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Q: A brick chimney goes up between two unheated rooms in our attic. For the last 35 years, creosote appears to have leaked out of the chimney and stained the wallpaper. Is this dangerous, and if it needs repair, what kind of technician handles this? 

A: The problem is dangerous, and it should be looked after. The creosote indicates there are openings in the chimney wall that extend to the flue. Incidentally, the black stains you see might not be creosote.  

Creosote is formed from the incomplete combustion of wood or coal. Instead, the material might be a sooty, oily film from the incomplete combustion of fuel oil. 

Regardless, both creosote and fuel oil film have corrosive elements.  

As flue gas rises, it cools to the point that a corrosive condensate might form and adhere to the chimney liner. This corrosive material attacks the flue lining and its mortar joints. If there is no flue lining, the corrosive deposits attack the brick and mortar. Eventually, cracks will open and flue gas will escape. 

When the chimney is in the house, rather than outside, flue gases can seep through the cracks and into the attic or living areas.  

This is a fire hazard if wood framing is near the chimney, and it’s a health hazard: flue gas contains carbon monoxide. 

The safest correction is to have a new flue lining installed. Many chimney sweep companies do this.  

Check in the Yellow Pages for chimney sweeps. The company should be certified by the National Chimney Sweep Guild. 

Q: I have received conflicting advice regarding vapor barriers for crawl spaces. Some advise leaving small spaces between the sheets to allow the ground to dry out under the barrier. Others advise leaving no gaps. Also, should the barrier be extended up the concrete walls? 

A: In a crawl space, the vapor barrier is usually 4- to 6-mil-thick polyethylene sheets with overlapped joints that are sealed with heavy-duty plastic tape.  

The sheets are run several inches up the sides of the foundation and are taped to the wall. 

You should not leave spaces between pieces of the vapor barrier.  

To be effective, it must be continuous. The spaces would allow moisture vapor into the crawlspace. 

Some background is helpful in understanding how a vapor barrier works. It is installed to stop the capillary rise of moisture in the soil from becoming airborne vapor.  

It can’t prevent water from collecting in the soil under the crawl space. 

If you find that the ground in the crawlspace is wet, you should take measures to dry it out. For example, the ground should slope away from the house on all sides, foundation drains should be installed to move water away from the foundation, and downspouts should discharge water far enough from the house so it doesn’t seep into the basement. 

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.