So this week, I had an idea on how to gain some extra insulation under my bus without losing any interior space. I spent a lot of time thinking it through and doing some research and measuring, and decided to document the idea in case it might work for someone else. I plan to implement this idea in my own bus.
Now this will work specifically for Thomas busses for sure. Other brands may do their floors differently. On all 3 of the Thomas busses in my yard, the flooring is done identically.
Basically, on a Thomas bus, the floor consists of C channels laid crosswise on top of the frame rail, and attached to each other somehow. They are 9.5" on center on unleashed, but I haven't measured any of the others yet as it is cold out there. In unleashed, the space "inside" the C channel is approximately 2" of vertical space by 9.25" of horizontal space. This space extends all the way across the bus, and is repeated many times from front to back.
Now, my idea is to somehow seal this volume and then fill it with slow expand cavity fill foam. Tiger Foam has some that is reasonably priced, and R7 per inch. This is a closed cell foam that stops air flow, moisture, and sound in addition to being great in a fire and better than RMax insulation value. It does need to be protected from UV though.
After installing a small insert into the opening of the channel, I will caulk it down, using the caulk to both adhere the insert and prevent moisture from getting up between the insert and the channel.
Once it is enclosed, simply drill a small hole from above, and fill with foam.
My plan is to use a thin (but not too thin) gauge of galvanized steel. I will figure out the size I need, and the number, and order it precut. I will order some extra, as I have some obstacles. I figure I can cut pieces down and overlap them and caulk them in order to get past obstacles.
In my floor, the only thing within the volume of the channels is the protrusions from above (nails holding the flooring) and the clamps that attach every so often to hold the whole floor/body to the frame rails.
This is not a replacement for insulating the floor inside. The abutting pieces are still made of steel, and will conduct heat quite nicely. It is just like a stud in a wall transferring heat from one side to the other. But if you combine this with an inch or two on the inside of your floor, you can get R21 to R28 with only 1 to 2 inches of height loss inside for insulation.
In my application, I plan to put at least 2 more inches on the inside of my floor, then put minimum 3/4" plywood on top of the insulation.
I'm also considering getting a rhino-liner type coating to put on the bottom after the galvanized insert and foam in place to help protect everything better. I figure it won't hurt anything.
I hope this idea helps others,
[edit: removed the incorrect fire rating information]