Phil writes from North Reading, Massachusetts: "I have 10 inches of loose fill insulation in my attic, but there is no vapor barrier. I plan on adding R-19 unfaced insulation perpendicular to the joists,for a total of 15 inches. Should I add a vapor barrier, and if so, how and where?"

Answer: When insulating on top of existing insulation we generally recommend the use of unfaced insulation. That's because the vapor retarder should be on the warm-in-winter side of the insulation, which in your case is under the existing insulation.

Most communities require a vapor retarder but its use and placement varies throughout the U.S. depending on humidity levels. Your local building code official makes those decisions and any exceptions. Some building codes do not require a vapor barrier in your situation if the attic space has sufficient ventilation.

If you need a vapor barrier, consider getting a "paint on" vapor barrier for the ceilings below the attic. This paint can be found at any local paint shop.