What is a Radiant Barrier Roof? Exploring the Legitimacy of Claims of Characteristics, Test Procedures And “R” Ratings for Thermal Insulations

Information and opinions by George H: (rbisys@juno.com)

The chart data enclosed is taken from a mechanical engineering handbook along with opinions from thirty years field experience.


Confusion about the performance of various insulation materials is not a recent phenomenon. Some of the confusion comes from the fact that various materials control heat energy transfer according to the specific physical properties of the materials and their assembly for use.

Another problem is that large manufacturers, with government sanction, literally control the methods used to test their product and competing products. This has been an ongoing fight for over fifty years in this country. Some products, commonly used here, are not allowed in other countries because of low performance and serious health issues.
The most common testing problems are:

(1) The tests do not reflect actual “installed summer / winter conditions”, which can reveal up to fifty percent difference in performance compared to “accepted tests”.
(2) Most tests favor conductivity resistance and limit the effects of radiant energy. Most homes have about 12-15% conductive surfaces, about 7% is convection and air spaces accounting for up to 80% radiant energy gain or loss.
(3) Some tests do not reveal the serious performance degradation from condensation, actually storing and increasing heat flow, and how it affects the interior humidity levels.
(4) Some tests do not reveal possible mold and other problems.
(5) Some tests, or labeling, do not reveal the health problems due to toxic chemicals. This information is classified as proprietary information and given only to the government.
(6) The tests or labels do not reveal the ratio of material to air volume This ratio can be as low as 1% mass to 99% air volume allowing radiant energy to travel through like an open door, plus air infiltration. The exception to this is radiant barriers which rely on the air space to perform efficiently. If insulation tests were performed with the best interest of the consumer at heart, there would probably be only two insulations available to the consumer.
(7) The other subject ignored by the bulk insulation manufacturers is the approximate 80% heat gain/loss in buildings through radiant heat, infrared energy. This can be expected because most bulk insulations are only about 10 – 20% efficient in rejecting radiant energy, compared to about 97% for radiant barriers.
(8) The “R” factor for bulk insulations are based on the reciprocal of a “u” factor, a conductive test ( for a material that is about 99% air spaces?). The efficiency of radiant barriers are based on a “k” factor. You cannot obtain a “R” value from a “k” factor.

The independent, non competitive, method presented here is based on long established data of energy exchange between two surfaces, ceiling/floor, at a given Delta T” (temperature difference between two surfaces) and will tell you what amount of heat energy is radiated into and out of the home summer and winter.

This method depends on no tests and incorporates the characteristics of the insulation, building materials and the effects of any climate condition. It can be performed by anyone with a thermometer. Conventional “R” factor calculations cannot tell you this, due to the problems mentioned above and that the calculations are usually for material only. With “R” factors you can calculate for one set of condidtions and then find out the calculations had no reference to what is actually going on in the structure.

The common denominator for all insulations is; what is the temperature of the drywall and the floor it is radiating to? This “in situ” method incorporates all the variables because the drywall temperature determines your heating / cooling costs. You can use either Btu calculations or temperature calculations. You can see why the manufacturer of low efficiency insulation will not want to use this method.

The drywall emission rate, about 90%+, is used in the following chart because that is the most commonly used material. The source of this information, and the following chart, is from an emissivity chart and formula of a mechanical engineering handbook. You may not be familiar with this source of information. It is a manual of materials charts, characteristics, formulas and numerous other factors used by engineers to manufacture most every thing you use. For many professional engineers it is the engineer’s “bible”.

THE HUMAN FACTOR: The average person believes that the air temperature is the dominant factor in comfort. This might be true if it wasn’t for the energy radiating into and out of the building with its effects. It is this energy ratio between the interior surfaces and the surface of the body that ultimately determines the comfort factor and energy consumption.

For maximum energy savings you want the lowest rate of absorption and re-radiation of energy. Lower is better. The determining factors of any insulation’s performance are:

1) The rate of absorption and re-emittance ( radiating ) of energy. From the “bible” we see that wood (cellulose), and glass (fiberglass) is about 90%+ efficient in absorbing and re radiating energy. Base foam materials are about 20% efficient. Aluminum foil about .03%.

2) Other than the basic material and its construction features, moisture, either from humidity or condensations can cause substantial energy flow. Using the ratio or 5% increase per 1% of moisture by weight, data published by the National Bureau of Standards shows that fiberglass and cellulose can increase energy flow about 45 / 72% due to moisture in an uninhabited structure. Even the relative humidity can account for a dramatic increase in energy flow. Increased humidity levels in an inhabited structure can cause even more energy lose / gain along with the 1,000+ btu used to convert vapor to liquid. Since radiant barriers do not cause condensation and are superior vapor barriers, the interior humidity levels can be lower than with other insulations.

3) The low quality of installation can also be a detriment to the effectiveness of insulation.

The following chart shows Btu transfer for various ceiling temperatures. Calculations for infiltration, doors and windows are separate as they will be the same for any insulation installed. To increase the envelope efficiency even more, Insulation Specialists has developed a simple method of installing radiant barriers to reduce to about 1% the conductivity surfaces of studs and ceiling joists from the normal 12-15 % surface area.

