Q: We have always had a problem with a mildew smell in our living room. The smell seems to come from the fireplace, which we have not used in more than 20 years because the chimney will not draw the smoke out. The roof next door is taller than the top of our chimney, and the houses are really close.

We had chimney companies come out only to tell us that this is not something they could help with. We had professional air-sealing companies tell us that this is not something they do either. No one seems to know the answer or a direction in which we should go.

We have a crawl space underneath the family room where the fireplace is. A room in the finished basement that has an access panel to the crawl space has the same mildew smell. Would it help to have the crawl space sealed properly, so that even though it has a low ceiling and is not livable, it will be sealed like the rest of the living spaces?

A: Sealing and insulating the crawl space would probably make your living room smell better, and it would also help it stay warmer in winter.

Although you notice the smell by the fireplace, the musty odor probably originates in the space under the room, said Matthew Hargrove, president of Total Home Performance in Easton, Md., and Michelle Burgee, chief executive of Complete Home Solutions in Upper Marlboro, Md. Both companies specialize in diagnosing and treating indoor air-quality problems and improving energy efficiency.

“The air in our homes moves from bottom to top,” Hargrove wrote in an email about your situation. “This is known as the ‘stack effect.’ ” He estimated that about 30 percent of the air that infiltrates a house to make up for exhaust via fans, chimneys and leaks comes from the crawl space or basement. A typical dirt crawl space is damp or at least has high humidity — a perfect situation for mildew growth and thus that icky smell.

Adding to the humidity problem, a typical crawl space also has vents in the walls. Although venting was intended as a way to keep air in the crawl space fresh, a better solution is to close off the vents and insulate the crawl space so that the conditions that allow mildew to grow do not occur. An excellent overview of the issues and specific steps is available on the website of Building Science Corp., a consulting company that helped lead the rethinking of how crawl spaces should be detailed.

Hargrove and Burgee recommended that you first address any ground moisture issues. This may mean installing a French drain, or ditch, around the perimeter of the interior with a sump pump to remove any moisture that collects there. Then you would need insulation on the foundation walls (not the ceiling of the crawl space), a sealed vapor barrier that covers the floor and is fastened up the walls, and a dehumidifier to control the humidity year-round. If there is significant mildew growth, you would need to have it removed before the sealing work is done.

Hargrove and Burgee estimated the total costs differently, so you should get at least two estimates before hiring someone. The size, condition, access and other issues specific to your house will make a difference. Hargrove said jobs like this usually cost $3 to $6 a square foot; Burgee said $2,000 to $4,000 for smaller crawl spaces and $5,000 to $7,000 for crawl spaces with more than 1,000 square feet.

Jeanne Huber is a freelance writer.