max
i use the doulbe bubble foil backed windshiled protectors under the dinette bench seats it does help some.
but the best was the 1" thick poly urethane foam boards , cut em to size and attatch with adhesive or screws and use GOOD foil ducting tape to seal teh edges. i did this with the subfloor over the basement and it really cut the chill factor in the bedroom
and you would be surprised how much air stoppage you can get just by a using a can of expanding foam and get al lthem wire chases and plumbing holes sealed up.