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My near-ground level kitchen floor was cantilevered, overhanging the foundation by ~2.5ft. The bottom surface of the floor joists and the gap between them and the ground was left open, allowing unneeded air flow (and cooling loss) from my dehumidified crawlspace, letting animals access the crawlspace, and exposing the floor joists to the elements.

I have installed insulation between the joists and sealed the bottom surface of the joists with OSB. I filled any large gaps with expanding foam, small gaps with exterior caulk, and sealed all exposed wood with Kilz Klear primer sealer. Here is a crude diagram of the situation and how I plan to cover the ground underneath the floor.

Diagram

This just leaves the unappealing gap between the floor and the ground, which I need to seal to keep animals from nesting underneath my floor. I haven't found other examples of a (questionable) floor design like this. I have some ideas, but I'm out of my depth in evaluating what would be best.

  1. Do I try to add some kind of framing underneath the floor to extend the siding to the ground?
    • If so, how would I waterproof it all?
  2. Do I line the gap with cinder blocks (or something more appealing)?
    • Maybe I could add aluminum flashing over it from the existing siding down to the ground?
  3. Do I cover it with some sort of sheet metal without the blocks?
    • My only concern here is that the sheet metal would end up bent or damaged without some kind of backing.

Additionally, he electrical and coolant hoses to my HVAC blower fan are run through this space, so I have to work around them somehow.

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    Aside: I have similar cantilevers where the bottom sheathing (Your OSB.) wasn't tight to the foundation. The gap varied between 1/2 and 3/4 inch. After the mice discovered it I inserted backer rod and caulked. That didn't last long. 1/8" thick aluminum covering the gap has proven to be much more resistant to rodents. (It wasn't absolutely clear where you used expanding foam and caulk. Neither will stop small, sharp teeth.)
    – HABO
    Commented Oct 2, 2023 at 3:11
  • Thanks, @HABO, I hadn't considered that. Given the additional obstacle I added--that my HVAC blower fan connections run through that space (and the OSB)--I'm not sure how I would totally teeth proof it. Fortunately, I don't have mice or rats (that I know of) and the chipmunks seem to prefer the stumps to the house. If that changes, I'll go after it again with some aluminum. Commented Oct 2, 2023 at 3:31

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Fill that skinny gap between your brick and the OSB with spray foam. Go nuts filling the whole gap so there isn't a nice warm and flat surface for rodents right next to potential crawlspace access.

Install a pressure treated 2X6 to the underside of the cantilever and with its outer face flush with your rim joist's outer face. Sandwich rubber flashing between the OSB and the 2X6. Sneak rubber flashing under the bottom of your siding and spanning over the edge of your OSB and 1" to 2" of this pressure treated 2X6. Sneak a galvanized steel flashing sheet between your siding and covering the rubber flashing plus 1/2" (if your rubber flashing wasn't very straight, then just trim off any spots where it sticks out from under the metal flashing). You don't want the galvanized steel sheet in contact with the pressure treated 2X6, as the 2X6's copper treatment will galvanically corrode the sheet metal if they're in direct contact (hot dip galvanized fasteners are fine for holding the steel sheet in place, though).

Quick drawing

Fasten your plastic vapor barrier to the underside of the 2X6 with staples. This is to maintain the fit up as you pinch the plastic between a pressure treated 2X2 and the pressure treated 2X6 with corrosion resistant fasteners. Be sure to stagger the breaks in the 2X2 from the breaks in the 2X6. Fold any excess plastic under the 2X2.

Fill the space under the 2X6 and plastic with 3/4" clean crushed rock (angular gravel that's clean of smaller gravel particles). Without the fines, plants will universally find this rock inhospitable and burrowing rodents shouldn't be able to traverse it either. The 3/4" clean crushed rock is ideal to mound up 1" to 2" on the outside of the 2X6 (short of the sheet metal). It won't let any moisture stand against the 2X6, where that 2X6 should last for over 50 years. If this is an aesthetically important area, then you can pile soil up against the 2X6 1" to 2" (again short of the sheet metal). This configuration will still last for 30 years plus.

Note that sealing this space with a vapor barrier is of critical importance if it is enclosed like this. Without air circulation, moisture from the soil could easily humidify the space and encourage mold growth on your OSB and beyond. Ideally the vapor barrier would also be fastened to the surface of your brick to thwart moisture intrusion, where I've seen people even go so far as to put butyl tape between the two. The butyl tape seems silly to me, but I don't live in a high humidity climate. In my climate I would use a 1X2 and Tapcons spaced every 2'-0" to pinch the plastic onto the brick surface.

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