Tests:

REFLECTIVITY AND AIR SPACES

In order to retard heat flow by conduction, walls and roofs are build 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.

The following test results illustrate how heat transfer across a given air space may be modified. The distance between the hot and cold walls is 1-1/2" and the temperatures of the hot and cold surfaces are 212 degrees and 32 degrees, respectively. In CASE 1, the enclosing walls are paper, wood, asbestos or other similar material. In CASE 2, the walls are lined with aluminum foil. In CASE 3, two sheets of aluminum foil are used to divide the enclosure into three 1/2" spaces.

Conduction 21 BTU's
Convection 92 BTU's
Radiation 206 BTU's
TOTAL 319 BTU'S


CASE 1, UN INSULATED WALL SPACE.

The surfaces of ordinary building materials, including ordinary bulk insulation have a radiation or emissivity rate of about 90%, a heat ray absorption rate of over 90%. Air has low density, so conduction is slight (only 21 BTU'sJ Convection currents transfer 90 BTU's.

Conduction 21 BTU's
Convection 92 BTU's
Radiation 10 BTU's
TOTAL »23 BTU'S
 
 
CASE 2, THE SAME WALL SPACE EXCEPT

that the inner surfaces were lined with sheets of aluminum foil of 5% emissivity and absorptivity.* Note the drastic drop in heat flow by radiation, from 206 BTU's to 10 BTU's. Conduction and convection are unchanged. The original total heat loss of 319 BTU's drops to 124 BTU's.
Conduction 23 BTU's
Convection 23 BTU's
Radiation 2 BTU's
TOTAL 48 BTU'S
CASE 3, TWO SHEETS OF (5% EMISSIVE)

ALUMINUM FOIL divide the wall space into 3 reflective compartments. Heat loss by radiation drops 94% from Case 1. The 2 interior sheets retard convection so that its flow falls 75%. Conduction rises only 2 BTU's; from 21 BTU's to 23 BTU's. The total heat loss drops 85% from Case 1.

Note: 65% (206) 577 *s of the total fin's going through this itall space is radiation. ^Aluminum has 3% to 5% emissivity and absorptivity.

Reflection and emissivity by surfaces can ONLY occur in SPACE. The ideal space is any dimension 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, that particular surface ceases to have radiant insulation value at the points in 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: a fact employed regularly by the NASA space program. In the space vehicle Columbia, ceramic tiles are imbedded with aluminum bits which reflect heat before it can be absorbed. "Moon suits" are made of reflective foil surfaces surrounding trapped air for major temperature modification.

HEAT LOSS THROUGH AIR

There is no such thing as a "dead" air space as far as heat transfer is concerned, even in the case of a perfectly airtight 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 surface, 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 over 2000%. Most aluminum insulation has only a 5%
absorption and emissivity ratio. It is impervious to water vapor and convection currents, and reflects 95% of all radiant energy which strikes its air-bound surfaces.

HEAT LOSS THROUGH FLOORS

Heat is lost through floors primarily by radiation (up to 93%). When ALUMINUM insulation is installed in the ground floors and crawl spaces of cold buildings, it prevents the heat rays from penetrating down, reflecting the heat back into the building and warming the floor surfaces. Since aluminum is non-permeable, it is unaffected by ground vapors.

CONDENSATION

Water vapor is the gas phase of water. As a gas, it will expand or contract to fill any space it may be in. In a given space, with the air at a given temperature, there is a limited amount of vapor that can be suspended. Any excess will turn into water. The point just before condensation commences is called 100% saturation. The condensation point is called dew point.
  
     
     
     
     
 
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