vapor barriers

[dbarr15]

Turning my garage to a workshop. It has been tyvek wrapped and a layer of 3/4 pink insulation put on and then the siding. I want to insulate the walls, ceiling and floor. It will be heated and a/c. I will be putting down a wood floor with PT sleepers, pink insulation in between and 3/4 ply on top. I have heard so many stories about vapor barriers and where to put them. Do I lay 6mm plastic on cement floor and then sleepers,insulation and then ply or do I lay down the sleepers and insulation then plastic then ply? I have seen both ways talked about. What about the walls? I will be using fiberglass batt (can not afford spray foam so not an option) do I buy faced fiberglass insulation or not? Do I put faced/unfaced in the walls and ceiling and then cover with 6mm plastic before sheetrock. I want to do this so there will not be any moisture problems. Thanks for your help. Doug

[crokett]

You only want the vapor barrier in one place. For the walls/ceiling I would use faced bats, paper goes towards the heated/cooled side. For the floors either put plastic down, then your sleepers, then unfaced insulation then floor, or put down the sleepers, then faced insulation then floor but don't do both the plastic and the faced insulation. If it were me I would just leave the floor as concrete. If you are looking for a more comfortable walking surface put rubber mats where you will do most of your standing.

[Uncle Cracker]

David is correct. A concrete floor of 4" or greater thickness need not be insulated. The heat gain or loss is not significant, particularly when compared to walls, ceilings or windows/doors. Put your money into the best R-value you can afford in the ceilings and walls, insulated windows/doors, and good weatherstripping. The reason he suggests no duplication of vapor barriers is that moisture will invariably get trapped between them, and mildew (possibly toxic) can form. Remember also that some air infiltration is better than none, as the space must breathe, albeit just a bit, so that moisture and contaminants are not similarly trapped within the building.