I have torn out the floor and insulation in the kitchen of an old house with a crawl space and am now in the process of installing the new subfloor that will support the insulation between the beams. I have added 45x220 on each side of the old 15x15 beams to fit a bit more wood fiber insulation between them and to level the floor. As a subfloor, I am considering using tongue and groove boards resting on 25x25 battens.
However, I have a problem at the outer wall. The foundation stones extend a bit into the room, preventing me from attaching the subfloor lower than 15 cm from the floor. Therefore, I've come up with four different options.
A. Lay the tongue and groove boards on the stone with sill paper in between and angle the boards for maximum insulation. I would then nail the boards from above with battens.
B. Place a floor beam right at the edge of the stone foundation and thereby lay the tongue and groove boards in two steps.
C. Attach battens at the bottom of the sill and lay the boards on those, then angle them down towards the 220 beam on the other side. (This results in only about 9-10 cm thick insulation along the sill.)
D. Determine the subfloor's depth according to the maximum depth of the sill and lay the subfloor level. (As mentioned, this will result in only 9-10 cm for 60 cm into the room.)
I am most interested in option A as it is the simplest and provides the most insulation, but I am concerned that the tongue and groove boards might rot against the foundation stone. Does anyone have an opinion?
Interesting, I am in the process of breaking up a similar floor myself.
I would probably also go with option A. If you are concerned about moisture against the foundation stone, a fifth option could be to attach a beam to the sill and attach the floorboards to the beam "hanging," with a small air gap to the foundation stone. Neither the beam nor the floorboards should have direct contact with the foundation stone.
Option A is probably the simplest. There is specialized blindbottenboard that is generally easier to install than råspont. There is also borbehandlad plywood. If the c/c distance between the joists varies a lot and you don’t have a good saw, råspont might still be easier. Note that you cannot increase the insulation capacity unpunished from, for example, 15 cm of sawdust to 22 cm of cellulose fiber in an old house with a crawl space foundation, as the residual heat from above to the foundation will then be significantly less. However, if there is a heated chimney passing through the floor structure, it often compensates for the heat loss.
I will go with option A then. As you say, there is a chimney going through the floor joist, so the crawl space will get heat from there. I have also prepared an inspection hatch for the crawl space under the stairs so that I can inspect the space at regular intervals. The crawl space is only under half the house as the rest has a basement that gets waste heat from the water heater, large freezer, etc. so there won't be a huge difference in temperature between the outside air and the crawl space in the summer.
