Question

Is radiant barrier sheathing worth using?

Asked by Betty Wyse
Saint Thomas, MO

We are building a new home in South Carolina, and the contractor recommends it to reduce attic temp. It is not used here in Missouri and we are unfamiliar with the product.

Answer

Cynthia Phakos

Answered by Cynthia Phakos

Los Angeles, CA

Koffka Phakos Design

December 19, 2012

I live Southern California, and our 2013 energy code, Title 24 2013(effective January 1), requires that all new residential wood construction use Radiant-Barrier Sheathing at the roof.

I have been an advocate of it use since I experienced firsthand the difference it makes. During construction on a house where we were remodeling and also adding space, the structure was stripped of finishes and the addition with the new radiant sheathing was significantly cooler than the existing home without.

With your new house in South Carolina, which is in a Southern climate, the roof of the house is the area most affected by heat gain from the sun, and so this area makes most sense to upgrade. 

As radiant-barrier sheathing is only one component of the roof and attic space, you might consider also:

  • upgrading the insulation to a higher R-Value,
  • insulating the ductwork,
  • installing a cool roof,
  • sealing any of the penetrations in the ceiling, as well as
  • installing a radiant-barrier sheathing.

How Radiant-Barrier Sheathing Works

This is important to understand so that it will be installed properly.

All materials either reflect or absorb radiant heat. Materials that absorb heat will then radiate heat – whereas a low emitting material won’t radiate it or emit it. These materials have a low emissivity and are commonly referred to as low-e materials. Low-e windows work similarly to a radiant-barrier, with a low-e coating on the interior of the outer pane of glass.

When the sun hits the roof, the sheathing warms up and radiates heat, which in turn heats up the rafters, air handlers, ducts and anything else in the attic space, making for a hotter attic and therefore a hotter home.

Radiant-barrier sheathing, made up of either plywood or oriented strand board (OSB) with a thin layer of aluminum facing on one side, is installed to the roof rafters with the aluminum layer facing down.

  • The aluminum is not used for its reflective properties but for its low emitting properties.
  • The sheathing absorbs the radiated heat, yet the aluminum facing does not allow it to emit into the attic space.
  • In the winter, when you are heating your home, the reflective property of the aluminum facing will reflect the heat back into the house and so the design of the radiant-barrier benefits your home during cooler times as well.

The Oak Ridge National Laboratory (ORNL) has done extensive research on radiant-barrier sheathing and has put together a Fact Sheet (here) that in detail lists potential energy savings for your home, as well as different installations and criteria for purchasing and installing it. Some of the facts they list are: Radiant-barrier sheathing emits 3%- 5% of the heat it absorbs and can reduce summer heat gain in the roof from 16 to 42%. 

Specifications

In selecting your radiant-barrier sheathing make sure that the label indicated that the product emittance is less than 0.1, as measured by ASTM 1371. Emissivity ratings are between 0 and 1 with 0 being the optimal performance for a low-e material.

Radiant barrier materials must also have a high reflectivity of 0.9 or 90% or more and must face an open air space of not less than 3/4” to perform properly, since anything that it comes in contact with it directly will heat up through conduction as will the roof rafters.

As many homes have ducts or air handlers in the attic, the radiant-barrier sheathing will contribute to your energy savings by keeping these elements cooler.

Cost

Depending on the company, compared to conventional sheathing, the radiant-barrier sheathing costs only about $5 per 4x8 sheet.

 

For more information:

Read "Should I include a radiant barrier in my new roof installation in Northern California?" a Q&A answered by Tammy Schwolsky.

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