Brad writes from New York: "I own a house that is approximately 90 years old. The walls are plaster with some sort of wood slatting behind it. i am uncertain if there is any insulation behind the walls but i assume not. How does one check the space behind the walls without drilling a bit hole in it? Is there some type of device to look into the wall to check the amount of space? Thanks."

Answer: You may be able to find evidence of insulation around electrical outlets or light switches after taking the face plates off the wall.

Otherwise, we can suggest working with an insulation contractor because adding blown-in insulation to existing walls is not a good do-it-yourself project. The work could damage interior walls if not done properly. You state that your home may not have any insulation in the walls but if there is even a small amount the existing insulation will likely block the new material from getting everywhere it needs to be. Bottom line: I recommend that you contact a local insulation professional.