Thermodynamics and Reflective Insulation


Reflective insulation prevents heat transfer by thermal radiation. It does not always secure versus heat transfer by conduction or convection. Why do you have to learn about this insulation? It might suggest thousands of dollars saved for many years for heating/cooling, in what you select for not just roof repair materials, however more importantly, what is put in between the roofing and building framing. Bear with me here.


Emissivity is expressed as a number between zero and one at a provided wavelength. The greater the emissivity, the greater the released radiation at a given wavelength. At an offered wavelength and angle of occurrence the emissivity and reflectivity values add up to 1 by Kirchhoff's law.


For those of us who never studied thermodynamics (the majority of us), Kirchoff did it for us over 150 years back. While he appears to have been a quite clever man, most of his work will go over the heads of us average folks. The essential thing to bear in mind is - a good absorber is a great emitter (if something gains heat readily it also provides it off easily) and a great reflector is a poor absorber.


Reflective insulation materials must have low emissivity (usually 0.1 or less) at the wavelengths at which they are anticipated to operate. For common building products, the wavelengths remain in the mid- and long- infrared spectrum.


It may or might not show high visual reflectivity. This is because while reflectivity and emissivity must add up to unity at an offered wavelength, reflectivity at one set of wavelengths (noticeable) and emissivity at a various set of wavelengths (thermal) do not necessarily add up to unity. However, it is possible to produce noticeably dark colored surface areas with low thermal emissivity. Visit our new website at .


To perform effectively, reflective insulation has to deal with open space (like air or a vacuum) through which there would otherwise be radiation.


Moving forward in time from Kirchhoff...


In the 1920's patents were filed on reflective surfaces being utilized as structure insulation. Current enhancements in technology had then allowed low emissivity aluminum foil to be commercially practical. Over the next 30 years, millions of square feet of reflective insulation were installed in the US alone. Noteworthy examples include tasks at MIT, Princeton, and homes such as Frank Sinatra's house.


So how does this work in a structure?


Glowing solar power strikes the roofing system, warming the roof (often shingles, felt paper and roofing sheathing or steel) by conduction, and triggering the underside of the sheathing and the roofing framing to radiate heat downward through the attic toward the attic floor (or within the building shell when no enclosed attic space exists). When a reflective insulation is put between the roofing product and the building interior, much of the heat radiated from the hot roof is shown back towards the roofing and the low emissivity of the underside of the insulation suggests hardly any radiant heat is given off downwards. This makes the top surface of the insulation cooler than it would have lacked a reflective insulation and thus lowers the amount of heat which moves into the structure.


According to a study by Florida Solar Energy Center, a white tile or white metal cool roof can surpass standard black shingle roofing system with a reflective insulation in the attic; however the black shingle roof with reflective insulation exceeded t


For setting up a reflective insulation under a pole structure steel roofing system, the product might be applied directly by draping over the roofing system purlins. Much more efficient is to install over the purlins, set up 2 4 furring strips on top of the insulation, then the roofing system steel. The furring strips make sure the reflective insulation deals with into an adequate air space to be effective. If an air space is not present or is too little, heat might have the ability to perform through the insulation. Since the metal in the reflective insulation is extremely conductive, the heat transfer would all be through conduction and the heat would not be blocked.


I did this on my huge 3 story gambrel style accessory building I built in South Dakota about 7 years back. I put 2 4's over the reflective insulation, and then used the roofing system steel.

For shingled roofs, reflective insulation might be applied over the rafters or trusses and under the roofing system decking (typically osb or plywood). This application approach has the insulation sheets draped over the trusses of rafters, developing a small air space above with the reflective insulation facing into the entire interior attic space below.


I even used this method in re-roofing our house 2 summers back. The shingles had seen their better use, so we applied 2 4's right over the shingles, put down reflective insulation and then applied white roofing system steel for a "cool roofing system" option. It's significantly cooler in summer season and much warmer in winter season, in addition to lower energy costs.


Another technique of using reflective insulation to the roof in new design would be to use it where it is pre-laminated to OSB or roof sheathing. While manufacturers of this installation technique often promote the cost savings in labor costs in utilizing a product which functions as roofing decking and reflective insulation in one, these products are typically considered by many to be "costly.".


One common mistaken belief concerning reflective insulation is the heat reflecting off the insulation back out the roofing has the potential to enhance the roofing system temperature and potentially damage shingles. RIMA (Roofing Insulation Manufacturers Association) International composed a technical paper on the topic, where they collected statements from the biggest roof manufacturers, and none said a reflective insulation would in any method influence the service warranty of the shingles.


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I even used this method in re-roofing our house 2 summers back. The shingles had seen their better use, so we applied 2 4's right over the shingles, put down reflective insulation and then applied white roofing system steel for a "cool roofing system" option.


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