PHYSICS OF Radiant Barrier Insulations
HEAT GAIN/LOSS IN BUILDINGS
There are three modes of heat transfer. CONDUCTION, CONVECTION and RADIATION (INFRARED). Of the three, RADIATION is the PRIMARY MODE; conduction and convection are secondary and come into play ONLY AS MATTER INTERRUPTS or INTERFERES with RADIANT HEAT TRANSFER. As matter absorbs RADIANT ENERGY, it is HEATED, develops a difference in temperature, and results in molecular motion (conduction in solids) or mass motion (convection in liquids and gas).
CONDUCTION is direct heat flow through matter (molecular motion). It results from actual PHYSICAL CONTACT of one part of the same body with another part, or of one body with another. For instance, if one end of an iron rod is heated, the heat travels by conduction through the metal to the other end; it also travels to the surface and is conducted to the surrounding air which is another, but less dense, body. An example of conduction through contact between two solids is a cooking pot on the solid surface of a hot stove. The greatest flow of heat possible between materials is where there is direct conduction between solids. Heat is always conducted from warm to cold never from cold to warm, and always moves via the shortest and easiest route. In general, the more dense a substance, the better conductor it is. Solid rock, glass and aluminum, being very dense are good conductors of heat. Reduce their density by mixing air into the mass, and their conductivity is reduced. Because air has low density, the percentage of heat transferred by conduction through air is comparatively small. Two thin sheets aluminum foil with about one inch of air space in between weighs less than one ounce per square foot. The ratio is approximately 1 of mass to 100 of air, most important in reducing heat flow by conduction. The less dense the mass, the less will be the flow
of heat by conduction.
CONVECTION is the transport of heat within a gas or liquid, caused by the actual flow of the material within itself (mass motion). In building spaces, natural convection heat flow is largely upward, somewhat sideways, not downwards. This is called " free convection ". Convection may also be mechanically induced, as by a fan. This is called " forced convection ".
Radiation is the transmission of electromagnetic rays through space. Infrared rays occur between light and radar waves, (between the 3 - 15 micron portion of the spectrum.) Henceforth, when we speak of radiation, we refer only to infrared rays. Each material whose temperature is above absolute zero (-459.7 d, F) emits infrared radiation, including; the sun, icebergs, stove or radiators, humans, animals, furniture, ceilings, walls, roofs, floors, etc.
All substances, including air spaces building materials, such as wood, glass and plaster, and insulation, obey the same laws of nature, and TRANSFER heat. Solid materials differ only in the rate of heat transfer, which is mainly affected by differences in density, weight, shape, permeability and molecular structure. Materials which transfer heat slowly can be said to RESIST heat flow. Resistance or (“R Value”) is the unit of measurement used to describe materials ability to RESIST Heat Flow primarily in the CONDUCTIVE MODE.
PERMEABILITY or the ability of a material to absorb moisture plays a key factor in the performance of materials' ability to RESIST Heat Flow. A material that absorbs moisture easily transfers HEAT faster. (Have you ever picked up a hot pot lid with a wet towel or pot holder?) If so, you know that it takes only a second for you to feel the HEAT. Insulation, such as Fiberglass and Cellulose that becomes wet from rain or HUMIDITY in the air, looses its ability to RESIST Heat Flow, and soon becomes a CONDUCTOR OF HEAT, rather than a RESISTOR OF HEAT.
Direction of heat transfer is an important consideration. Heat is radiated and conducted in all directions, but convected primarily upward. In all cases, RADIATION is the dominant mode. Up to 93 % Down (Summer Heat Gain), Up to 75 % Up (Heat loss thru Ceiling / Roof), Up to 85% Side (Heat loss thru walls).
The chart below shows direction and percentages.
%’S OF HEAT FLOW ARE UP TO THE AMOUNTS INDICATED ON CHART BELOW IN ALL DIRECTIONS OF HEAT FLOW.
Summer & Winter Winter Summer
|Conduction 5% - 7 %||Convection Up to 45 %||Radiation Up to 93 %|
HOW DO WE STOP THIS HEAT TRANSFER?
Up till now, we have used (mass) insulation such as Fiberglass, Cellulose, Styrofoam, etc. to " SLOW DOWN " (RESIST), (R-VALUE), the CONDUCTION portion of the heat transfers. (See Thermal Values)
We NOW CAN use REFLECTIVE FOIL INSULATION and RADIANT BARRIERS to " REFLECT " or “ NOT ABSORB & EMIT “ (E value) the RADIATION portion of the heat transfer process BEFORE IT IS ABSORBED by the matter (Bricks, Wood, Concrete, Metal, etc.), so that the matter does not " build up " heat and pass it along into the cooler areas. Two sided Reflective Foil Insulation and RADIANT BARRIERS " Reflects " heat outwards in summer and inwards in winter keeping your building more COMFORTABLE with less energy use.
