One of the joists is also doubled. The lumber we bought was kinda crappy, so doubling the worst sticks was a way to compensate. The subfloor will have a seam there, so it adds more nailing surface, which we wanted because we're such framing newbies. Also, we're expecting an extra load in that area, so we're glad to have the extra strength. With better lumber and without the expected point load, I could have skipped the penultimate joist and left a 24" space instead.
Originally, we planned to lay the 4'x8' sheets of plywood subfloor along the joists. With that in mind, we started to install blocking to support the seams of the plywood. The 8' edges are tongue-and-groove, but the 4' edges are not, so they need the help. It's a little stronger to run the plywood the other way, so we changed the plans to lay the plywood across the joists. Still, we installed the blocking anyway, to help stabilize the 12' joists.
Here's the plan:
Once the floor system was framed up, we added insulation (before the subfloor). We chose 2" polyisocyanurate foam panels, in part because that's exactly what is under the yurt, so it's familiar. Also, rodents won't nest in it, like they might in fiberglass, and when I work in the crawlspace I won't be breathing fiberglass. We have a lot of wooden stakes (Buffy would be proud) so we attached those to the sides of the joists, 2" below the top, to support the foam. Then we cut the foam to fit each joist bay. A few cans of Great Stuff expanding foam insulation filled in the gaps.
Looking back, I wish I had set the insulation a little lower, by maybe 1/4" or 1/2", so it wouldn't have a chance to interfere with the subfloor if it stuck out somewhere. Also, I could have cut the panels 1/2" smaller and filled in with Great Stuff for a tighter seal than trying to make panels fit tightly, and ending up with a gap that's too small to get the foam in to. Still, I think it will be fine. After the Great Stuff cured I went back with a saw and cut off the extra.
We could have waited wait until after the subfloor was in place before installing the insulation. Cut it to be 1/2" undersized, then glue it to the underside of the subfloor. Random-sized pieces can be fitted together to fill the bays. Great Stuff fills in gaps. You can even wait postpone this work until the building is complete, so you can see how the building works without insulation to decide how much to put in later.
Another alternative is to put the foam panels on the underside of the joists. You can use the sheets whole, with only a few cutouts for posts or whatever. The almost continuous insulation would be more effective. And you could fill the joist bays with batts or loose fill insulation, and get a super-insulated floor. That's out of place in this building, however.
Once the insulation was done, it was time to install the subfloor. I used 3/4" T&G Sturd-I-Floor, which is rated for 24" joist spacing, for extra stiffness. The yurt floor is 1 1/8" on 4" joist spacing, which is within spec but very springy. (If I could go back, I would install 2x4 joists under the yurt floor, across the existing joists. Even if they were only 24" O.C., it would be much stiffer.)
We applied subfloor adhesive to the joists, then dropped on a sheet of plywood, then wiggled it in to position. It's easy to move when it's floating on a bed of wet glue. When it was in position we started screwing it down. The board sheet wasn't completely flat, so I used my weight to press down the plywood as each screw went in. Here's the first board:
Because our joist spacing is a little inconsistent (16" O.C., except the last 2 that are 12" O.C.) we had to be careful about how we set out the plywood to get the edges right over a joist. The layout is 8' + 2', 4' + 6', and 8' + 2'.
Getting the tongues in to the groove was harder than I expected. We used a sledgehammer, but the insulation panels made it hard to get a good hit. It might have helped to spend some time with a water spray on one side and the sun on the other to take out the warp before installing.
Once all the subflooring was down, we went back and cut the panels again. They were a little long. I wanted them to be slightly short, so I could be sure they wouldn't interfere with the wall sheathing that connects to the joists. We measured carefully, cut, measured again, cut again. Like the old saying.
The resulting floor is really nice to walk on. Very firm and quite level, except one corner where a beam had a bend up at the end.
Looking back, I wish we had used a 3rd beam down the middle of the floor. We could have dropped the joist size down and gotten a stiffer floor. No joist hangers required. I am considering adding it later.
I know it's a good floor because it attracts beautiful women:
