus on **air sealing**. This is where most people fail. Those eave floors usually have joist bays that run straight under your bedroom floor. If you don't block those off, cold air just whistles right under your feet, making the insulation on the walls almost useless. Cut some rigid foam board to fit between the floor joists directly under the knee wall, and seal the edges with canned spray foam. It's a tedious, hands-on crawl, but it makes a massive difference in how that mini-split performs.

Since you have ducting in there, you should also wrap those silver lines in R-8 duct insulation if they aren't already. If you decide to insulate the roof rafters instead of the knee walls to bring the ducts into a 'conditioned' space, you must leave a 2-inch gap between the insulation and the roof deck for ventilation. Most people find it way easier to just stick to the knee walls and the floor. Just make sure the batts are snug against the drywall and secured so they don't sag over time.

Don't forget the small stuff while you're in there. Caulk the wire penetrations and seal around any electrical outlets on those knee walls from the backside. It feels like overkill when you're doing it, but stopping the actual movement of air is what stops the temperature swings you're feeling now. Wear a good respirator and long sleeves, because that old 1950s dust is no joke.

This is exactly the kind of stuff I've been looking for! I'm just getting started with my first home and honestly I'm so hyped to finally stop wearing three sweaters inside. My house has the same wierd half-story layout and it's been a total puzzle. Quick question though—since you mentioned that 1950s dust and sealing everythign tight, does that actually help with the 'old house' smell? Or am I just going to trap that funky attic scent inside my bedroom walls forever if I seal it up too well? I'm low-key obsessed with making this place smell like a normal house!

Listen, back in the day we used to just throw some rockwool or even sawdust back there and hope for the best, but those days are long gone. If you want to kill that 'old house' smell, you gotta get that 50s rockwool and mouse-nest-filled fiberglass out of there entirely. That stuff acts like a sponge for every smell and spill from the last seventy years. Once it's bare, hit those floor joists with a shop vac before you seal. And don't forget the knee wall door—it’s just a big hole in your house. I ussually glue a piece of rigid foam right to the back of it and add some heavy-duty weatherstripping.