Attic ventilation has a big role in properly insulating house

If you're planning to insulate the attic this fall, be sure it has enough ventilation or you could run into a problem. Without sufficient attic ventilation, warn energy-conservation experts, you can add to the discomfort of the house, increase the cost of air conditioning in the summer, and even risk structural damage to the roof and the loss of insulation value in the winter.

Why insulate the attic, anyway? Because that's where a house loses the greater part of its heat in the winter. A properly insulated attic, says I. B. Spangler Jr. of Alcoa Building Products Inc., a subsidiary of the Aluminum Company of America, will provide:

* Greater comfort for you and your family.

* Cash savings on heating and cooling as well as home repairs.

If the attic is sealed tight, however, the homeowner is asking for trouble.

"In an unventilated attic, the summer sun can raise the roof temperature to 160 degrees F. or more," says Mr. Spangler. "Even adequate insulation on the floor of the attic can't stop some of this heat from being radiated to the ceilings of the rooms below and then down to the rooms themselves and the people in them.

"An effective attic-ventilation system keeps the attic temperature within 10 or 15 degrees of the outside temperature, even on the hottest days."

When attics and ceilings are overheated, the air-conditioning system has to work much harder and, as a result, wastes energy and money. An overheated attic also can cause roof shingles to deteriorate and even to buckle and come loose.

In the winter, of course, moisture from the house can easily work its way through the ceiling insulation and into the attic unless you have an adequate vapor barrier to prevent it. Even then, the moisture very often has a way of getting through.

Obviously, if the moisture gets into the cold air of the attic, it can condense into droplets of water, soaking the roof sheathing, rafters, and other structural members of the house. If the insulation itself gets wet, it loses a lot of its effectiveness.

The result? Costly repairs to the roof, peeling paint on the outside, and higher heating bills because of the less-than-optimum performance of the insulation.

All insulation materials are not equal. When buying insulation, don't talk about inches. Instead, talk about R-value. The R-value, or the ability of a material to resist the passage of heat, provides an accurate measurement of its insulating ability.

"The higher a material's R-value," says B. G. Woodham Jr., techical services manager of Owens-Corning Fiberglas Corporations's insulation operating division, "the greater that material's ability to resist the passage of heat through the walls and attic."

One brand of insulation may be thicker than another brand, for example, but if they're both marked with the same R-value, they'll both perform the same in resisting heat flow in a building.

Before you decide to insulate the attic, remember that insulation is available in two basic forms: Blanket, or batt, and loose fill.

Blankets are easiest for the do-it-yourselfer to use and, if applied as directed on the package, will provide the same insulating value over the entire surface to which they are applied.

Loose-fill insulation is poured from a bag or blown under pressure into a wall cavity between the studs and joists.However, if poured onto the floor, loose-fill insulation can easily be blown about the attic. In other words, it may not stay in place.

Insulation experts say it is much more difficult to get an R-value figure with a loose-fill product.

You can consult a building-materials dealer for suggestions on the recommended thickness, or R-value, for various insulation jobs in your part of the country.