It makes sense to insulate a garage if you are going to be heating the space. The same kinds of insulation materials can be used in the garage that is used for the rest of your home. However, there are some materials that are better than others, depending on whether or not your garage is finished. You should also consider insulating your garage door, and it has different installation requirements compared to what is needed on ceilings or walls.
In garages, the most commonly used kind of insulation is fiberglass (it is also the most popular kind that is used in houses). It is sold in the form of long blankets and pre-cut batts that fit in between the ceiling joists and walls studs. There is also loose-fill fiberglass that is available. It is well-suited for blowing into a garage attic area over a finished ceiling.
If the ceiling and walls are going to stay open (not covered up with plywood or drywall), then a good option is to use encapsulated or paper-faced fiberglass bats wrapped up inside a plastic film. That will provide a bit of a more finished look to the walls, and there will not be itchy insulation fibers exposed that are constantly catching dust.
Cellulose is a type of loose-fill insulation that is becoming increasingly popular. It is made out of recycled newspapers primarily and then a fire retardant is added to treat it. Usually, cellulose is blown into ceiling cavities and walls using a special blowing machine that aerates and fluffs up the cellulose as well. Blowers are available for rent at many home centers and tool rental stores. If you purchase cellulose from them they will sometimes loan you a machine for free.
Since cellulose is loose-fill, it is only suitable for finished garage ceilings and walls. If the garage is finished already (but not insulated), then cellulose can be installed by cutting strategic holes inside the wall material and then spraying the insulating in the cavities that are between the framing members. The final step is to patch the holes up.
Rigid Foam Insulation
There is rigid foam insulation that is available in 4×8-foot sheets with thicknesses ranging from 1/2 inch up to 4 inches. Polyisocyanurate, extruded polystyrene, and expanded polystyrene (which is similar to Styrofoam) are the most common types of materials. Rigid foam provides a high R-value per each inch of thickness. It can also be cut to fit practically any space. It is a good option for insulating garage doors and for thin walls. If you are planning to transform your garage into a full-time workspace or a living area and would like to insulate the floor, rigid foam covered in a subflooring material such as plywood is one option that you can use.
Spray Foam Insulation
Spray foam works very well for air-sealing and has a great R-value. It is a high-end material that is normally used in energy-efficient construction. For a majority of garage projects, using spray foam is actually overkill. However, if you are planning to convert your garage into a living area, then it may make sense.
Garage Door Insulation
You should not insulate your garage ceiling and walls unless you insulate your large garage door as well. Insulation kits can be purchased for regular metal garage doors. You can also cut rigid foam insulation pieces to fit each of the door sections or panels. Just remember that garage doors have structural metal ribbing that is a strong conductor of heat, and normally that is not insulated. This results in the door’s overall thermal performance being far below the insulation’s rated performance.
Air-sealing is especially important when it comes to garage doors. A special garage door that has an integrated weather seal strip can be used to create seals along the top and sides of the door. Use a bottom seal or rubber gasket to seal along the bottom part of the door. They are available in different sizes for covering large or small gaps between the garage floor and the door.