Hello, I am seeking advice on a piece of flooring I would like to insulate myself.

The situation: we have a 1928 home in Seattle that we've been in for about 10 years. The house has a kitchen nook that is cantilevered about 4 feet out from the foundation and is hovering about 3"-4" above the soil/rocks. In the winter time the nook, and in particular the floor in the nook, can be quite cool. I'd like to insulate under the floor/between the joists to reduce the amount of heat we lose out of that room.

My working plan based off what I have read and what I think is feasible: access the cantilevered space through the openings in the basement and then insulate between the joist with 3-4" of Polystyrene closed cell foam board reaching about R-15 to R-20. I am planning to cut the 48" board to match the width between the joist and then use an adhesive (and maybe a few screws with large washers) to adhere the board to the underside of the floor. I am also considering to use a closed cell spray foam to fill in some of the inevitable cracks I'll miss with the foam board. Lastly, I'd seal the space between the cantilevered section of the house and the basement to prevent critters from finding their way into the basement. Because the cantilevered section is just 3-4" above the soil I cannot think of a way to close the underside of cantilevered joist so I was just going to leave it open, as it has been for the past 100 years.

When I began reading further about all of the ways in which moisture can cause problems without the correct insulation considerations I began to second guess my methods and thought I should seek some expert advice.

Please feel free to rip apart my plan and advise how to insulate this space.

Thanks all!