Radiant barrier vs blowing in more cellulose for a 100-year-old house?

I dealt with this exact setup in my 1910 build over in Winona. Radiant barriers sound like a quick fix, but honestly, the heat isn't just coming trough the roof deck; it's migrating from the attic floor through every wire penetration and top plate you haven't plugged yet. I wasted a summer trying those foil rolls and it barely moved the needle. What acutally cooled my bedrooms down was crawling into those knee wall spaces with a couple of cans of Great Stuff Pro and some rigid foam board. I blocked off the floor joists under the knee walls—that's where the hot air from the eaves realy sneaks into your floor system. Once I sealed those gaps and blew in about 15 inches of GreenFiber cellulose, the AC finally started cycling off. If you just pile on insulation without sealing the air leaks first, you’re basicly just putting a sweater on a guy who’s standing in a draft. Focus on the air bypasses first, then go heavy on the R-value.

I think you are looking at it the wrong way if you are just focusing on the floor. In my 1920s place, I spent a fortune on R-60 cellulose and it just felt like a giant heat sponge by 4 PM. Once that mass gets hot, it radiates into the bedrooms all night long, and your AC will never win that battle against the thermal lag. Instead of more bulk, I would look at getting a powered attic fan. I put in a gable-mounted unit from Air Vent Inc. and it was the only thing that actually dropped the temperature in those cramped knee wall spaces. You need to pull that 140-degree air out of the peak. If you do not have enough intake from the soffits, the fan will just pull your expensive AC air up through the walls, so make sure you clear out any old bird nests or painted-over vents first. That active airflow makes a much bigger difference in Minnesota humidity than just adding another foot of fluff.

I'm still a bit lost on the logistics of all this. If I'm crawling into those tiny knee wall spaces, how do I acutally find those air leaks or 'top plates' you mentioned? Is there a specific tool for that, or do I just look for dusty spots? I honestly don't know much about the diferent materials yet, but is cellulose relly that much better than the pink fiberglass rolls at stopping the heat? I just don't want to spend a weekend up there only to realize I missed the most important spots beacuse I didn't know what I was looking at.

Look for the 'dirty' fiberglass. In my 1920s place, any insulation that turned grey or black was a dead giveaway for an air leak. It acts like a filter for the house dust moving thru. The real game changer for me was the sloped ceilings. They’re basicly giant radiators. I used a hole saw to dense-pack those rafter bays with cellulose from the attic side. If your slopes are hot to the touch, you’ll never feel cool regardless of what is on the floor. That stopped the 'oven' effect instantly and finally let my AC catch up.

Oh man, this is exactly what I needed to read! I'm just starting to look at my own attic mess and I'm super pumped to finally fix the sweatbox situation. The idea of air leaks turning the insulation grey is wild—I definitely have some 'dirty' looking stuff up there. Speaking of old houses though, has anyone here ever tried using those thermal cameras that plug into your phone? I'm obsessed with the idea of seeing the heat bleeding through the walls in real-time, or is that just a total waste of money for a DIYer?

Get the FLIR ONE Edge Pro for your phone. It’s a game changer for spotting those leaky top plates and hot studs in 1.5-story homes. While you’re at it, don’t ignore the knee wall doors; they are usally just uninsulated slabs of wood. I glued 2-inch R-Tech rigid foam to the back of mine and used some MD Building Products weatherstripping to kill the draft. If those doors aren't airtight, all that GreenFiber you blow in won't mean a thing when the stack effect kicks in.