Better to prioritize sealing drafts or adding more attic insulation?

I had that exact same issue with my 1994 ranch. I spent a fortune on extra blown-in cellulose in the attic and barely noticed a change in the humidity. It wasn't til I sat on the floor by my back patio door and felt the literal breeze coming thru the bottom corner that I realized where the cool air was going. If you can see even a sliver of daylight around your door frame at noon, your AC is just fighting a losing battle. I replaced all my old, flattened vinyl seals with Schlegel Q-Lon foam seals and it made a massive difference. Most people don't realize that those adjustable screws on the door threshold are actualy there to be used. I raised mine about an eighth of an inch so the sweep on the bottom of the door actually made a tight seal. I also added some M-D Building Products adhesive corner seals to the bottom of the jambs.

Doing those small adjustments to the front and side doors dropped my bedroom temp by three degrees almost instantly. It’s way cheaper than another foot of fiberglass and actually stops the humidity from creeping in from the porch.

I actualy disagree that it’s just about the gaps. In my expirience with 90s builds, the real culprit is often the door material itself. Those standard contractor-grade steel doors they used back then act like giant radiators. You can replace every inch of weatherstripping with the best silicone seals on the market, but if that metal skin is sitting in the sun, it’s transferring all that heat directly into your entryway. I used an infrared thermometer on my old front door and the inside surface was hitting 105 degrees while the AC was set to 72. Instead of just fighting the drafts, I’d look at the R-value of the door panel. I swapped my old door for a Therma-Tru fiberglass unit with a solid polyurethane core and the difference was night and day. It stopped that heavy, radiant heat from rolling into the foyer every afternoon. It’s a bigger upfront cost than a roll of foam tape, but if your door feels hot to the touch from the inside, no amount of sealing the edges is going to fix your humidity and cooling load issues.

You shoud realy examine the shim-space between the jamb and the rough opening becasue 90s builders notoriously stuffed those gaps with fiberglass batts wich, due to thier fibrous nature, allow air to pass thru while only providing minimal thermal resistance. Replacing that outdated material with a high-quality, low-expansion polyurethane foam specifically formulated for fenestration will effectively eliminate the hidden air leaks that your Ecobee is struggling to compensate for by sealing the entire perimeter against pressure differentials.

I'm realy new to this and honestly didn't even know you coud adjust the bottom of the door with screws. I always thougt those were just to hold the metal plate down! Does that adjustment work for any door, or do you need a specific type of threshold for it to move? Also, I keep hearing about 'door sweeps'—is that the same thing as the foam seals on the sides, or is it a seperate piece I need to buy? My front door has a tiny window in the top half, and I'm wondering if that glass is as bad as the steel doors people are talking about. How do you check for leaks if you can't acutally see daylight trough the cracks?

Don't forget the door's glass if you've got it. I've got a 90s place too and those side-lites or the glass inserts in the front door are usally just single pane. They'll bake your entryway in minutes. I didn't wanna swap the whole door, so I just grabbed some Gila heat-control window film from the hardware store. It's way cheaper than a new door and keeps the sun from turning my foyer into a sauna. If you've allready done the weatherstripping and it's still hot, check if that glass is burning to the touch. Makes a huge difference.

If you really want to kill those drafts and stop the heat transfer, you need to look at the Therma-Tru Smooth-Star entry door system. Those old 90s doors have terrible thermal breaks. I swapped mine out for a Smooth-Star with their specialized compression weatherstripping, and the difference in my hallway temp was night and day. It’s fiberglass, so it won't rot or conduct heat like the steel ones mentioned above. Plus, the way the door integrates with their rot-resistant composite sills makes a much tighter seal than any DIY adhesive strip ever could. definatly worth the upgrade over just patching up an old door.

So I tried checking the shim-space like suggested. Wow, those old fiberglass batts were basially just filters for hot air. I replaced them with foam and adjusted the threshold screws. The difference is night and day! The humidity is way down and the bedroom temperture finally matches the rest of the house. I'm glad I didn't just dump more insulation in the attic.