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HEAT TRANSFER

To effectively control the temperature indoors a steel building requires insulation to be installed along exterior walls and roof. When warm air and cool air are both present in an enclosed steel building, a temperature differential occurs. The heat will work its way from warmer areas to colder areas until the temperature inside the building stabilizes. At that point, the cooler air will be present at lower elevations and warmer air will be present at areas near the roof. Insulation is the key in stabilizing the indoor atmospheric condition at more desirable levels. During hot summer days, insulation prevents heat from transferring into the building. On the other hand, insulation keeps the heat inside during winter.

Heat transfer occurs in three ways:

Conduction - Conduction occurs when heat from an object, whether solid or liquid, is transferred by touch to another object. A perfect example is heating a pot on a stove by conductive heat transfer from an electric coil.

Convection - Convection occurs with the physical movement of air. There are two types of convectional heat movement – natural and forced or mechanical convection. Natural convection occurs when hot air rises, displacing the cold air and moving it down. Forced or mechanical convection occurs when an object, for example, a fan, physically moves and “forces” the air to move.

Radiation - Radiation occurs when the objects temperature is opposite than the temperature of the air around it. The sun’s radiation is a specific example. The sun is hotter than everything else around it. The heat waves that it radiates travels through the air and is either absorbed or reflected by the surface of the object it comes in contact with.

To prevent heat transfer by conduction or convection, traditional forms of mass insulation such as fiberglass, is used. However, fiberglass has a lesser effect on radiant heat. The approximate amount of radiant heat striking fiberglass that will either pass through or be “emitted” ranges from 80% to 90%. Typically, radiant heat transfer has as much impact on the temperature inside the steel building, as conduction and convection.

INSULATING AGAINST RADIANT HEAT

Fiberglass, a traditional form of insulation, is effective in preventing heat transfer by conduction and convention. However, it has less effect on radiant heat. As much as 80% to 90% of radiant heat striking fiberglass will pass through it. Transfer of radiant has as much impact in the building’s temperature as conduction and convention.

The thermal properties of insulating materials are known or can be accurately measured. The performance of insulation, or the amount of heat flow through any combination of materials can be calculated. To be able to do this, it is necessary to know and understand certain technical terms.

Thermal Resistance (R-Value) . The R-Value measures the ability of a material to resist heat flow. This means that the higher the R-Value, the greater the thermal efficiency. It is also defined as the reciprocal of the amount of heat energy per area of material per degree difference between the outside and inside temperatures. Its units of measurements are:

       (square feet x hour x degree F)/BTU in the English system
       (square meters x degrees C)/watts in the metric system

Overall Heat Transmission Coefficient (U-Value) . The material’s U-Value serves as the basis for determining transmitted heat loss. It is measures heat passage through a complete building section, including air films.

Thermal Conductivity (K-Value) . The K-Value is the measure used to express the amount of heat that passes through one square foot of a homogenous material that is exactly one inch thick and has a temperature difference of one degree Fahrenheit between the inside and outside surfaces. As the K-Value decreases, the amount of heat permitted to pass through the material also decreases.

Thermal Conductance (C-Vlaue). The C-Value is the measure of the amount of heat that passes through any insulation material of any thickness.

Surface Air Film Coefficient (F) . This measures the amount of heat flow between an exposed surface of a material and the adjacent air. Basically, F is the measure of the conductance of heat through the air film that clings to all surfaces.

The table below shows typical materials used in constructing steel buildings.

Material

R-Value

Material

R-Value

Metal Panels Negligible 0.6 lb. Density Fiberglass 3.0" 10.0
Brick - Fale 0.44 0.6 lb. Density Fiberglass 3.5" 11.0
Concrete Block 4" Cylinder 1.11 0.6 lb. Density Fiberglass 4.0" 13.0
Concrete Block 8" Cylinder 1.72 0.6 lb. Density Fiberglass 6.0" 19.0
Poured Concrete 6" 1.33 Dead Air Space - 4"  
Gypsum Board - 3 / 8" 0.32 - Roof Heat - Flow Up 0.94
Gypsum Board - 1 / 2" 0.45 - Wall Heat - Flow Out 1.01
Plywood - 1 / 4" 0.30 Surface Air Film  
Plywood - 3 / 8" 0.47 - Inside Roof 0.61
Glass - 1 / 8" Clear 0.035 - Inside Walls 0.68
Acoustical Tile - 1 / 2" 1.19 - Outside Roof and Walls 0.17
Acoustical Tile - 3 / 4" 1.78 1# Thermal Block at Structural Member 1.5

 

The U-Value is commonly used in commercial building construction. U-Values can be obtained from “K” or “C” or “R” factors. The calculated “U-Value” is computed by adding up all the applicable “R” values of the steel building assembly using the table above, and getting the reciprocal.

For example, if the total “R-Value” factors is 12.18. The calculated “U-Value” is 0.082. Heat transfer is obtained by multiplying the calculated “U-Value” by the number of degrees temperature difference between the inside and outside surfaces.