In the post above, two up, you can see that some of the cross frame members have a 1/2" strip of plywood above them. This is where the joint in the floor panels occurs. I elected to put in solid foam insulation, which required an adjustment for that 1/2" step.
I started by press fitting 1-1/2" thick foam into the webs of the main frame members (see photo in previous post). This provided a nice solid wall to end-butt the large horizontal pieces of foam.
The first horizontal layer was 1/2" thick and fit precisely between the edge of the plywood step and the next cross frame member. Below that I press fit in a piece of 1-1/2" foam, cut to fit the entire space between the cross frames and from main frame to main frame (some were pieced together). All surfaces were coated with liquid nails. In addition, 3" deck screws and 3/8" thick particle board washers were used to retain the foam. This allowed the deck screws to have 5/8" purchase into the plywood above. You can make the washers with a hole saw with the guide bit removed (the guide bit hole is too big to retain the head of the deck screw--gotta put that hole in later).
After all that, I injected space-filling foam along the edges and into any gaps. This serves two purposes. First, zero air infiltration. (Necessary? probably not.) Second, this stuff is great glue and I want those pieces of foam to stay put [forever]. You can just see a little of the space filling foam oozing out along the left edge in this photo. You can also see that I used cable mounts to firmly attach the brake wires to the wooden frame members--I always worry about them bouncing around and fraying.
What's the down side? All this stuff is waterproof and now pretty much air tight. A liquid spill that soaks the floor or finds it's way under the floor is going to be trouble--very long time to dry. I guess it's a tradeoff. Ask me in 5-10 years.
I put an extra layer of 1-1/2" foam under the back access area. There are lots of pipes back there that make me crazy when the outside temperature goes below 25 or so. I want all the protection possible for that space. By the way, the belly pan is 5" deep, so you can put a lot of insulation in there. But it's probably wise to keep it out of contact with the belly skin, since this area isn't sealed and you can expect some water to get in a run around on the skin.
You can't see it in these photos, but the POR 15 has been overcoated with Rustoleum Metalic Brite Finish--no name, but the cap is a sparkly aluminium. I did not use the POR primer, since the cure has only been going for three days. I did sand the heck out of it to get the bugs down from something like 60 grit to more like 120. Hey, the bumper and frame aren't a 57 Chevy, OK?
Next post will be the belly skins and we find out if my hole-finding technique flies!