Radiant Barriers
Science Vs Hype

By
Ron Birrell
Master Solutions
Copyright 2006.

Heat Movement in the Attic

"...conduction, radiation and convection are the primary mechanisms of heat transfer in a structure." ---Department of Energy

Convection - or transfer of heat through air

Hot air transfers heat to surrounding material (rafters, ducting, insulation) heating the attic space thereby effecting the living space.

The Answer - Soffit and ridge vents.

Conduction - or transfer of heat through material

Hot roof transfers heat from the surface of the shingles through the material to the underside of the roof heating the attic space or through the insulation, thereby effecting the living space.

The Answer – Batts or blown insulation.

Radiation - or transfer of heat through electromagnetic (i.e. radio) waves

Radiant energy transfers through space in straight lines until it absorbed (causing heat) by rafters, ducting, insulation, and on into the living space.

The Answer – Radiant barrier.

Radiant Energy

What is Radiant Energy?

The energy transported by electromagnetic radiation.

Radiant energy is commonly called light energy. But light energy is only one kind of radiant energy. All waves emit energy. Radio and television waves are other types of radiant energy. So are micro waves and x-rays. Light waves do work by wiggling the receptors in the back of our eyes.

When this radiation hits an object in it's path, then it may get absorbed partly or completely and transformed into some other form. For example, when you are standing in front of a fire, the heat you feel is transmitted by thermal radiation. Heat will move by radiation even in a vacuum, as the sun’s heat is transmitted to earth through space.

What Does Radiant Energy Do?

When radiant heat is viewed as a form of energy, a wave length, rather than a type of heat movement, it becomes easier to understand how it behaves with fibrous insulation products. The radiant energy from the sun travels through the roof, insulation, and into the living space. Depending on how much gets absorbed by the particular materials will result in the amount of heat generated.

When radiant heat can be reduces from entering a home, conductive and convection also decreases proportionately. Therefore, if you can reduce the amount of radiant energy, you reduce convective and conductive heat loads which increase comfort and significantly reduces the need for energy.

Reducing radiant heat can be accomplished by placing one or more highly reflective surfaces between the space being cooled (your home), and the source (the sun or you ceiling during the winter). Keep this in mind: Heat always moves from a higher temperature to a lower temperature.

Radiant Heat Facts:

• Studies have demonstrated that 75 to 94 percent of the heat gain is from radiant energy.
• Radiant heat travels in a straight line from the source (the sun for instance), to the absorber (the roof and walls of our homes and offices).
• Radiant heat travels at 186,000 miles per second (speed of light)
• Radiant heat can only be absorbed or reflected. The amount of which is determined by the surface it is striking, angle to the source and absorbtivity of the surface.
• Radiant heat strikes the earth, our homes, our cars, etc., which in turn heats the air. A good example of this is our changing seasons. Even though the sun is much closer to the earth during the winter, the angle at which radiant heat strikes the earth is a lower angle which bounces; much of this radiant heat back into space.

Why Doesn’t More Insulation Work?

The “R” Value.

The R-value of Conventional fibrous batt and blown insulation is a laboratory measurement that does not consider all 3 methods of heat transfer, only measures resistance to conduction. Basically, the higher the R factor, the slower heat travels through it

Effect of Radiant Energy on Insulation.

Insulation absorbs radiant energy, thereby heating up the insulation. Since insulation has an R factor, heat gets trapped for many hours in the insulation against the ceiling creating a demand for cooling energy many hours after the sun has gone down. You can see this by going into your attic several hours after sunset. Notice the attic is still hot! Where is the heat coming from? Now feel the insulation...it's warm.

Without protection from radiant heat, insulation traps, stores, then releases heat for hours consuming energy, reducing comfort, costing you energy dollars.

Radiant Barrier

What is a Radiant Barrier!

A radiant barrier is a highly reflective surface that easily changes the direction of infrared heat instead of having it absorbed into your insulation. Why slow heat down in your home or office when you can change the direction with a low cost, proven, high performance Radiant Barrier? The performance is unmatched and comfort and savings are immediate.

Scientific Studies on the Benefit of Radiant Barrier.

