Q: A company has proposed installing "Global Energy Shield" in the attic area of my home over the blown in insulation and covering all areas including the garage ceiling, which has blown in insulation and over the sky light areas, the cathedral and other raised ceilings. The material is a heavy gauge foil perforated with very small holes approximately half-inch apart.

The specifications on the material state: reflectivity 97 percent, emissivity 3 percent, flame/smoke rating 25/30 Class 1 Class A, tensile strength 73 lb./sq. in., puncture resistance 52 lb/sq. ft., weight 31.8 lbs./1000 sq. ft., thickness 1.4 mil.

AJC STAFF (ENLARGE)

Ducts in this attic are wrapped in radiant barrier material, which can save on cooling bills. But there is a risk of trapping moisture in your insulation.

My question? Is this a good insulation to add over the blown in insulation?

A: The material you are referring to is commonly known as a radiant barrier. To understand radiant barriers it is helpful to understand a little about radiant heat transfer.

All materials give off some energy through thermal radiation. The amount of energy they give off depends upon the temperature of the material and one of the properties you listed above, "emissivity." Simply, emissivity is the amount of radiation a material will emit. The lower the percentage the better. The materials that your roof is made of (roofing and roof sheathing) have fairly high emissivity, so when the hot summer sun hits your roof the roofing and sheathing absorb and then emit radiant heat into your attic. Some of this heat transfers through your insulation and into your house increasing your cooling bills.

Another property of materials is called reflectivity. This refers to the ability of a material to repel or reflect radiant heat. Radiant barrier products should have a low emissivity and a high reflectivity. Typically you would look for an emissivity of less than 5 per cent and a reflectivity of greater than 95 per cent. The ratings of your material meet these criteria.

The idea of a radiant barrier is to reflect radiant heat and lower your cooling bills.

There are several places that a radiant barrier can be installed. Common ones are on top of attic insulation as you describe, against the under side of the roof framing members (rafters), and on the bottom side of the roof sheathing. The method your contractor has recommended is the most common because it is the easiest and, therefore, least expensive installation method. It is also the most effective — at first.

The problem with this method is that, over time, dust will settle on top of the barrier and reduce its effectiveness. Installing the barrier on top of the insulation also has the potential of trapping moisture in the insulation. Although the material, as you noted, is perforated to allow moisture to escape, experts are unsure of the number of perforations necessary to keep from trapping moisture.

Most energy experts feel that a radiant barrier can save between 5 percent and 15 percent on cooling bills. If your monthly cooling bill is $200 you would save between $10 and $30. These estimates are based upon an attic that is insulated to R-19. (This is equivalent to approximately 8.6 inches of loose fill insulation). If your insulation is deeper than this, the effectiveness of the radiant barrier will be less.

To summarize, there is definitely a potential for saving on your cooling bills by installing a radiant barrier. However, the effectiveness of the installation method proposed on your house may reduce over time and carries an uncertain risk of trapping moisture in your insulation.