Thermodynamics and Reflective Insulation
The reflective insulation prevents the heat transfer by heat radiation. It does not necessarily protect against heat transfer by conduction or convection. Why do you need to know about this isolation? This can mean that thousands of dollars have been saved over the years for heating / cooling. This applies not only to roofing materials, but also to what is between the roof and the building frame. Carry me here.
In today’s science lesson …
All materials release energy due to heat radiation due to their temperature. The amount of energy radiated depends on the surface temperature and a property called emissivity or emissivity. Emissivity is expressed as a number between zero and one at a given wavelength. The higher the emissivity, the greater the emitted radiation at a given wavelength. A related material property is reflectance or reflectivity. This is a measure of how much energy is reflected by a material at a particular wavelength. The reflectivity is also expressed as a number between zero and one (or as a percentage between 0 and 100%). At a given wavelength and angle of incidence, the emission and reflection values add up to 1 according to Kirchoff’s law.
For those of us who have never studied thermodynamics (most of us), Kirchoff did so for us over 150 years ago. While he seems to be a pretty smart guy, most of his work will go beyond the heads of us average people. The most important thing to remember is that a good absorber is a good emitter (if something gains heat easily, it will also dissipate it easily) and a good reflector is a bad absorber.
Reflective insulation materials must have a low emissivity (typically 0.1 or less) at the wavelengths at which they are to function. For typical building materials, the wavelengths are in the middle and long infrared spectrum.
It may or may not have high visual reflectivity. This is because the reflectivity and emissivity at a given wavelength must add up to unity, which does not necessarily add reflectivity at one set of wavelengths (visible) and emissivity at another set of wavelengths (thermally) to one unit have to. However, it is possible to produce visibly dark colored surfaces with low thermal emissivity.
To ensure proper functioning, the reflective insulation must face the open space (such as air or vacuum) that otherwise emits radiation.
From Kirchoff into the future …
In the 1920s, patents were registered for reflective surfaces used as building insulation. Recent technological advances have then allowed low-emissivity aluminum foil to be commercially viable. Over the next 30 years, millions of square feet of reflective insulation have been installed in the US alone. Notable examples include projects at MIT, in Princeton and in homes like Frank Sinatra’s house.
How does it work in a building?
Solar radiant energy strikes the roof and heats the roof system (often shingles, felt paper and roof sheathing or steel) by heat conduction. The underside of the shroud and the roof frame radiate heat down through the attic towards the loft (or inside the building envelope if there is no enclosed loft). When a reflective insulation is applied between the roofing material and the interior of the building, much of the heat radiated by the hot roof is reflected back to the roof, and due to the low emissivity of the underside of the insulation, very little radiant heat is emitted downwards. This makes the top of the insulation cooler than without reflective insulation, thus reducing the amount of heat entering the building.
This is different from the “Cool Roof” strategy, where solar energy is reflected before heating the roof. Both, however, serve to reduce the radiant heat. According to a Florida Solar Energy Center study, a white tiled roof or white metal cooling roof can surpass a traditional black shingled roof with reflective loft insulation, but the black shingled roof with reflective insulation has surpassed the red brick cooling roof. One option is to install both reflective insulation and a cool roof for optimal performance. When deciding to use a cool roof and / or reflective insulation, a variety of factors must be considered.
For the installation of a reflective insulation under a post steel roof, the product can be applied directly by draping over the roof purlins. Even more effective is the installation over the purlins, the installation of 2 × 4 plaster strips on the insulation and then on the roof steel. The fur strips ensure that the reflective insulation surfaces are in sufficient airspace to be effective. If an air space is not present or too small, heat may possibly be passed through the insulation. Since the metal in the reflective insulation is highly conductive, the heat transfer would be by conduction only and the heat would not be blocked.
I did this on my massive 3-story Gambrel-style building that I built in South Dakota about 7 years ago. I’ve placed 2 × 4 over the reflective insulation and then applied the roof steel. My wife opted for black steel, so I added the 2 × 4 values to counteract their color choices and make the most of the insulation value. Our heating / cooling calculations are phenomenally small. In fact, this 84 x 60 x 20 inch building is one third cheaper to heat / cool than our single story 30 x 60 inch house across the street!
For shingle roofs, reflective insulation can be placed over the rafters or trusses and under the roof terrace (usually Osb or plywood). In this method of application, the insulation panels are wound over the rafter binders, creating a small airspace above, with the reflective insulation in the entire interior of the roof below.
Even two summers ago I rethought our house with this method. The shingles had seen their better use, so we placed 2 x 4 directly over the shingles, put on the reflective insulation and then applied white roof steel for a “cooling roof” solution. In summer, it is noticeably cooler and much warmer in winter, along with lower energy costs.
Another method of applying reflective insulation to the roof of a new building is to use it where it is prelaminated on OSB or a roof shell. While manufacturers of this method of installation often praise labor cost savings when using a product that serves as a roof terrace and reflective insulation in one, these products are considered “expensive” by most.
A common misconception with regard to reflective insulation is that the heat reflected by the insulation can raise the roof temperature and possibly damage shingles. That’s just not the case. Performance tests at the Florida Solar Energy Center conclusively showed that the temperature increase at the hottest point of the day was no more than 5 degrees F. In fact, this study showed that the reflective insulation had the potential to lower the roof temperature when the sun went down because it prevented heat loss through the roof. RIMA (Roofing Insulation Manufacturers Association) International has prepared a technical document on this subject, gathering statements from the largest roofing fabricators, and saying that reflective insulation would in no way compromise the shingles’ warranty
Wrapping the walls of a building with reflective insulation can reduce space requirements for air conditioners by 10% to 20% and save on energy and construction costs.
Reflective insulation is also very effective in floor systems over unheated basements and crawl spaces. It can either be stapled under the floor beams, creating a single reflective air space, or between the beams, followed by a kind of sheathing. The reflective insulation works very well in this application for two reasons. First, reflective insulation, which is not perforated for breathability, acts as an excellent vapor barrier. This means that the soil moisture can not pass through the reflective insulation in the living room. Second, the floor is the only part of the building where the heat flow is always directed downwards, as opposed to a roof where the heat flows downwards in the summer and upwards in the winter. When the heat flow decreases, 93% of the heat is radiant heat. This is exactly what the reflective insulation should prevent.
Reflective isolation IS actually isolation and has actual R-values in tested systems. It is highly directional, which means that it is better for heat gain than heat loss. In tests, the rate is up to R-14 depending on the installation.
However, reflective insulation is not the “end of all” insulation. I prefer to use it for something that is great – an isolated vapor barrier.
There are many commercially available vapor barriers. These include Tyvek® (a registered trademark of DuPont Corporation) and other similar household packaging. Even polyethylene films (think Visqueen by British Polythene Industries Limited) are a vapor barrier.
Although none of these products have fine vapor barriers, this is not an effective method of controlling condensation. To control the condensation, a thermal separation must be generated. It is the layer of air cells interposed between the layers of reflective insulation (most commonly a white interior trim and an aluminum exterior trim) that provides the necessary thermal separation.
If the sole purpose of heating a mast building is to interrupt the cooling with a slight heat for a few hours (for example, by turning up a space heater), reflective insulation may be an affordable option. For buildings that are to be air conditioned, other options are more practical.