Radiant Barriers

Radiant Barriers

There are three methods by which heat can be transferred.  Heat is transferred between materials by conduction, (like touching a hot surface;) convection, (like touching the air near a hot surface,) and by radiation, (the warmth that you feel directly from the sun that is especially noticeable on days with low air temperatures.)

Attic insulation is installed to slow heat transfer by conduction.  A radiant barrier is installed to slow heat transfer that occurs by radiation.  Radiant barriers use reflective foil to block radiant heat transfer from the roof. Radiant barriers may be installed in a couple of different configurations. The easiest is on the attic floor, which involves laying the radiant barrier on top of the existing attic insulation. Another way to install a radiant barrier is to attach it just beneath the roof, separated by a small air space.

The paragraph below is from an article written by Mario A. Medina that appeared in the Home Energy website:

http://homeenergy.org/archive/hem.dis.anl.gov/eehem/00/000915.html

Heat transfer across the ceiling from the attic space, or across roofing directly into the living space when a home has cathedral ceilings, contributes significantly to the cooling load in residential buildings.  Ceiling heat flows originate with the incident solar radiation that is absorbed by the roof.  The amount of heat that is not re-radiated or convected from the roof is conducted across the attic decking material.  Of the heat that arrives at the opposite surface of the deck, part is convected to the attic air and the rest is radiated to the ceiling frame.  The net heat that is absorbed by the ceiling frame during the radiation exchange is both convected to the attic air and transported in the direction of decreasing temperature into the conditioned space, where it becomes part of the cooling load.  In general, except for the solar energy that is absorbed by the roofing materials, heat transfer processes in the heating season are similar to those described above, but in the reverse direction.

Installation

In an attic, a properly installed radiant barrier that faces an air space can block up to 95 percent of the heat radiating down from a hot roof. This helps reduce heat gain (via conduction) in the conditioned space caused by the solar radiation on the roof in the summer (a significant driver for cooling load). However, the associated energy and cost savings depend on numerous factors, including the installation location (that is, whether the barrier is ceiling or floor-mounted), the square footage of attic space, the amount of existing insulation and ventilation in the attic, the color of the roof, thermostat settings, outdoor temperature, and the tightness of the building envelope. The greatest opportunities for energy savings from a radiant barrier are in applications where the existing insulation is minimal. However, the same can be said of adding insulation, which benefits the customer in both hot and cold seasons.

How much will a radiant barrier reduce the attic load?

The more attic insulation installed the lower the percentage of load reduction brought about by the radiant barrier.  Figures can be found for a reduction in load of all the way from 5% to 45%.  Since ceiling heat gains only represent about 20 to 30 percent of the total cooling load for a house, a radiant barrier will reduce cooling energy consumption by about 3 to 9 percent (multiply the percentage of the total cooling load by the percentage reduction in ceiling heat flow). A study conducted by the Oak Ridge National Laboratory and the Florida Solar Energy Center concluded that “If your roof accounts for less than 20 percent of your cooling load, then an attic radiant barrier can’t possibly save more than 20 percent [as some manufacturers claim] on your bills.”

Problems/Concerns:

Improperly installed radiant barriers may increase the temperature of roofing materials.

Improperly installed radiant barriers may block airflow currents needed to ventilate attic spaces.

The dusty environment found in the attic may reduce its long term effectiveness.

In retrofit applications installing a radiant barrier can be very difficult.  Unlike spraying in additional insulation, installing a radiant barrier requires a substantial number of hours in labor to correctly install the material.  It would not be practical or even safe to install this during summer months.

Installing a radiant barrier into an existing structure would disturb the blown in attic insulation to a degree that it would require additional insulation to be blown in.

Additional thoughts

It is much easier to deal with radiant barrier that has a foam backing or is layered with a bubble-wrap type material.  Radiant barriers that are foil only can tear very easily making a difficult installation even worse.

If an individual wanted to dramatically reduce energy consumption in new construction a better alternative is to use spray on foam insulation on the underside of the roof and treating the attic as a conditioned space.<

The installation of a radiant barrier may make the most sense for someone wanting to reduce the load on a portion of the roof that is causing a problem in conditioning an interior space, such as a bonus room.  Again, blown in insulation would likely need to be added after installation of the radiant barrier.

For more information on radiant barriers, you can visit websites maintained by ORNL and the FSEC. Additional information on reflective barriers is available from the Reflective Insulation Manufacturers Association (RIMA) at 1-800-279-4123 or www.rima.net.