You’ve probably read advertisements or received sales calls about the huge energy savings created by installing an attic radiant barrier. The savings claimed often are the very maximum possible in a perfect situation and are exaggerated for a typical retrofit installation.
Having said this, proper installation in a specific house can yield a reasonable payback and better comfort. The savings from installing a radiant barrier in the attic varies considerably, depending upon your climate, house design and construction, 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.
An attic radiant barrier provides little positive or negative effect during the heating season. If your electric cooperative offers time-of-use rates, the savings may be somewhat higher.
It’s important to understand the basics of heat transfer
You need to know a house loses and gains heat, so you can fairly evaluate whether your home is a good candidate for a radiant barrier. The most important basic fact to know is that the rate at which heat flows from a hot area to a cold one is a function of the temperature difference between the two spaces.
Conduction is heat flow through a solid object or 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. This generally increases the rate of heat flow compared to plain conduction through a solid. An example is how your skin loses heat faster during winter on a windy day, causing a wind chill factor that in effect creates a lower temperature.
Radiation is heat flow directly from one object to another through a vacuum, air or glass. It’s not dependent on touching or fluid flow. This is how the sun heats the Earth or you feel warm sitting in front of a raging fire.
Radiant energy is unique
What makes radiant energy special in this discussion is that it’s much more affected by a 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 a radiant barrier is most often used in the attic to block heat flow through the roof. On a hot summer afternoon, the temperature of a dark shingled roof easily can reach 150 degrees. This hot roof conducts heat to the roof sheathing. From there, conduction takes over the radiant heat and carries it down through the insulation, to your ceiling and into your house.
A radiant barrier requires an air gap to prevent it from touching the hot surface; otherwise, it becomes a conductor like any other building material. Reinforced aluminum foil was originally was used as a radiant barrier, but now many barriers use plastic films with reflective surfaces.
In addition to reflectivity, emittance is a property of radiant barriers. It should be lower than 0.25 – 25 percent – in order to be an effective barrier. Aluminum foil is well below the 0.25 level. There also are reflective paints, such as Low/Mit (www.solec.org), that can be sprayed underneath the roof sheathing. Definitely check the emittance spec before signing any contract for installation of a radiant barrier.
However, to get the best payback from energy savings, it makes sense to install the radiant barrier yourself. Companies such as Innovative Insulation, Inc., (www.radiantbarrier.com) sell double-sided reflective foil for about $130 for a 4-foot x 250-foot roll. Invest in a hand construction stapler, a utility knife and a long straightedge and you’re ready to install the material.
The easiest method for installing a radiant barrier is to cut it into lengths and staple it underneath the roof rafters. It’s really not important how neatly it’s installed, but having adequate attic ventilation is critical – preferably a combination of soffit and ridge vents. When installing single-sided foil, face the reflective side down to take advantage of its low emittance.