Texas A&M University, Nevada Power Company,
Florida Solar Energy, Northeastern Illinois University,
TennesseeValley Authority, University of Mississippi,
The Oak Ridge National Laboratory,

All conducted studies on radiant barrier systems. All found conclusively that radiant barriers help keep cooling energy costs down.See attachment

If Radiant Barriers are so great, where are they used?

All over the US and in Canada
California and Florida mandate Radiant Barrier for new construction.

Definitions.

Emissivity - how much heat is emitted from a material.

Aluminum has a very low emissivity, meaning very little heat is radiated from its surface.

Reflectivity - how much energy is reflected, not absorbed by a surface.

Aluminum has a high reflectivity.

The lower the emissivity of a surface, the higher its reflectivity and the more energy efficient it is.See attachment for addition ratings

Radiant Barrier Products

Product Comparisons

There are several radiant barrier building materials on the market. These products should be carefully evaluated based on emissivity, ease of handling, installation, durability, and cost

• Foil-faced plywood or Oriented Strand Board (OSB) roof sheathing is installed like ordinary roof sheathing with the foil layer facing the interior of the attic. Product should be incised, this allows the sheathing to "exhale" moisture through the aluminum face
  
• Single or double-sided foil is either draped over trusses before the roof deck is installed or attached to the underside of trusses or sometimes installed over standard insulation on the attic floor. The foil is reinforced with various materials to compensate for its tendency to tear easily, but care must be taken not to puncture or tear the foil during installation and routine attic maintenance. Dust build-up on the surface of the foil increases emissivity and reduces effectiveness.
  
• Foil-faced insulation and blown-in foil chips are used on attic floors where dust buildup on the reflective surface increases emissivity and reduces effectiveness.
  
• Radiant Barrier paint is designed to be applied to the underside of roof decks or roof tops. It goes on like an ordinary latex paint (brush, roller or spray). Last the life of the roof deck when installed on the underside. No dust buildup, therefore, emissivity and effectiveness remain the same. Can be sprayed into areas unreachable with foil blankets.

Product	Material	Emissivity	Base	Cost
Radiosity 3000®	Paint additive	NOT RATED	Borosilicate Glass Beads	$50/ to mix with 5 gallons of paint
Lo/Mit™I*	Paint	.20	Xylene	35 - 65¢ per sq. ft. installed
Lo/Mit™II*	Paint	.20	DI Water	35 - 65¢ per sq. ft. installed
Radiance™*	Paint	.25		89¢ per sq. ft.installed
Radiance™*	Paint	.49	Zinc Silfide	$45/gal coverage 250 sq. ft.
E-Barrier™	Paint	.32	Vinyl Acetate-Ethylene Copolymer	$55/gal coverage 350-400 sq.ft.
Techshield™	Foil bonded to OSB	.03	Aluminum	13¢ Sq. ft. over the OSB
ELK Echotherm™	Foil	.03**	(Aluminum)	37¢ Sq. Ft.
K-Shield™	Foil	.03**	(Aluminum)	$1.50 Sq. Ft.
*Only installed Professionally **Based upon standard ratings for Aluminum Foil, Product not tested.

Radiant Barrier Ratings

The radiant barrier should have an Emissivity Rating.This is the efficiency value calculated or the result of testing. Testing should be done by a third party laboratory to either testing standard ASTM E-424-71 or ASTM E903-96 or using a Solar Spectrum Reflectometer.

The radiant barrier should be an Energy Star Product. ENERGY STAR is the symbol for energy efficiency, as designated by the U.S. Department of Energy (DOE) and the U.S. Environmental Protection Agency (EPA).

The radiant barrier should conforms to at least one of the follow as well as any applicable safety standards;

ASTM C 1313-00 Standard Specification for Sheet Radiant Barriers for Building Construction Applications

ASTM C 1158-01 Standard Practice for use and Installation of Radiant Barrier Systems (RBS) in Building Construction

ASTM C 1321-98 Standard Practice for use and Installation of Radiation Control Coating Systems (IRCCS) in Building Construction“American Society for Testing and Materials”