Saving money on energy bills is a good thing, especially in very warm or cold climates where the AC or the furnace have to run a lot. It can costs hundreds of dollars a month to make the living space comfortable.
However, the garage is a different story. Trust me, I found out the hard way. I will never recoup the money I spent insulating. On very cold days, the paint and other supplies still get ruined.
Here in Colorado, temperatures regularly drop into the single digits. I just wanted to keep the temperature above 32°F so paint and other stuff like caulking tubes wouldn’t get ruined.
Home Depot had a special on fiber glass insulation, so I spent about $1,500 insulating the walls, ceiling and the door itself. I knew I wasn’t going to make it super warm like inside the house, because I wasn’t actively heating it with natural gas or electricity.
Unless your garage is heated, insulation will have little affect on the temperature inside your garage in cold climates. Insulating the garage won’t make it warmer.
The main purpose of insulation is to slow the conduction of heat from the walls and ceiling to the outside, and vice-verse. Insulation does not generate heat. If your garage is already cold, the area will virtually remain the same temperature whether you have insulation or not. When a garage door is opened, the air is recycled in a matter of seconds then traps new cold air.
The same is true of your house. If you were to turn off the furnace, the house is going to cool down rather quickly.
And no, things like a refrigerator, freezer, or lights will not heat the garage to make the insulation worthwhile. A car engine does add a little heat, but it won’t last long.
Some say the wall(s) which separate the main house from the garage allow heat to pass, warming the garage. But that shouldn’t be the case! The walls should be insulated to prevent heat from conducting into the garage from the house. If those walls are not insulated enough, such that your garage is warm without a heater, then that is something I would fix immediately. You want to retain as much heat as possible in the living space of your house. Your heater is working hard to keep it warm where you spend most of your time.
In warm climates, I’m not sure why you’d want to insulate at all. Bringing a hot car engine into an insulated garage on a hot day will just make it warmer. I have that problem in the summer and usually open the garage door to cool things down. This defeats the purpose of insulating.
If I spent a lot of time in the garage and added a heater or A/C, that’s a totally different story. In that case, insulating the garage is definitely worth it because I’d want as much of the heat or cool air to stay in the garage for as long as possible. Running a heater or air conditioner costs a lot of money.
Now, if you do want a warm garage, then I would add a heater and insulate the ceiling, walls, and garage door. Depending on the size of your garage, insulating the walls with batts of R-13 will typically run you about $400. The ceiling might cost another $600 and the garage door will cost about $600 if you do it right.
You can use the pink Styrofoam stuff from Home Depot, but that will only insulate your door to about R-6. Buying a whole new insulated garage door might be better, but it will cost about $1500! So if you do just the walls and the ceiling, it’s probably going to run you $1,000. A bit pricey if you don’t spend any time in the garage.
The best way to make your garage a little more comfortable is to stop any cold air infiltrating into your garage from outside. Put new weather-stripping down where the garage door makes contact with the ground. Make sure the seal is intact and no air is coming in around an exterior door frame. Use expanding foam to fill in gaps. These are good first steps that will only cost you about $80 and can make quite a difference. Even a slight draft can make your garage feel much colder than it really is, especially when it’s 10°F outside.