Expert: Dan Griffin - 1/17/2008

Question
I want to build a new sub floor over the existing cement floor in a cottage< ineed to raise the floor 7inchs so I thought I would use 2 by 6 frame ontop of the cement, insulation and then 5 by 8 plywood, question is should I put a vapour barrier in there if so where and what kind...am I looking at this the right way?
Is is correct that the isulation should not touch the cement? thanks Rob

Answer
Raising a floor 7" sounds unusual in an existing structure as it will violate doors, place window sills pretty low and other anomalies.

If you still want to proceed:
I would put down a vapor barrier on the slab.  Visqueen or tar paper would be satisfactory.
Bed any sleepers in plastic roof cement.
Consider using treated lumber for joists.
I would use 6" Fiberglass batts in the joist chambers or rigid styrofoam, I wouldn't worry about either one touching the vapor barrier.   The foam can rub and creak if the joists have movement.
Glue and nail/screw the subfloor.  5x8 would be unusual.  I would use 1 1/8" tongue and groove decking.

2x6 are 5 1/2" + 1 1/8" decking makes 6 5/8", is the other 3/8 critical?