You don't want it air tight. Chickens poop puts of ammonia, and their respiration, etc., puts off moisture. This all needs to flow out through vents.
You want lots of ventilation up high, even in the freezing winter.
They can tolerate cold very well.
No drafts, on the chickens but good ventilation up high so the ammonia and moisture waft out.
I'd consider covering the eaves with 1/2 hardware cloth screwed in.
Then 3-4 windows that you can open in the warmer months, with the same hardware cloth screwed in
on the inside.
Thanks! I will post more as I go. I live in Central Oregon, so we really don't get a lot of rain. The inside doesn't have any water marks from leaks, but I'll keep that in mind. I am planning to swap out the lattice eves with boards to cut down on wind drifting through. How airtight should it be? The cedar siding has gaps between the boards (1/8" or smaller) that will let air in. Will that be a problem?
I agree with this, you need lots of ventilation and that ventilation up high is great, already done for you

so I'd definitely leave the lattice but do as was suggested and attach hardware cloth to the inside so nothing can get through the gaps.
As for the gaps in the shingles, I actually think that may be more of an issue than the lattice. The gaps may be small enough that it might not be an issue at all but drafts are the biggest concern and I'd be worried that might cause them. I'd say to test it out and make sure there aren't drafts.
But your walls are high enough that you can leave that lattice and have the ventilation be well above their heads. From what I've read you want them to be 12-18 inches below any ventilation so that it's not blowing right on them and then about the same distance off the floor or a bit higher and 18 inches off the wall. Of course this isn't entirely exact, do what works best, but the off the wall thing is because they need to have room to not hit the wall. But your coop is plenty big enough and plenty tall enough that of this should be no issue. The main thing with the roosts is to make sure they are ABOVE the nest boxes because they will sleep in the boxes otherwise. They want to be as high as possible. Now there's no need to make the roosts like 8 feet high, in fact I highly discourage that, but even just a few inches above the boxes is fine.
As for flooring, which you mentioned in other posts above, I currently have pine shavings in my run but they kick them out a lot. But I had nothing until a few weeks ago so I do not recommend that. What a lot of people do is put sand in the run with or without a layer or gravel first. Another popular idea is sort of a deep litter type thing but less work. You put down about 3 inches of wood chips (so skip the gravel and CHIPS not shavings, the different particle sizes are much better) and then 3 inches of leaves, grass clippings, pine needles, etc. And you don't even take the poop out because it will turn into compost and the chickens will turn it. And the best part is it's healthier for them and bugs and things will come and create an environment instead of just a run material and they can eat the bugs and have a grand time. The only thing you have to do is when it starts smelling, add more dry material. And you can constantly add to it.
Now, I'm no where near an expert, in fact, I've just regurgitated all this from other people from other threads but I very recently learned about this and it is the method I am going to use for my new run. Sand wouldn't work for us because the run is going to be very big plus I liked the natural environment feel especially since I can't always free range. Oh and that's another thing, it's a free option. Well, if you access to the stuff that is. We have a dump with piles of stuff plus half our property is woods so continous access to the leaves and stuff. Woodchips are one of the piles at the dump but could always rent a chipper or hire a tree company to take down some trees and ask them to leave the chips.