This is only excerpt of the full article, but I think it supports my claims:
http://www.patentstorm.us/patents/6248433/description.html
The amount of radiation emitted is a function of the emissivity factor of the source's surface. Emissivity is the rate at which radiation (emission) is given off. Absorption of radiation by an object is proportional to the absorptivity factor of its surface which is the reciprocal of its emissivity.
Although two objects may be identical, if the surface of one were covered with a material of 90% emissivity, and the surface of the other with a material of 5% emissivity, there would result a drastic difference in the rate of radiation flow from these two objects. The lower the emissivity, the lower the radiation. This is demonstrated by comparison of four identical, equally heated iron radiators covered with different materials. Paint one with aluminum paint and another with ordinary enamel. Cover the third with asbestos and the fourth with aluminum foil. Although all have the same temperature, the one covered with aluminum foil would radiate the least (lowest, e.g. 5%, emissivity). The radiators covered with ordinary paint or asbestos would radiate most because they have the highest emissivity (even higher than the original iron).
Painting over the aluminum paint or foil with ordinary paint changes the surface to 90% emissivity and thus increases the radiatiors's radiant heating capacity.
[ed] this supports my contention that most other materials has higher emissivity than aluminum allowing heat to 'pass through'
Materials whose surfaces do not appreciably reflect infrared rays, for example paper, asphalt, wood, glass and rock, have absorption and emissivity rates ranging from 80% to 93%. Most materials used in building construction (brick, stone, wood, paper, etc.) regardless of their color, absorb infrared radiation at about 90%.
The surface of aluminum has the ability not to absorb, but to reflect, 95% of the infrared rays which strike it. Since aluminum foil has such a low mass to air ratio, very little conduction can take place, particularly when only 5% of the rays are absorbed.
In order to retard heat flow by conduction, walls and roofs are built with internal air spaces. Conduction and convection through these air spaces combined represent only 20% to 35% of the heat which pass through them. In both winter and summer, 65% to 80% of the heat that passes from a warm wall to a colder wall or through a ventilated attic does so by radiation.
The value of air spaces as thermal insulation must include the character of the enclosing surfaces. The surfaces greatly affect the amount of energy transferred by radiation, depending on the material's absorptivity and emissivity, and are the only way of modifying the total heat transferred across a given space. The importance of radiation cannot be overlooked in problems involving ordinary room temperatures.
Reflection and emissivity by surfaces can only occur in space. The ideal space is any dimensioned 3/4" or more. Smaller spaces are also effective, but decreasingly so. Where there is no air space, we have conduction through solids. When a reflective surface of a material is attached to a ceiling, floor or wall, the at particular surface ceases to have radiant insulation value at the points in contact. Therefore, care must be exercised, when installing foil insulation, that it be stretched sufficiently to insure that any inner air spaces are properly opened up and that metal does not touch metal. Otherwise, conduction through solids will result at the point of contact.
Heat control with aluminum foil is made possible by taking advantage of its low thermal emissivity and the low thermal conductivity of air. It is possible with layered foil and air to practically eliminate heat transfer by radiation and convection.
There is no such thing as a "dead" air space as far as heat transfer is concerned, even in the case of a perfectly air-tight compartment such as a thermos bottle. Convection currents are inevitable with differences in temperature between surfaces, if air or some other gas is present inside. Since air has some density, there will be some heat transfer by conduction if any surface of a so-called "dead" air space is heated. Finally, radiation, which accounts for 50% to 80% of all heat transfer, will pass through air (or a vacuum) with ease, just as radiation travels the many million miles that separate the earth from the sun.
Aluminum foil, with its reflective surfaces can block the flow of radiation. Some foils have higher absorption and emissivity qualities than others. The variations run from 2% to 72%, a differential of 2000%.
Most aluminum insulation has only 5% absorption and emissivity ratio, is impervious to water vapor and convection currents, and reflects 95% of all radiant energy which strikes its air-bounded surfaces. The performance of most aluminum insulation is unsurpassed for upward winter heat and it has an added efficiency for downward summer heat because of the absence of convection currents. Aluminum prevents the heat rays from penetrating the surface covered therewith while reflecting the heat back, for instance, into the building.
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