Some 25 years ago the previous owners of our CZ4, 1950 home enclosed a screened-in porch, creating a 10+10 den with custom cabinetry on the east wall. Most of the room sits above a brick foundation. However, there is no foundation underneath a 3X10 section one one side of the room. For this section, there is only 1-2″ clearance above the dirt. By using a phone camera, I can confirm that 1) there is no sealing or insulation underneath the subfloor, 2) from what I can see there is no rot, 3) this space can be dug out to gain access.

Above this space sits a propane fireplace nested in full-wall custom cabinetry, and there is a great amount of cold air leaking in from underneath the fireplace as well as the wall itself. A 30,000BTUH fireplace is mostly heating the dirt underneath and back yard to the side. 

How would you solve these problems?

1. Cold and leaky floor. My plan is to dig a 2′ trench to gain access to the subfloor beneath the fireplace. Then, I will airseal the few floor bays, insert either fiberglass or rockwool batts, and seal with pressure treated/sealed plywood. I have read Martin’s counsel to place rigid foam underneath the joists, but given the clearance issues I don’t think this will be possible. And, my above plan should make a huge difference to begin with. I realize that this will not be a perfect solution – who knows what is under the brick foundation? – but this seems like a manageable DIY project and should have a huge ROI in comfort. I haven’t seen anything like this on GBA, so:

a. What materials should I use? I would use ccsf to seal the edges of the joists but am concerned that batts would no longer fit snugly.
b. What do you recommend for moisture control? Given the (so far) lack of evidence of rot, I am keen to simply replace the dirt as it is now, and perhaps cover it with 20mm polyethylene. 

2. Cold and leaky wall. This is likely due to an unsealed gas fireplace chase, which we cannot access from the inside due to the cabinetry. The exterior siding is due for paint or replacement, so we can come in from the outside. Is there any information here on GBA on how to seal and insulate that kind of chase, or has anyone done this before? Once the chase is sealed, the other bays in the wall would be perfect for dense-packing, but there are only a couple bays so caulk+batts might be a better option. We can’t add rigid foam on the outside because of a corner on the house.

Thank you.