Thoughts on floor insulation

[Phssthpok]

So, spring is here (finally), I managed to get the road up to my place 'dozed, and a couple of pads cut for building. I'm now at my friends place disassembling my cabin for moving. I was lucky that my source of (free) solid foam insulation was overflowing and managed to get another load (similar to my previous load).

With this load I should have enough to insulate not only the gambrel roof but also the floor. I've had enough of trying to make 'exact' cuts in solid foam to 'press fit' between studs, so this got me to thinking about the best/easiest way to do the floor joists.

The best theory I've been able to come up with so far is to run some wire or string on the underside from joist to joist, cut the insulation short (say 1/2" in each direction for about a 1/4 gap around the perimiter), and let the wire/string support it while I glue it in/seal the perimiters with that 'foam in a can' stuff. A friend had suggested doing the same with heavy plastic as a vapor barrier, but I'm afraid it would 'sag' under the weight of the 5.5" thick foam leading to a sloppy installation.

Another, more ambitious, friend suggested assembling the floor joist, tacking on plastic, then some 1/2 plywood, THEN flipping the whole assembly over and installing the foam.

I'm all for overbuilding things, but I don't think he's got a grasp of how heavy 12'x16' of 2x6PT skinned with 1/2" ply would actually be!

So... is there a better way of doing this, or is the 'support' idea the best option?

[John Raabe]

If you are going to skin the bottoms of the joists and then drop in the foam boards from above I would suggest using a air barrier but not a vapor barrier - a material like Tyvek or Typar. To keep critters out and to prevent sagging finish off the bottom with lath strips or chicken wire or (best) both to support the insulation and air barrier.

Then you have good support in place and you can the fill the joist cavity with the scrap foam board (not mattering too much if it fits). Finish off the voids with attic type pour-in cellulose insulation. It will fill the cavity and be easier and cheaper than canned foam. Get the deck on as quickly as possible and try to keep it dry (thus the reason for the air barrier - a poly barrier would just trap moisture).

In fact that is the main reason not to do this. You can trap moisture in the floor system if it rains before the roof is on.