We are moving into a house that has an enclosed porch area off the rear of the house. We assume it was once an open area that was later enclosed. It is now used as a pantry. Above the enclosure is a porch for the second level. The main home is solid brick (built 1910) and the porch enclosure appears to be wood framing with vinyl siding. It sits about 2 feet off the ground - e.g. you could walk under and see underneath to the joists. We haven't moved in but during the inspection I looked under and realized there was no insulation between the joists. And it is clear when you walk in this pantry area it is cold. I assume - but haven't been able to confirm - from the inside the floor has only wood decking with vinyl. The floor area is roughly 10' wide (along the house) by 6' deep (going out from the house). There are no utilities or anything else going through the floor.
My question relates to the best approach for insulation. I wasn't clear if this should be treated like a typical crawlspace given it is exposed as it is - open on the three sides and off the ground - almost like a deck. It seems putting fiberglass batts would not be a good idea - at least I assume. I was hoping for advice on the best approach. My first thought was foil faced rigid foam board with the foil to the floor side and then use canned foam to seal. I realize there would be thermal bridging. Or perhaps this small area is a great candidate for spray foam? I just wasn't sure how to treat this area. But it is clearly a drain on energy. This house is in the Washington, DC area.
I attached a very crude sketch of the structure - not to scale nor full detail. Thank you for any advise or guidance.