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mph7794 06-19-2011 10:33 PM

Converting 3 season porch to 4 should I do the floor?
Hi there. I am starting a DIY 4 season porch In Minnesota and I am not sure how to finish the floor. The current joists are 2" x 8", 16" on center. The bottom of the joist are only about 12" off the ground, so it is not possible to get underneath to insulate. So, we are planning on running 1x6 deck boards on the underside of each joist (full length). Then, I will need to drop down some plywood first before insulating. My question is, what would be recommended for the plywood. I found this and was wondering if this would be sufficient:

Moisture Resistant Plywood Underlayment

Or do I have to use some sort of treated plywood? Also, would you put some plastic down before the insulation? As for insulation, I am thinking of doing a layer of sheathing before throwing in some fiberglass insulation. Is this all overkill? I am open to suggestions and tips. I am sure more questions will follow!

Ron6519 06-20-2011 07:42 PM

How were you planning on controlling moisture from the ground?

mph7794 06-22-2011 11:23 PM

So, after speaking with the inspector, I will have to get an r-30 insulation in the 7.25" space. He recommended spray foam but admitted it is high in price. He said we could use 6" of rigid foam to get the r-30 as well. We also need r-44 in the ceiling (also 2 x 8), so I got my first quote for spray foam and it was $1790. That is twice as much as my doors and windows!

Anyways, if I were to do the rigid foam, can the 6 inches of foam be suspended by wire zig zagging the joists (1.25" from the bottom of the joist) rather than using any plywood? does a vapor barrier need to be between the ground and the foam or between the foam and sub-floor. Or, is the foam itself which claims to be "impervious to moisture" provide the barrier? Thanks for the help!

Ron6519 06-24-2011 10:58 AM

I'd put a 6 mil plastic over the dirt to keep the moisture at bay. I'd keep it tight to the rooms perimeter so moisture doesn't escape at the edges.
I'd also install 2 foundation vents for cross ventilation.

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