"R" stands for resistance...resistance to the transfer or movement of heat. "R" is a number that is the arithmetic reciprocal of the thermal conductivity of a substance. Thermal conductivity is the rate at which a substance conducts heat away from a heat source and is expressed in terms of BTU/inch/ hour/degree F/5Q.FT. The resulting number from this equation is usually expressed in decimal form since the value can not exceed "1". This means the smaller the number, the lower the rate of heat transfer and therefore the better the insulating property. As a scientific notation, this is a completely satisfactory expression, however, by utilizing the reciprocal value, a lumber greater than " I" is expressed and...the higher this number, the better the insulation property. This presents a more understandable rating system to the non- technical individual and has become "the" index of insulation performance. Since the majority of you reading this publication are well versed in engineering and scientific disciplines, it is most likely that up to this point, you haven't read anything new or different. But, don't quit now because we're going to look closely at the fact that there are really three (.3) "R" values that come into play when any insulation material is used to resist heat now [transfer) from any heat source to the surrounding atmosphere.  

     Heat energy is dissipated from the source in a radically outward pattern. For the purpose of heat loss calculations, the radial paths are converted into three paths of vector equivalents which are (I) horizontal, (2) vertical upward and (.3) vertical downward. These three paths are far more significant than they may first appear to be. Let's call these three paths the "horizontal mode," the "up mode" and the "down mode." Now...visualize your casting beds and you quickly realize it is not possible to provide an insulation cover for the down mode because you can't raise the bed and get a cover under it. This is really unfortunate because virtually all insulation media has its highest "R" factor in the down mode. What remains then, is the opportunity to insulate for the horizontal and the up mode. Again, visualizing your casting beds, you can see the "R" rating of your insulated cover is absolutely critical in the up mode. 
Unfortunately, the up mode "R" factor is the lowest of the three modes. This tells us what you really want to know about your insulated covers is, "What is the certified "R" value of the cover in the up mode?" Please take careful note of the fact that the question of the up mode "R" value is specifically stated to be that of the complete cover, not the generally published "R" value of the insulation media plus the arithmetic sum of the "R" value of the other layered materials comprising the assembled cover. Theoretically, "R" values are additive, however, they are directly additive only so long as interface contact does not provide conductance paths from layer to layer and also, only so long as transient convection paths are not created.  

What nobody tells you
about insulated covers.

     The most common misunderstanding of the real difference between theoretical and actual "in use" product "R" value is the result of several different reporting and testing systems that are in current use. There are two generally accepted ASTM test procedures that are used to determine thermal conductivity and the resulting "R" value. One test method provides for the media being tested to surround a rod which is accurately calibrated for heat input. Careful instrumentation is done on the outside, or "cold" side, of the media being tested to determine the rate of heat transfer . This is certainly a valid test procedure and, when professionally conducted, will provide an accurate "R" value. However, you the user of I the product so tested, must recognize that the "R" factor determined by this test method is a composite of the three (.3) "R" modes and IF you are unable to take advantage of the down mode component of the composite value (because you can't get your cover under your work piece), the "R" you are counting on for energy cost saving simply is not there. Remember, the down mode is the highest "R" value of the three modes (in some insulation media by a factor of 2X the up mode) and it isn't working for you at all.  

     The second ASTM test method, a guarded hotbox, uses a heated chamber or box inside which carefully monitored heat input elements are mounted. The specimen to be tested is positioned to form I the "closure wall" of the heated chamber and the outside or "cold side" of the test specimen is instrumented to accurately detect the temperature rise which results over a controlled time span. Five of the chamber walls are constructed from materials of known thermal conductive values and in addition, the internal temperature of the chamber is separately monitored throughout the test period. This assures an accurate accounting of the rate of transfer through the test media by virtue of a redundant verification of input-vs-output. With this method, it is possible to accurately identify the up mode, horizontal mode and the down mode "R" value of the test article. This is done by allowing the test specimen to be respectively the up, horizontal or the down "closure panel" of the test chamber and provide separate and independent determination of "R" values. 

     It is not always possible to determine which test procedure has been used for the published "R" value of the insulation media in general use for insulated covers 1 throughout the industry. One can I even suppose that only the most favorable "R" mode (the down mode) would be a preferred value to reference in marketing related dialogue concerning the insulation used in the covers. To avoid any misunderstanding, from whatever cause, the real and significant information you need is the system "R" value of tile finished product you are buying, and...insist on a certified guarded hotbox R" value in the up mode. With this information and the proper positioning of the covers over your work piece on the bed, you can fairly and consistently determine -in advance -the true performance cost of the covers you buy and factually realize the energy cost savings you are paying for .

     Remember...most cellular and fibrous insulation material is represented to have a certain "R" factor per inch of thickness and therefore, anything less than one inch thick will have a proportionally lower "R" performance factor . Since it is not practical for most of you to even consider handling a one inch
( 1 ") thick cover, don't be misled by statements of "R per inch."

     Ask for and get a certified system "R" value, per ASTM C 2.36 conducted in the up mode. Other representations may be ''as good" but... why gamble? This certification assures you of real world performance. Why accept less?

- more on The 3 R's of  Insulation Technology...