How I finally figured out why my blown-in attic wasn't keeping the house warm

Can vouch for the air sealing part. I did the insulation top-up in my 1962 ranch two winters ago and skipped sealing, and the difference was underwhelming. Then last fall I spent a weekend with a couple tubes of caulk and a can of Great Stuff foam working around the wiring penetrations and the old recessed lights in the hallway, and the upstairs rooms finally stopped feeling like an open window was cracked. That first pass of sealing did more than the six inches of new cellulose did.

The attic hatch was my worst offender too. Mine was just a piece of plywood sitting loose in the frame, so I glued some rigid foam board to the back of it and stuck weatherstripping around the frame edges. Held a lit incense stick near it before and after and the smoke told the whole story. One thing I'd add for anyone doing the top-up: rake out the old insulation near the walls before you start blowing so you can acutally see the top plates of the exterior walls. I found a gap the width of my hand along the whole north side of my house that way, and filling that made a noticable dent in the draft coming down behind the baseboards in the kids' room.

One thing about the depth check is that the material up there matters as much as the depth itself, because blown fiberglass and blown cellulose do not settle at the same rate or give the same R-value per inch. I glued a strip of wood with inch marks on it and leave it poking up through the insulation near the access opening so each fall I can eyeball how much it has dropped without crawling around. If you are unsure what the previous owner used, a small pinch pulled out and looked at close will tell you pretty fast, since fiberglass looks like spun sugar and cellulose looks like grey shredded paper.

One thing I learned the hard way up there: only step where the joists are. Put a foot between them once and went halfway through the ceiling into my kid's room. Now I toss a couple of scrap boards across the joists and kneel on those instead. Gives you a path to crawl along, keeps your weight spread out, and you're less likely to crush what's alredy down while you're working.

Reading this makes me wonder if my situation even applies. I've got cathedral ceilings in the upstairs rooms, so there's no open attic space to blow anything into, just a narrow gap between the ceiling and the roof deck. I get cold spots along the ceiling edges in winter and I've never figured out if that's the insulation being thin in there or air coming in around the rafters. Is there even a way to check what's in that gap without opening up the ceiling? Feels like half the advice on here assumes you've got a crawlable attic and I've got about eight inches of space.