Features

Q: My two-story house is 40 feet wide on each side, and has three 12-inch-square vents on the roof’s south side.  

There are four soffit vents on the house’s north and south sides. During the winter, frost collects on the attic side of the roof deck and on the rafters. The frost melts and drips on the insulation and seeps through the ceiling. What can I do to reduce the attic moisture in the winter? 

A: Your problem is typical of an attic that has excessive moisture buildup and inadequate ventilation. The unobstructed attic ventilation should be one-threehundreth of the attic floor area. 

Based on your data, the vent openings are about 20 percent less than the recommended amount.  

If there are insect screens covering the vent openings, then the percentage is even more. Insect screens reduces the effective opening by about 40 percent. 

To increase moisture reduction, the roof deck between the rafters should be “washed” with cool dry air.  

This can be achieved with continuous ridge and soffit vents. If these vents cannot be installed, then you must use additional roof and soffit vents. Frost tends to develop on the roof’s north slope, there are no vents presently located there, so install the vents on the north side. 

Also, moisture can migrate into the attic through wall cavities because water can collect in the basement or crawl space after a rain. Keep those areas dry. 

Q: The cathedral ceiling in my 10-year-old house was OK until five years ago, when we noticed black spots appearing through the sand finish. I painted it and it looked good but the black spots came back. No matter what I do, they come back. I have asked many carpenters what causes it, but none seem to know. Can you help? 

A: The black spots are probably mildew spores. Mildew is a fungus and unless you kill it, it will come through a new layer of paint, especially a water-based paint.  

Try washing the ceiling with a solution of bleach, detergent and water. After the ceiling is dry, paint it with a mildew-resistant paint or use a fungicide additive in the paint. 

Q: The plans I have for building a year-round doghouse state that low-radiant heat can be used during the cold winter months. Just what is low-radiant heat and where can I get the parts needed to install it? 

A: This type of heat radiates directly to objects so it does not have to heat the air around them to have a warming effect. Low-radiant heat is usually supplied by electric cables embedded in floors or ceilings.  

To protect the cables from damage by the animals, lay them in a bed of sand over polyurethane insulation and a vapor barrier followed by a minimum of 3 inches of concrete.  

Some low-radiant heating cables can be laid directly in the concrete, but insulation is still required to keep bottom heat loss to a minimum.  

Use Styrofoam panels for this purpose. Protect all wiring in metal conduit so your dog can’t chew it. 

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.