You may think it’s a good idea to insulate a garage door. As always, you need to do the math. (Darla Johannsen)

I live in a condo with an attached garage. The current garage door insulation is rated at R-4. Even with that, it’s cold in the winter and hot as Hades in the summer. Local companies have given me estimates to increase the insulation by installing a new door. This would cost more than \$1,000. I’ve seen kits that allow you to add insulation to the inside of the door for one-tenth the cost of a new door. Are they worth the price? Will the kit last? Will it fall off the door onto my new car when the door is in the up position? What would you do?Darla J., Findlay, Ohio

I know all about cold and hot garages. My current home has a large three-car garage that’s attached to the house with a small connector hallway. My previous home had a very large garage that was 95 percent detached from my home. Even though the walls and ceilings of both are insulated, they still aren’t comfortable when the temperatures get extreme.

I think many people expect too much from insulation. Based on the constant flow of e-mails I get, it seems that many feel if something is insulated, then that means the space will be comfortable almost all the time.

What you, and others, need to know about insulation is that it simply slows the transfer of heat. Heat wants to travel rapidly from where it’s hot to where it’s cold. This is one of the simple laws of thermodynamics you may have studied in high school physics.

The rate of heat transfer is the R number you referenced. The higher that number, the better the insulating material performs at slowing down the heat transfer. A product with a rating of R-38 will retain heat much longer than a product that has a rating of R-5.

The issue with most garages is they’re not heated or cooled. Very few people can afford to spend the money to pump heat or air conditioning into a garage to keep them the same temperature as the inside of a house.

Because of this, garages tend to act much like outdoor sheds. What little heat there is in a garage eventually starts to transfer through the walls and ceiling to where it’s cooler. This means as you go from summer to winter, at some point your garage is just a few degrees warmer than the outside air temperature.

The actual temperature of your garage in the winter depends on many things. For example, how well is the garage door weatherstripped? Are there other air leaks around the walls or ceiling of the garage? How much heat from your house or condo leaks into your garage? How often do you drive your car, and do you park it right away in your garage so the heat from the car and engine help heat the garage space?

Here’s the bottom line: If you don’t supply heat or cooling into your garage, the garage space temperature will eventually start to be quite close to whatever the outdoor air temperature is, no matter how well insulated the garage walls, ceiling and garage door are.

All that said, the insulation kit you saw at the home center or hardware store could be a great value. It all depends on how well the rest of your garage is insulated. If your garage door is the weak link in your insulation with respect to the walls and ceiling, and the kit allows the door to now have as much insulation as the rest of the space, then you’re headed in the right direction.

However, if the insulation gives the door an R-factor much higher than the walls and ceiling, then the heat in the garage will transfer faster out of the walls and ceiling and you’ll still have a cold garage in the winter. I know all this is confusing, but think of your garage like your body. If you have a warm coat and pants on but no hat, then you’ll probably lose lots of heat out of your head and feel colder faster.

If you decide to purchase the kit, you need to make sure the insulation stays on the door. You’re right to be concerned that when your garage door is in the open position and your car is in the garage, the new insulation could detach and tumble onto your car. It’s up to you to determine how well the new insulation bonds and is connected to the existing insulation.

If the new kit relies on some sort of adhesive, be sure your new and old insulation are dust free before applying the adhesive. Perhaps do a test. Just glue on one piece of insulation, allow the adhesive to dry and cure and then raise the garage door to see if the insulation stays put.

Tim Carter is a columnist for Tribune Media Services. Contact him through his Web site at www.askthebuilder.com.

Catch up with some of Tim’s previous columns:

What you need to know before knocking down that wall in your home

Will exposure to rain hurt home’s framing lumber?

How to redirect water around a damp garage