TESTING THERMAL VALUES *
U FACTOR is the rate of heat flow in BTU'S in one hour through one sq. ft. area of ceilings, roofs, walls or floors, including insulation (if any) resulting from a 1 degree F. temperature difference between the air inside and the air outside. U = BTU's flowing in ONE sq. ft. for ONE degree change.
R FACTOR or RESISTANCE to heat flow is the reciprocal of U; in other words, 1/U. The smaller the U factor fraction, the larger the R factor, the better the insulation ability to stop CONDUCTIVE HEAT FLOW. NOTE: Neither of these factors includes RADIATION or CONVECTION flow.
E FACTOR (VALUE) ( Emmissivity ) is the amount of RADIANT HEAT ENERGY absorbed by a surface, CONDUCTED thru it’s mass, and then emitted or re-radiated thru the opposite side. The lower the E VALUE, the greater the insulating protection from RADIANT ENERGY TRANSFER.
REFLECTIVITY is the opposite of Emmissivity, i.e., The ability a surface has to REFLECT RADIANT HEAT ENERGY. The higher the reflectivity, the less potential a material has to absorb radiant heat, THE LESS HEAT IS Conducted thru it’s mass, and emitted or re-radiated thru the opposite side.
RE - Value is the equivalent heat stopping ability of Reflective Foil Insulation and Radiant Barriers as compared to Fiberglass, Cellulose and Foam insulation, i.e.: mass insulation’s under identical conditions within a system. The RE - Value of Foil insulation’s takes into consideration the total BTU's/H/per sq. ft. of heat transfer by Conduction, Convection & Radiation and considers “ Real Life Use “ conditions not considered in accepted "R" and "U" Value Calculations. RE - Value = Apparent System “ R “ Value. ( EFFECTIVE RESISTANCE )
BTUFOIL tm REFLECTIVE INSULATION and BTUBUSTERS tm PREMIUM RADIANT BARRIER
THERMAL PERFORMANCE IN DIFFERENT SYSTEMS APPLICATIONS
APPARENT SYSTEMS “ R “ VALUE / “ RE “ VALUE ( EFFECTIVE RESISTANCE )
Performance of Reflective Foil Insulation and Radiant Barriers
The past 25 years has been very exciting in the Energy Conservation business. With the introduction and worldwide use of Radiant Barriers and Reflective Insulation products, the excitement has increased. Although the sales of these types of products have increased exponentially year after year, the consumer is sometimes confused about the PERFORMANCE and VALUE of such products. There have been over the years many unrealistic claims made about the performance, many salespeople have in their excitement have made overconfident statements concerning what these products can and cannot do.
We are providing to you, the “ Tools and Ammunition “ (Information) for you to study the REAL FACTS & FIGURES, and make your own decision if these products are for you. In Addition, consider this SIMPLE TRUTH, “ Your Very Own Common Sense “ will guide you to the Correct Answer. Although a great deal of the information we have provided may seem to be too “complicated” or “high tech”, It’s actually very easy to understand why & how these products work, and how they will benefit you.
“ HEAT CONTAINMENT “.
That is, Keeping the
in the summer
and Keeping the
in the winter.
Doesn't it make good sense that if the heat cannot come inside in the summer, your A/C will not have to work as hard and as long to keep you cool and comfortable, and doesn't it also make sense that if the heat doesn't escape outside in the winter, your heater / furnace will not have to work as long and as hard to keep you warm and cozy in the winter ? Of course it does, and this also means much lower heating and cooling cost all year long.
Most all of the materials your Home or Business is constructed with ABSORBS & TRANSFERS Heat. Common building materials ABSORBS & TRANSFERS 80% - 95 % of the HEAT. Heat is ALWAYS transferred from the warmer side to the cooler side. Reflective Insulation and Radiant Barriers “ REFLECT 95 % - 97 % of the HEAT back to where it came. Only 3% - 5% can be transferred from the warmer side to the cooler side. Stop and think about this fact, Is it better to have insulation products that ABSORBS & TRANSFERS 80% - 95% of the HEAT, (causing your Air Conditioner and Heater to work longer and harder) or to have insulation products that REFLECT 95% - 97% of the HEAT, thereby reducing the need for your Air Conditioner and Heater to work so long and hard. (Q) Which one do you believe will produce LOWER UTILITY BILLS? (A) The insulation that reflects 95% - 97% the heat. --- See, I told you it was easy to understand.
