[MovedGoalPosts]

You are laying this boarding directly over the existing board. Thus the thickness of the added board will not be an issue for strength as any load is directly transferred to the existing boards. A thin plywood would be more than sufficient to block the draughts.

Wood products offer some thermal properties, but for the thicknesses you are talking of either as existing flooring or added boarding, not enough to make any real difference. If you want insulation, as opposed to draught proofing then you need an insulating layer. But given that your walls and roof don't have that high a performance, any significant expense on the floor, is not in my view justifiable.