Enclose existing patio
Hello. I have a 16’x28’ patio that I want to enclose as part of house remodel to make more living space. The existing patio is an 8” thick, 5,000 psi concrete pad (with 1/2” rebar in 1’x1’ grid). The edge furthest from the house sits on a 12” thick, 4’ deep wall with a footer. In each side at 8’ and at the edge of the house there are 6”x6” post that sit on 24” wide, 4’ deep concrete pillars under the pad. There is a 2”x12”x16’ header extending out from the house over the posts that supports the roof, which is tied back into the main roof. I am looking to enclose this with 2”x6” walls with a couple windows and sliding doors. On the concrete pad I am going to put plastic down for a vapor barrier and frame 2”x8” PT floor joists. I plan to fill the pockets with spray foam and 3/4” T&G will be the subfloor at the main house level. Inside will be conditioned space (heat and AC). My code officer is concerned because the original pad was not poured to be used for conditioned space. I do not have full walls with rigid foam down the sides or under the pad. Is that a big problem? Anything I can do to help make an existing pad more frost free?
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Depending on your local adoption of the various building codes, your building inspectors "concerns" could be mandatory requirements. Most modern energy codes require either underslab insulation or perimeter insulation extending down at least to the footings. The perimeter insulation is easier for retrofit work. If you are in a cold-winter area, you would be well advised to spend the effort to dig down and install perimeter insulation for energy savings and moisture management, if you can install drains to daylight. Alternately, you could insulate the floor of the new space with either foam between the concrete and joists, or batting in the joists. I prefer using the rigid foam as batts in this location are likely to become damp.
Your post also suggests that the concern is related to frost damage to the foundations. The risk of frost damage is not adversely affected by finishing the space, and the foundation you describe would provide more than enough freeze protection up to climate zone 6 or so.
Thank you very much for the comments. Yes, Zone 5 - New York. I do have access to the three sides and could excavate and do something along the edges. I have a local engineering looking into it as well.
Instead of framing subfloor I would just lay down rigid foam and have a floating floor with 2 layers 3/4" tg ply with staggered seams, glued, as you would to retrofit a basement slab.