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As long as you use the "window and door" foam (blue can), leave a space (6") every so often.
My question would be though.....drafts don't often come via under the siding, especially since you mentioned the draft is coming via the floor boards.
Does your house have a crawlspace ?....if so more likely you need to add more or fix existing floor insulation, it may have fallen out, or not be there at all.
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