The Oak Ridge National Laboratory estimates that annual savings from installing radiant barriers can be about $150 for very hot climates to $40 for cold climates.
Photo by James Dulley
Question: My air-conditioning costs were high last summer, and we still felt too warm at times. Ads for radiant barriers say they help save a lot, but I don’t know how they work. How much does one cost, and will it save much?
Answer: Advertisements predict huge energy savings from installing attic radiant barriers. The savings claimed are often the maximum possible and exaggerated for a typical retrofit installation. But, having said this, proper installation can yield a reasonable payback and better comfort.
The savings from installing a radiant barrier in the attic vary considerably, depending on your climate, your home’s orientation to the sun and other factors. The Oak Ridge National Laboratory estimates the air conditioning cost savings can range from about $150 annually for very hot climates to only $40 for cold climates. If your electric cooperative offers time-of-use rates, the savings may be somewhat higher. During heating seasons, attic radiant barriers provide little positive or negative effect.
You need to know some basics about heat transfer—how a house loses and gains heat—to understand how radiant barriers can help in a home.
Heat flows in three ways: conduction, convection and radiation.
Conduction is heat flow through a solid object or through several objects touching one another. This is how the handle on an iron skillet gets hot on the stove. The walls and ceiling of a house also lose or gain heat this way, because the building materials are all nailed together.
Convection is where heat flows through a moving fluid or gas. An example is how your skin loses heat faster during winter in the wind.
Radiation is heat flow directly from one object to another through a vacuum (space), air, glass or other such medium. It is not dependent on touching or fluid flow. This is how the sun heats the Earth or how you feel warm sitting in front of a fire.
What makes radiant energy unique is that it is much more affected by the temperature difference than the other types of heat flow. For conduction and convection, if the temperature difference between indoors and outdoors doubles, the heat flow also doubles. With radiation, the heat flow is 16 times greater when the temperature difference doubles.
This is why radiant barriers are most often used in the attic to block heat flow through the roof. On a hot day, the temperature of a dark shingle roof can easily reach 150 degrees. This hot roof conducts heat to the roof sheathing; the heat is then carried down through the insulation, to your ceiling and into your house.
Reinforced aluminum foil, which reflects heat, has typically been used as a radiant barrier, but now many barriers use plastic films with reflective surfaces. In addition to reflectivity, emittance is an important property of radiant barriers for inhibiting heat transfer. It should be lower than 0.25 (25 percent) to be an effective barrier. Aluminum foil is well below the 0.25 level. Another option is reflective paints, which can be sprayed underneath the roof sheathing. Definitely check the emittance spec before signing a contract or making a purchase.
A certified installer can install a radiant barrier in your home. Or, to get a better payback from the energy savings, you can install the radiant barrier yourself. You can buy double-sided reflective foil for about $130 for a 4-by-250-foot roll. You’ll need a hand construction stapler, a utility knife and a long straightedge to install it. Follow the manufacturer’s instructions carefully.
The easiest method to install the radiant barrier is to cut it into lengths and staple it underneath the roof rafters. It is not important how neatly it is installed, but it is important to have adequate attic ventilation, preferably a combination of soffit and a ridge vent. Radiant barriers require an air gap to prevent them from touching a hot surface; otherwise, they become a conductor, just like any other building material.
When installing single-sided foil, face the reflective side down to take advantage of its low emittance.
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These companies offer materials for installation of radiant barriers and radiation-control coatings:
SOLEC - (609) 883-7700
Innovative Insulation, Inc. - (800) 825-0123