How should I insulate a daylight basement with 2x6 framing in Snohomish County?

That 1-inch gap you have is actually a huge advantage. You should definately slide **XPS foam board** (the pink or blue stuff) behind those studs against the concrete wall. Tape all the seams with a solid flashing tape. This creates a thermal break so your warm interior air isn't hitting that cold concrete and condensing. If you just stuff R21 batts in there against the concrete, they’re going to get damp eventually.

Since you're in Snohomish County, stay away from the standard 6-mil plastic vapor barrier on the interior of those below-grade walls. It's a common mistake that traps moisture in the wall cavity and leads to rot. Instead of fiberglass, look into **Rockwool mineral wool**. It handles the humidity of Western Washington way better than fiberglass, it doesn't lose R-value if it gets a bit damp, and it's much easier to cut around outlets and wires. For the parts of the wall that transition to wood siding above grade, you can keep the same foam and rockwool setup for consistency. It makes the drywall phase easier if your wall depth is uniform. Just make sure you're paying attention to **fireblocking**. Inspectors around here are sticklers for it. You’ll need to seal that 1-inch gap at the top plate and every 10 feet horizontally so fire can't race trough the void behind your framing. Check the county website for thier handout on finished basements. They usually have a specific diagram for how they want the insulation and fireblocking layered. Doing it this way costs more upfront than the cheap batts, but it’s the right way to build in a climate as wet as ours.

That fireblocking part sounds like a bit of a headache. I hadn't even thougt about the gap at the top of the wall. Is that somethig I can just fill with that orange fire-rated spray foam, or do I need to cut actual wood blocks to fit in that 1-inch space? I’m also wondering about the Rockwool—since it’s heavier than the cheap fiberglass stuff, do I need to do anything special to keep it from sagging in those 22.5-inch bays, or does it just friction-fit well enugh on its own?

You can ussually get away with the orange fire-rated spray foam for a 1-inch gap, but check if your local inspector prefers mineral wool stuffed in there tight—they can be picky. As for the Rockwool sagging, it’s acutally the opposite. It’s stiff enought that it stays put much better than fiberglass ever will.

One thing you didn't mention is your bottom plate. Since you're on a slab, make sure that bottom 2x6 is pressure-treated or you’ve got a sill sealer underneath. If it's raw wood touching concrete, the inspector will flag it immediately for moisture concerns. It’s way easier to slide a gasket under there now than to swap a plate later.

That bottom plate absolutely has to be pressure treated since it's on the slab. If it's alredy framed with regular lumber, you might need to slide some sill sealer under it or use a specific adhesive if the inspector allows it.

Also, keep an eye on your total R-value. In Snohomish, they’re usually looking for R-15 continuous or R-21 in the cavities. The 1-inch XPS only gets you about R-5, so you’ll need those thicker batts to meet code. definatly seal the bottom edge of the foam to the concrete with some canned foam to stop air from drafting behind the wall.