Vapor Barrier Helps Protect Insulation

<i> From Popular Mechanics</i>

QUESTION: Can you explain to me exactly what a vapor barrier is and how it functions in a house?

ANSWER: Every house generates water vapor from cooking, bathing and human respiration. Slightly higher air pressure inside the house pushes this vapor out through walls and crevices. When vapor-containing warm air hits a cold surface like a wall, the water vapor condenses, wetting the wall and soaking the insulation. Wet insulation is relatively ineffective, paper peels off wet walls and wet wood rots.

Vapor barriers--called retarders in the industry--resist water vapor movement. They’re rated in perms. Any material with a rating of 1 perm or less is considered adequate. A 6-mil thick polyethylene sheet has a perm rating of 0.06--super adequate.

The vapor barrier, whether separate or permanently attached to the insulation, goes on the warm side of the insulation to prevent condensation wetting the insulation whenever possible.


Since no vapor barrier is perfect, some warm moist air will always reach the sheathing and condense. This makes adequate ventilation just as important as the vapor barrier, especially in the attic. Free-moving air evaporates the moisture before it can do any harm.

If you are insulating the attic roof and walls as part of a remodeling project, insulating batts or blankets, with kraft paper or aluminum foil vapor barrier, are the wisest choice. The 16- and 24-inch standard widths fit snugly between rafters and staple easily in place.

However, if there are gaps between the edges of the batts and the framing, you can lose 50% or more of the insulation’s effect. Make the job neat and tight but allow at least one inch between the non-vapor-barrier side of the insulation and the roof sheathing for air circulation.

Make sure each rafter cavity is vented both at the soffit and roof ridge. Vent area should equal one square foot for each 150 square feet of attic floor space. If there is already some insulation in place and you want to add more, do not install a second vapor barrier. It could trap condensation in the insulation layer between the two vapor barriers and ruin the effectiveness of the outer layer.


Insulate Attic Floor Up to Top of Joists

Q: We just purchased an old house that we plan to repair and update in stages. One of our first projects is to insulate the attic floor as the existing insulation consists of a thin blanket of rock wool. What is the best method of insulating this area?

A: What you want to do here is fill the area between the joists to the top of the joists with insulating batts. You’ll need a vapor barrier with the insulation on the side facing the floor. If you want to add more insulation, install unfaced batts or blankets across the top of the joists. This deepens the insulation layer and greatly reduces heat loss through the joists. Loose fill insulation should be installed to the weight volume printed on the bags.

Remember, the National Electric Code warns against covering light fixtures and electric motors with insulation. It requires a three-inch clearance on all sides to keep them from overheating and creating a fire hazard. Build a sheet-metal frame around such fixtures to keep the clearance.