Which way does the insulation go?

Insulation between the floor joists in the crawl space under half of our house was installed with the foil side toward the ground instead of toward the floor. i understand that this traps moisture in the insulation and reduces its effectiveness. We also have mildew in the bathrooms that are over this crawl space. We've been told by local hardware and building-supply people that everyone installs batts upside down under the floor because they're so much easier to handle -- so don't worry. What do you say? Barbara D. Kirkpatrick Huntsville, Ala.

"I say the vapor barrier should always be on the hot side," says Fred Creswick at the Oak Ridge National Laboratory in Tennessee.

"You install insulation in the attic with the vapor barrier -- or foil side of the insulation -- toward the floor of the attic," he adds. "Why is it any different under the house?"

This is the standard practice, as I understand it.

Of course, mildew in the bathroom could be from other sources. You could try to reverse the insulation at least under the bath, suggests Mr. Creswick, who is in charge of conservation at the Oak Ridge lab.

Be sure to ventilate the bathroom after a hot bath or shower so as to disperse the steam.