To get snugness you seek, include a vapor barrier when you add insulation


August 08, 1992|By Karol V. Menzie and Randy Johnson

Spring, summer, fall, winter -- it seems people are always thinking about insulation. It's not a bad thing to put a little thought into because the choices are so varied and the difficulties (and expense) of going back later and redoing an area can be so great.

A reader in New York wrote to ask us about insulating the floor of a summer-cottage addition, built on treated wood pilings.

"The question is the floor," the reader writes. "It is one thickness of 3/4 inch plywood resting on 2 by 6 crosspieces over the pilings . . . We plan to cover it with wall-to-wall indoor-outdoor carpet. Should we add another thickness of plywood over the existing 3/4 inch floor before putting down the rug? The contractor said it is not needed. Other people (not builders) have said it should have another layer to stop moisture coming in, especially where the 4 by8 plywood butts together. Some also said we should put down a vapor barrier plastic sheet between the existing floor and a top layer of plywood."

The contractor is right -- it isn't necessary to add any more thickness to the floor under carpet, unless the existing floor has a problem, such as unevenness or gaps in the plywood. If there is a question about the gaps between the existing plywood sheets, it wouldn't hurt to add another 3/8 inch thickness, with the sheets running perpendicular to the existing layer.

If the reader were installing tile or sheet-good flooring, it would be necessary to add underlayment to the existing floor. Heavier tile, such as slate or ceramic, requires thicker underlayment, up to 3/4 of an inch.

The place to deal with a potential moisture problem, however, is not on the floor. It's on the ground. Bare ground in crawl spaces or basements is a major source of moisture vapor. The easiest solution is to cover the ground with building paper (15-pound felt) or heavy plastic, weighted down with rocks or gravel so it doesn't wash or blow away. A permanent solution is 2 inches of gravel, a plastic vapor barrier and 4 inches of concrete.

However, the reader said the cottage addition was on a hill, on pilings. If the space underneath the addition isn't enclosed, covering the ground won't make much difference.

In either case, the floor needs to be insulated, underneath, between the joists, with paper-faced fiberglass batt insulation, with the paper facing up. (That makes it harder to install, but the vapor barrier must face up, toward the space that's heated on the odd cold spring or fall night.)

To help keep the insulation up, and to protect it from weather and bugs, the bottom of the joists could be enclosed with plywood sheets -- sort of a sandwich, with the floor on top, the plywood under the joists on the bottom, and the insulation in the middle. The bottom plywood could be primed and painted, for additional moisture protection.

Although the cottage is not used in the winter, on any chilly day the two largest sources of heat loss are the ceiling and the floor. Those are the most important places to insulate.

Another insulation question comes from a Baltimore reader, who lives in a two-story brick rowhouse with an uninsulated 3-foot-high crawl space under the roof. "I want to insulate this space with blown-in insulation, but I am confused as to which to use, between fiberglass or cellulose fiber." He's been told that cellulose has a tendency to settle and that the high humidity in the Baltimore area will cause it to lump together. But he's also been told there's a health hazard with fiberglass.

We can't really say which type of insulation is best. Like a lot of products, each has advantages and disadvantages. Choosing between them is largely a matter of what you're comfortable with.

Avoiding problems with blown-in insulation is mostly a matter of making sure the space it will be blown into is sealed top and bottom, and that any recessed ceiling fixtures are boxed in to keep excess heat from building up in the fixture.

Before the insulation goes in, the attic space must be completely closed, with all cracks, access doors and other openings sealed. There are two reasons for this. First, you don't want the insulation material to get into the living area of the house and

second, you don't want warm, moist air from the heated part of the house getting into the attic, where it could cause condensation problems. Also, the roof should be checked to make sure there are no holes or leaks, since wet insulation loses its insulating value.

Health risks seem greatest to the installer; most manufacturers recommend procedures that will keep such risks at a minimum.

Next: Summer rain and a leaky roof.



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