In summer you can measure the drywall temperature which can reach up to 110 degs on a 95 deg day with the lower efficiency insulations and no roof shading. If the floor temperature is 75 degs the ceiling, using temperature figures, will radiate about 99 degs/sf/hr. The 110 deg ceiling temperature is about 25 degrees hotter than a winter radiant heat system, causing the air conditioner to run continuously to try to compensate. Without the air conditioner the interior temperature could exceed 100 degs.

If the radiant barrier is 110 degs it will radiate about 2-3 degs /sf/hr. In a properly designed ranch home the interior temperature, with radiant barrier, will be about 80-81 degs without air-conditioning. The humidity levels can also be lower as the radiant barrier does not cause condensation which can be forced into the home by the high temperatures in the structure as with some of the lower efficiency materials.

Question; if the indoor temperature can be hotter inside than outside without the air-conditioned, how can the manufacturer claim their material is insulation?
As you use the chart keep in mind these two questions;

1: If bulk insulations are about 99% airspaces and radiant energy travels through space at about the speed of light, and the base material absorbs and re-radiates the energy at about a 80-90 percent efficiency, how can a manufacturer claim their material is an insulator? More importantly how can an “R” value be assigned to them?

2: If the function of a radiant barrier is to reflect energy, how can an “R” factor be assigned to it? How can the government and the manufacturers of bulk insulations legitimately force the use of “R” factors in evaluating radiant barriers? More importantly, why?

3: Why has the US Senate interfered with, at least twice, the governments fair trade polices, including FTC regulations, when it comes to insulations? Regulations which would have provided for a fair playing field. Answer: Over $100,000,000,000.00 tax revenue per year due to the excessive use of energy.
Because of this and other reasons the American home owner is using up to two to three times the amount of energy to heat a cool a home than what should be used.

In summer you can determine the temperature of your ceiling drywall by taping a thermometer to the drywall surface.

This chart is based on a 75 deg floor temperature. The chart can be validated by using the emissivity data and formula from Mark’s Mechanical Engineering Handbook. FG values are for insulation between joists and include joist heat transfer. The radiant barrier value is for the joists surfaces covered with the radiant barrier and a furring strip to separate the radiant barrier from the drywall. “A” is the dry wall temperature. “B” represents the Btu’s radiated for the FG installation. “C” represents the Btu’s radiated for the radiant barrier installation. “D” the Btu difference between the fiberglass and radiant barrier. Although the mechanics for side walls will be slightly difference this method can be used for approximate comparisons.

The 110 deg is high lighted to represent a 95 deg day. The 30 line is highlighted to show the similarities of the summer winter conditions. Note the jump when the temperature gets down to zero degs. Because of the rapid drop off in FG efficiency as the material thickness is increased it is difficult to extrapolate the radiant barrier and FG data for “R” value comparison. Compared to the advertised “R” value for FG the RB “R” factor could exceed “R”100 value by a considerable amount, and it is impossible to have a “R” value of 100 much less 100 plus.

Myth: Dust adversely affects the radiant barrier performance. A: Dust has little or no effect on a horizontally installed radiant barrier with airspace both sides. The top surface could be painted black and the bottom surface might emit 1 or 2 extra Btus. Most ceiling installations have one or more layers, so any increase in heat flow is doubtful. There is little or no dust on vertical installations. Even with dust present the radiant barrier is superior to other materials. These comments never reveal the test material type or test method or actual performance differences.
Myth: Holes adversely affect the radiant barrier performance. A: Some radiant barriers are manufactured with vapor escape holes. I know of no laboratory tests showing an increase in heat flow, particularly in multi layer installations. Obviously you don’t want large holes, these should be repaired.
Myth: Radiant barriers are not as efficient on up heat (winter) as summer. A: The engineering handbook does not make such a distinction. The mechanics of up heat vs down conductive heat flow are different; therefore any given material may exhibit slight differences for winter. However these comments never note that the radiant barrier is still superior to other materials.
Myth: Aluminum corrodes. A: Pure aluminum, such as the 99.9% pure foil used in radiant barrier, does not corrode under normal atmospheric conditions.
A light, invisible, oxidation does occur preventing any further oxidation. You would not want to breathe the fumes that could cause severe corrosion. Corrosion can and does occur in some unfinished alloy aluminum because of the dissimilar metals used for alloying the metal.
Myth: Radiant barrier loses its insulation values over time. A: Since radiant barriers do not corrode, the answer is self evident. I know of installations over 30 years old that work just fine.
Myth: You can’t use radiant barriers in very cold climates: A: When Perry and other scientists went to the poles they use aluminum foil to insulate the structures. The Navy SEALS used multi-layer foil (mfg’d to mil spec HH I 1252) in 1964 in the Artic buildings where the mineral wool was failing. Radiant barriers are used quite extensively and exclusively, in severely cold conditions, such as, cryogenics and space platforms.
Myth: Radiant barriers are not very efficient in attic add-on application. If the application is not proper then this is a true statement. However, the retrofit tests so far conducted are not the most effect application method. I have found that a double layer installation directly over the existing material can reduce a/c run time 50% or more. Why test the most inefficient method?