Published on May 3rd, 2014 | by Henri


Insulating bedroom floor over unheated garage

Q.    My bedroom is located over an unheated garage. The unfinished garage ceiling is insulated with two layers of rolled bat insulation. The paper vapor barrier is facing the garage, not the bedroom floor.

I would like to seal off the garage ceiling with foam boards to stop any cold air from coming in through the bedroom floor and to provide some additional insulation. My understanding is that the vapor barrier should be next to, or on the side of, the heated room. Does this hold true for my bedroom/garage situation? Must I reinstall the insulation with the vapor barrier next to the bedroom floor before I put up the foam board?

A.    Ideally, yes but we know that paper vapor retarders are not very effective. Moreover, the addition of at least 1-inch thick rigid insulation will warm the joist cavities and reduce the chance of condensation within them. It will also reduce any heat loss through the floor joists.

And if your sub-floor is plywood, that in itself is an effective vapor retarder. However, pull down some of the fiberglass insulation in a few widely scattered spots, particularly next to the outside walls, and check for any signs of moisture or mold development. If you find any of these, you should seriously consider pulling the insulation down wherever these conditions are visible, allow the areas to dry and put the insulation back (if it is dry) with the paper vapor retarder facing the bedroom floor.

Be sure that the entire joist cavities are filled with fiberglass insulation to prevent heat loss and cold air from infiltrating between the top of the insulation and the subfloor. You are unlikely to find that the entire area has been affected.

Also be sure that you use extruded polystyrene rigid insulation (XPS — blue, pink, grey or green). Remember that rigid insulation on the garage ceiling will have to be covered with fire-code drywall, properly taped.

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