To further complicate and confuse things, the established “ rating systems “ that test and rate insulation products and systems have not been able to agree on an established, accurate method of rating these products. It is true that because of climatic conditions thru out the world, the testing method would be nearly overwhelming. Performance varies from warm / hot climate to cool / cold climates.
HUMIDITY also plays an important role in the performance of conventional insulation products, i.e. fiberglass, cellulose, but has little or no effect on Radiant Barriers and reflective insulations. The R-value of mass type insulation is reduced by over 35% with only 1.5% moisture content. i.e. from R-13 to R-8.3 and it should be noted that the moisture content in mass insulation materials typically exceeds 1.5%.
The primary reason that Radiant Barriers and Reflective insulation product insulates better than “ conventional insulation “ is that Radiant Barriers and Reflective insulation “ REFLECT “ infrared heat energy rather than ABSORBING it (like conventional insulation products do). Conventional insulation then allows the ABSORBED infrared heat to be CONDUCTED thru it’s mass and RE – RADIATED thru the other side.
NOT ALLOWING THE INFRARED HEAT ENERGY TO BE ABSORBED AND THEN CONDUCTED thru it’s mass.
This function is called “ Reflectivity “. The Portion of the heat energy that is absorbed (3 % - 5 %) is called “ Absorbilility “.
This 3 % - 5 % is ABSORBED AND CONDUCTED thru it’s mass and is then re-radiated thru the other side.
This is called “ Emmissivity / E - Value “. Therefore, only 3 % - 5 % of the total infrared heat energy can enter and pass thru the insulation, and into / out of the building. The net result is more ENERGY EFFICIENT building.
Q. Will Reflective Insulation and Radiant Barrier work
where I live & work?
Q. How well will they work for me?
A. Reflective Insulation and Radiant Barriers will work the same no
matter where they / you are. They will reflect HEAT back to its source.
They will help maintain more comfortable interiors both summer and
winter. They will help keep Utility Bills Lower.
Q. Can every existing home, office or commercial metal building use
A. Most all structures can, however, depending on the construction type and difficulty in “Retro-fitting”, it may cost more to install these products in some cases than in others.
Q. Can every new home, office or commercial metal building use these products?
A. Yes, Installation in new construction is generally easier, faster, cleaner, safer and more cost effective to the owner / builder than conventional insulation products.
Q. Do I still need to use conventional insulation products when I use Reflective Insulation and / or Radiant Barrier in my new home or commercial building?
A. In some cases, yes. Due to the current inadequate, out-of-date energy codes that only recognize “ R Value “ you may be forced to use other products. Even so, it is both cost effective and advisable to use our BTUFOIL tm REFLECTIVE INSULATION and BTUBUSTERS tm RADIANT BARRIERS as well. They will enhance the performance of conventional insulation products. In other cases, you may be able to use our products in place of conventional insulation products. A vast majority of our satisfied customers over the past 25 years have used these products as “stand alone” products to insulate their homes and business.
THE NEED FOR “ R - VALUE “
Should we rely solely on “ R-Value “ to insulate our Homes and business? The short and CORRECT answer is NO! The reason is that “ R- Value “ does not consider all the heat transfer methods as described. R-Value falls very short of accurately predicting the Energy Efficiency of Walls, Ceiling, Floor and Attic assemblies. Radiant Barriers and Reflective Insulation are rated in “ Systems R- Values “. This takes into consideration all the components within the Wall / Ceiling / Floor/ Attic assembly, including the air spaces within and adjoining.
Although Radiant Barriers and Reflective Insulation generally have lower R - Values assigned by virtue of these test, THE TRUTH IS THAT RADIANT BARRIERS AND REFLECTIVE INSULATION PERFORM BETTER than “conventional insulation“ with higher R - Values assigned under the same conditions.
Why, because the testing methods used only test for CONDUCTIVE ( 5% - 7 % )heat flow, and ignores the greatest heat transfer process called ( 65 % - 93 % ) “ RADIATION “ (infrared heat transfer).
SO, WHAT “ VALUES “ SHOULD WE CONSIDER
# 1. E - Value (Factor) - See Definition on page # 2, the lower the better
# 2. Reflectivity - See Definition on page # 2, the higher the better
# 3. Absorbilility - See Definition on page # 2, the lower the better
# 4. Perm Rating - The ability of a material to resist water vapor transfer, the lower the better.
# 5. “R “ & “U” Values - See Definitions on page # 2