Is there anything wrong with placing dow cornings r-3 rigid foam insulation that is a 3/4 inch (blue) 4 x 8 panels. Under 3/4 t&G plywood flooring 4x8 panels. Directly on top of joists. So in other words you would have the joists, then insulation, then subfloor. This material is very rigid and does not seem to give with weight on top of it. My biggest question would it degrade over time. This would be a concern because your walls are on top of this.
Another question I have is the T&G 4x8 panels are not quite 4 foot across. I am building a 12 foot wide cabin. That leaves 1/2 inch short on the back and front walls. Is that a concern since again the walls are at a 90 degree angle to the floor/rim joist edge.
It will provide more insulation value on the bottom of the joists as it will not be squashed between the floor and joists, will trap a dead air space and will give you a solid connection between your subfloor and joists. I don't know about other concerns but I know John is pretty well up on this.
This would also eliminate concerns about degradation over time.
Chris asks a question that has been bugging me: What is the allowable compressive strength of foam insulation?
I did a bit of phoning and was helped to the following (typical) location for a PDF file of the properties of OwensCorning pink Formular #250 insulation:
http://www.owenscorning.com/comminsul/documents/Foamular250.pdf
Looks like 25 psi is allowable for this product. Not much! This particular product would not be usable under floor tile ... atop subflooring. (If I read correctly). Disappointing.
Now here's a high compressive strength foam that does the job of supporting floor tile, (but not walls):
http://www.owenscorning.com/comminsul/documents/EXTFoamular.pdf
Bet it'll cost you two extremities!
I greatly appreciate all the feedback. I too have researched Corning's site. Seems that they are all into pink these days.(????) The insulation I have is blue 3/8 inch thick in 4x 8 panels. Runs $8.00 sheet. Which I thought was expensive enough for
3R. Seems like it has a thin strip of plastic on one side.
It sounds like you are using Dow Blueboard which is a closed cell extruded foam (R5 per inch). The highest use of this product is below grade (such as the inside of foundation walls or under slabs) — as this foam doesn't absorb moisture. It is also used as an exterior wall insulation over the structural sheathing and under the siding.
You could go underneath the assembled floor system and install panels to fit between the joists and staple the cut foam panels to the bottom of the subfloor. This would eliminate the question of compressive strength and give you almost the same working R-value. But why wouldn't you do the normal and cheaper thing of using batt insulation between the joists — much higher R at lower cost?
Not sure what the big picture goal is here.