Old addition w/o footer needs rebuilding

[rai]

I have a problem room which needs renovation, and would appreciate some insight.

The room is an open addition built on a slab which is one step down from the adjacent main house basement. Digging next to the slab, it appears to be only 4~6" thick, no footer. There is a mortared flagstone kneewall on the slab and then framed walls above it to a shed roof. The walls are uninsulated and are largely filled by jalousi windows. It is roughly 18 feet wide and extends maybe 12 feet from the house.

Since there's no foundation under it, the whole room has sunken at least an inch since it was built. This is evident from marks where the slab used to be against the house and also the base of the frame walls has separated from the top of the kneewall near the house (the walls are attached to the house).

What I'd like to do is to rebuild and finish this room, and open up the adjacent basement room with a continuous, level floor. There are some load bearing wall issues, but this post is about how to deal with the slab and kneewall. The kneewall is actually attractive from the outside and blends with a retaining wall in the yard, so I wouldn't mind keeping it. It's generally in good shape (no cracking or mortar deterioration).

The slab itself is really AT grade. I don't have drainage problems, but it's really close. That makes me nervous about how to handle the under floor area.

I thought about excavating and adding a footer around the slab. I'm not crazy about this due to the shear amount of labor required. It would also mean framing the floor within the kneewall area on sleepers. This isn't exactly a bonafide crawlspace... and what about ventilation?

I also thought about sinking piers through the slab and building upon those. This is much less work than putting in a perimeter footer. I'd frame the walls up from the beam/joist floor and flash the wall exterior to the top of the kneewall. The same crawlspace ventilation concern applies.

I think the 'right' thing to do is to demolish the whole thing, excavate a proper footer and build up from there (slab, proper crawlspace or whatever); but I think that's prohibitive budget-wise.

I'm leaning towards the pier approach. Any thoughts on this and keeping it ventilated properly?

Thanks in advance!

[RobBase]

I'm sort of finding it hard to understand some of your descriptions.

Is the elevation of the slab in this addition one step down from the 1st floor in the main house or one step down from the basement floor? Sort of sounds like this addition used to be a patio until someone decided to enclose it and make it into an addition............unfortunately.

I'm also not sure of what you're trying to ventilate.

I'm pretty sure that what you said near the end of your post is your best and pretty much only option if you desire this addition to not become a recurring headache..........................

Tear the whole thing down, dig past the frostline, pour a footing and do it right.

[rai]

The slab is one step down from the basement floor level, and Yes, it was apparently just a patio originally. It was subsequently turned into a patio room with frame walls and roof attached to the house.

As for the ventilation... My understanding is that if I'm building a wood floor over an unconditioned space, that space should be ventilated and the floor should be like 18" above the ground in that space.

If that's the case, then building an 8" floor over sleepers or on footers would not allow enough vertical height and ventilation.

It just occurred to me, though, that putting a wood-subfloor over a concrete slab isn't that different. I'm just talking about a much thicker sub-floor...

[RobBase]

Yes, technically when building a wood floor over subgrade you want as much distance as possible between the moist ground and the joists. Ventilating it would be done by installing louvered vents in the foundation that the wood sits on. It's known as a crawlspace, but you're project sounds like it's past that stage.

If you put sleepers over the slab, and your slab doesn't exhibit signs of moisture infiltration (tape a 12" square piece of clear plastic to the slab with all 4 sides duct taped to the concrete, come back 1 day later and see if there is condensation under the plastic), then just put them so the new wooden subfloor is at the same grade as the basement floor.

[rai]

I'm sure that the slab isn't dry, per se.

I'd have to put down a moisture barrier and run it up the stone kneewall. I'd also have to insulate between the joists. To vent it, I'd probably do something like vents on one end and a 'smart' (humidity/temp sensing) force air vent on the opposite end.

Alternatively, I'd try to seal and insulate the slab and kneewall, not the floor. I'd then condition the 'crawlspace' by connecting HVAC supply and return air to it (or just force air through it from the room above, using a fan). I'd have to think about which side of the insulation the vapor barrier should go...