We have a house in NE PA, a rancher built in 1989 that we’ve had for a few years. There is a 15×15 addition on the back of the house that started as a regular deck, had a roof added and became a screened in porch and probably about 15 years ago had walls added.  

Walls and ceiling are insulated, floor is not.  The floor has plywood and underneath that is the original PT decking with 2×10 floor joists supporting the structure. The room is exposed underneath about 18” off the ground.

Without insulation, the room is basically unusable in the winter.

 Was thinking about getting closed cell spray foam but it’s $$$$$.  Next thought is to cut and cobble 2” rigid foam up against the floor and fill the rest with rock wool followed by plywood on the bottom.

Searched the FHB site and so many results show that are ancient and generally talk about fiberglass.

Any suggestions and do I have my order of installation right?

Thanks,

Bob