Published on June 13th, 2013 | by Henri


Insulating the floor over garage

Q.    I live in a split-entry brick home. The garage is in the house. The back bedrooms are very cold. I have insulation between the joists with the vapor barrier up against the floor of the bedrooms. There is some heat in the garage but I don’t want it too hot because of the salt from the roads on the cars. Should the vapor barrier face the garage or should the insulation be unfaced?

A.    You did it right: the vapor retarder should always be on the winter warm side of the insulation but, in your case, the insulation should be tight against the floor above. The exception to this rule is in climates where air-conditioning is prevalent over heating — the deep South.

Keep in mind that the bedrooms (which I assume are over the garage) have an added exposure to the cold — their floor. In such cases, the insulation should completely fill the spaces between the joists. If the fiberglass batts are left exposed in your garage, they are subject to heat loss through air movement (fiberglass is a filter, after all). In that case, do whatever is needed to fill the spaces between the joists with more unfaced insulation, fasten 1-inch thick rigid insulation to the bottom of the joists and cover it with fire-rated gypsum board, making sure to tape the joints.

If there is already gypboard on the garage ceiling and the fiberglass insulation does not completely fill the spaces between the joists, your choices are to have dense-pack cellulose blown in or the gypboard removed and additional fiberglass put in. If the spaces between the joists are filled with fiberglass, consider adding 1-inch thick rigid insulation to the bottom of the joists, even if the ceiling is covered with gypboard, and cover it with another layer of fire-rated gypboard.

You don’t say what your heating system is but it may need to be better balanced. If your system is warm air, insulate any ducts in the garage space and balance the system to feed more warm air in the bedrooms. If you have a hydronic system, and the bedrooms are not on a separate zone, you may need to add radiation in each of them.

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