Features

Q: My home has a heating system with a metal chimney that runs up through the attic and terminates above the roof. Since the temperature of the chimney in the attic is not high enough to ignite wood, why is it necessary to have a 2-inch clearance between the chimney and the wood framing in the attic? 

A: The ignition temperature of wood is defined as the temperature at which it begins to burn. Wood and other combustibles undergo a physical change when continually exposed to elevated temperatures. This reduces their ignition temperatures. 

Wood normally begins to burn at about 400 F to 600 F. However, when it’s continually exposed to temperatures between 150 F and 250 F, its ignition temperature can become as low as 200 F. 

The lowering of the ignition temperature of wood and other combustible materials can take years to occur. When it does, should the wood’s temperature coincide with its lowered ignition temperature, it will ignite and burn spontaneously. This is the basis for the clearance requirements in building and fire codes. 

Q: A musty odor that gives us headaches is coming from under our sink. A plumber checked the drains and vents. The city has checked the sewers and there are no leaks under the house. Nor are there any dead animals under it. This odor suddenly appeared two months ago in my 51/2-year-old house. It’s worse when it rains. Any suggestions? 

A: As you’ve had your sewage system checked thoroughly, this is only one option. After turning off the water, remove the base cabinet under the sink.  

Then remove the drywall behind the sink, as there are cases where rodents have entered the walls from either attics or basements and, when they cannot find their way out, they die and decompose. This might tie in with the two months you mention.  

You might also want to get the opinion of a reliable exterminator before you tear your kitchen apart. 

Q: I have just noticed this since I had my rooms remodeled, and I had insulation put on first. Now I find mold spots forming inside glass picture frames on the shelves. My basement is a dirt floor under the living room. Could the dirt floor be causing this problem? 

Answer: Yes. Even when the dirt feels dry to the touch, it wicks up subsurface water, and this is released into the area under the living room and eventually into the living room itself. 

Before you remodeled your rooms, there apparently were enough open joints in the walls through which the moisture could escape to the outside. After you remodeled, those joints were sealed, causing the moisture to remain in the rooms. To control the moisture buildup, you should cover the dirt floor in the basement with a vapor barrier, such as 4- or 6-mil-thick polyethylene plastic sheets. Overlap the sheets and tape the joints shut. 

To submit a question, write to Popular Mechanics, Reader Service Bureau, 224 W. 57th St., New York, NY 10019. The most interesting questions will be answered in a future column.