Pick a furnace filter, then be sure to replace it

Q. I’m confused by the many types of filters sold at home centers for forced-air furnaces and air conditioners. I want to keep the air in our house as clean as possible.

The salesperson wasn’t much help, but said the blower motor can be damaged by improper filter use. Can you help?

A. There are basically three types of replaceable filters for central heating and cooling units, although there are so many different sizes that sizing alone can be confusing.

The basic types are fiberglass filters, which are the least expensive, often costing only a dollar or two each, and do the poorest job of filtering out dust and dirt, capturing only the largest particles in the air stream; pleated filters, which look something like an old-time washboard and capture much smaller particles because of their extra filter area and tighter construction (average price about $5 each); and electrostatic or high-efficiency filters, which can capture very small particles and cost the most (up to about $20 each).

Some dealers also sell washable filters, which cost the most and should be cleaned about every six months, following the directions supplied with the filter.

Pleated filters are a good choice for most systems. However, if someone in the house has respiratory problems, electrostatic filters, with their ability to capture tiny particles such as mold and pollen, can be the best buy.

The main danger of filters is failing to change them regularly. If the filter becomes overly clogged, it can put extra strain on the fan motor and also cause extra fuel consumption.

Always read the directions for your specific filters, but in general fiberglass filters should be changed once a month; pleated and electrostatic filters will last for about three months.

Before buying new filters, check the size of the existing filter and note the correct direction of the air flow, indicated by arrows on the rim of the filter. When I change a filter, I mark the date of insertion on the rim and check occasionally to see how much dirt is being trapped.

Q. My basement has insulation on all the walls. It is about 3 inches thick and has a silvery covering facing the inside. It is nailed to the block walls.

I want to finish the basement by installing framing and drywall. Is it OK to use the insulation as it is? The basement has no water problems.

A. The insulation appears to be fiberglass blankets with a foil vapor barrier. The vapor barrier is in the correct place, facing the warm side of the walls.

It is unusual to insulate a basement this way unless there are no plans to finish it. Normally, the framing would be constructed against the walls and the insulation would be stapled into the cavities between wall studs.

It should be OK to install framing inside the existing insulation as long as there are no moisture problems in the basement, but of course you will lose more floor space.

It would be a good idea to check behind some of the insulation to make sure it is dry and there is no hidden seepage. You could save a little floor space by using 2-by-3 framing instead of 2-by-4s.

Q. Part of the asphalt roof of our house, which is shaded by a large maple tree, has developed gray lichens. I don’t like the look of them, but I am convinced that any attempt to remove them would do more harm than good.

We like the tree’s shade and don’t want to remove it. Any ideas?

A. There is certainly some controversy about whether mold-killing roof cleaners damage shingles, but the consensus seems to be that most cleaners are safe if used properly, which usually includes thorough rinsing after cleaning.

Asphalt shingles, which are used on most houses in the U.S., are very tough and need to be to survive the sun, heat, cold, wind, rain, snow and hail that comes their way. One expert even claims to have used a pressure-washer at high pressure, held close to his shingles, and reports no damage.

I don’t recommend pressure washing unless done at low pressure (about 500 psi) because I believe it can dislodge protective granules from the shingles. The best bet is to test any cleaner on a very small area, or on scrap shingles, and check the results after a few weeks to see if there is any damage.

The most dangerous thing about roof cleaning might not be the cleaners, but do-it-yourselfers who risk serious injury because they are unskilled about work on roofs. Wet roofs are especially dangerous, so it pays to have an insured professional handle the job.