Turning a slab into climate controlled interior space

[juddspaintballs]

I have a 12x28' sun porch area on one end of my house. It has a poured slab foundation that is ground level in the back and about 3' above ground level in the front. I am going to remove all of the glass doors on the sunporch and frame it up, insulate the walls, sheath and drywall it in. I'll insulate the attic space (common roofline with the house). The concrete floor sits 14" below the floor of my house. Since I have the ceiling height in that area, I want to keep the floor level with the house, which means I need to raise the floor with joists. I'm also putting my washer/dryer out there and that will make plumbing the drain easier. I figure I can rest the 2x8 joists on 2x6 ribbon boards at the studs and maybe throw a 2x6 center beam in for extra support under the center of the joists. Sub floor on the 2x8's, then flooring material on top of that.



How do I insulate and vapor barrier this floor? House is in eastern WV and I am going to put a mini split out there to keep the temperature close to what the house is. There will still be a door between the main house and what is essentially going to be our mud and laundry room. There is no access under the concrete slab.

 

[juddspaintballs]

Looks like I should lay down 6 mil plastic sheeting over the whole floor and then put my ribbon boards and joists over that. Insulation? I could hang faced fiberglass batt between the joists for insulation?

 

[kaymccampbell]

Sounds good, but you should fill the joist bays full of insulation.

 

[rjacobs]

You could paint the slab with something like redguard, which would also water proof it, although I dont know about its properties as a vapor barrier.

Otherwise, yea use the thick plastic sheeting.

As far as insulation, thats a tough one because it sounds like you will have no air movement in the joists in question so IMO mold and mildew could become a problem. Can you vent each joist space with a small vent? If not, I would probably do spray foam.

 

[juddspaintballs]

I checked with a handyman friend of mine. He said 6 mil plastic on the concrete would be fine, and if I extend it past the concrete, I can frame the walls right on top of it without needing a pressure treated bottom board. Then wrap the plastic up a few inches on the outside of the framing and staple it on. Run the 2x8's as the ribbon boards and also a center support board, then string the 2x6's across those. There will be a 7.5" space between the plastic and the bottom of the 2x6's in that case, but it won't affect anything since it's a sealed space. I'll put rockwool insulation in between the 2x6's and then subfloor over that. There won't be a vapor or moisture barrier between the subfloor and rockwool, so the insulated space and the void space under it can still "breathe" through the subfloor in the unlikely event any moisture finds its way in there.