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Ventilate and Insulate

An insulated, finished garage will stay cooler in the summer and warmer in the winter.

First, be sure the attic is well-ventilated. Check to see how many attic and roof vents you have. In general, you’ll need one square foot (144 sq. in.) of vent opening per 300 sq. ft. of attic, divided between the soffit and roof vents. For a typical 20- by 22-ft. garage, you’ll need about six 4-in. by 12-in. soffit vents and two standard square roof vents.

Install air chutes

Make sure your ventilation is effective by installing vent chutes between the trusses. Vent chutes have a channel that prevents blown insulation from blocking the airflow from the soffit vents to the attic space.

Plug the area under the vent with wood blocking or plastic and a chunk of fiberglass insulation. This prevents wind from blowing up through the insulation, or insulation from filling the soffit.

Add a vapor barrier

Protect walls from interior moisture and reduce air leaks with poly sheeting. Overlap corners, being careful not to create a “bridge” that would interfere with the drywall. Use only enough staples to hold the poly while you install the drywall.

We recommend filling the stud spaces with friction-fit fiberglass batts, covering the walls and ceiling with a 4-mil poly vapor retarder, and blowing insulation (cellulose is a good choice) into the attic after installing the ceiling drywall.

Start by insulating the walls. Buy unfaced R-13 batts for 2×4 walls and unfaced R-19 batts for 2×6 walls. Match the width of the batt (15-1/4-in. or 23-1/4-in.) to the stud space. Cut batts carefully for a tight fit.

Next, staple 4-mil poly to the walls and ceiling. Embed the poly sheeting in the caulk around the perimeter of each piece to create an airtight seal. Seal the seams between sheets and seal the poly to electrical boxes with caulk or special sheathing tape. (It’s typically red and looks like packing tape.)

After installing the drywall on the ceiling, you can blow insulation into the attic. Home centers and some rental stores sell blow-in insulation and often offer a free or reduced-rate rental on the blower if you purchase insulation from them.

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Weather-Strip Doors and Windows

Install weather stripping

Seal out drafts (and bugs!) with garage-door weather stripping. This is an important part of a finished garage. You don’t want to do all this work only for the garage to be drafty or filled with pests.

Cut the strips to length and nail them into place. Press the weather strip against the door as you nail it to create a tight seal.

Leaky doors and windows let in uncomfortable drafts and increase heating and cooling costs. Check your garage service door to be sure it has good weather stripping and a threshold that seals tightly to the bottom of the door.

In most cases, if your service door is missing a threshold and weather stripping, it’s more efficient to replace the door with a new, weather-tight version. You can buy an inexpensive prehung metal exterior door at home centers and lumber yards.

The overhead garage door can be a big source of drafts and heat loss. Recently installed doors usually include a weather-stripped stop around the perimeter. But older doors may be lacking a weather strip.

Luckily, it’s an easy fix. Garage door weather stripping is available at home centers and hardware stores, and installation is straightforward. If your overhead door is uninsulated, search online for “garage door insulating kits.”

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