Garage Insulation Basics
It pays to insulate your garage if you’re adding a garage heater, whether on a permanent or as-needed basis. If you’re not adding heat, there’s little point in insulating. It’s a popular misconception that insulation adds warmth. In reality, insulation merely slows the transfer of heat through the insulated barrier (wall, ceiling, or floor), which is good for hot and cold climates. Some say that insulating an unheated garage that is attached to the house may offer an additional thermal buffer between the exterior of the home and the outdoors. But no state requires the entirety of the garage to be insulated as part of an energy-efficiency mandate. It’s also unlikely that this minimum improvement in energy transfer will offset the costs of adding extensive insulation. The walls that are shared with the house, however, should always be insulated to their maximum value.
Air Sealing
It’s also important to realize the value of air sealing in conjunction with insulation. Garages typically aren’t built to be airtight and have lots of air gaps to the outdoors. You can insulate the walls, ceiling, and door of the garage to the highest R-value (the higher the R-value number, the better the material’s insulating effectiveness) possible, but if you fail to fill those remaining air gaps, you’ll still be wasting a lot of heat. So, before insulating, go around the garage with a can of low-expanding spray foam and seal all gaps and cracks that let in the daylight. Waiting to do this after the insulation is installed tends to be a messy job. Also, make sure weatherstripping along the bottom of the garage door, window, and door frames are intact to seal off drafts. Below, we’ll break down the pros and cons of each of the five types of insulation and the factors you should consider when choosing which one is best for your garage.
Get a Quote Fiberglass is the most commonly used type of insulation in garages (just as it’s the most popular type in homes). It’s sold in precut batts and long blankets that fit between wall studs and ceiling joists. You can also get loose-fill fiberglass, which is suitable for blowing into a garage attic space above a finished ceiling. If the walls and ceiling will remain open (not covered with drywall or plywood), it’s a good idea to use paper-faced or encapsulated fiberglass bats that are wrapped in a plastic film. These will give the walls a slightly more finished look, and you won’t have the itchy fibers of the insulation exposed and ready to catch dust at all times. Cellulose is a loose-fill insulation that is growing in popularity. Made primarily from recycled newspapers and treated with a fire retardant, cellulose is usually blown into wall and ceiling cavities with a special blowing machine that also aerates the cellulose and fluffs it up. Blowers can be rented at many tool rental stores, and home centers will sometimes loan you a free one if you buy your cellulose from them. Because it’s loose-fill, cellulose is suitable only for finished garage walls and ceilings. If the garage is already finished (but uninsulated), you can install cellulose by cutting strategic holes in the wall material, spraying the insulation into the cavities between framing members, then patching the holes. Rigid foam comes in 4-by 8-foot sheets and 1/2-inch to 4-inch thicknesses. The most common materials include expanded polystyrene (similar to Styrofoam), extruded polystyrene, and polyisocyanurate. Rigid foam offers a high R-value per inch of thickness and can be cut to fit almost any space. It’s a good choice for thin walls and insulating garage doors. If you’re turning the garage into living space or a full-time workspace and want to insulate the floor, one option is to use rigid foam covered in plywood or other subfloor material. Spray foam (beyond the low-expanding canned product) is excellent for both R-value and air sealing. As a high-end material typically used for energy-efficient construction, spray foam may be overkill for most garage projects. But it might make sense if you’re converting the garage to a living space. Don’t insulate your garage walls and ceiling without insulating the big garage door, too. You can buy insulation kits for standard metal garage doors, or you can cut pieces of rigid foam insulation or Reflectix sheets to fit each door panel/section. Keep in mind that the structural metal ribbing of garage doors is an excellent conductor of heat, and this typically doesn’t get insulated. As a result, the overall thermal performance of the door will be well below the rated performance of the insulation itself. Air sealing is particularly important with garage doors. Create a seal along the sides and top of the door with special garage door trim with an integrated weather-seal strip. Seal along the bottom of the door with a new rubber gasket, or “bottom seal.” It’s available in different sizes to cover small or large gaps between the door and the garage floor.
