STEEL BUILDINGS - R-VALUES
The popularity of steel buildings is due mainly to their low cost and quick construction as compared to traditional buildings. However, because steel buildings are made from thin sheets of metal fastened to a frame, the temperature of the interiors can be difficult to control. To address this problem, thermal insulation is installed to help regulate the temperature within the steel building’s interiors.
Thermal insulation is intended to decrease the transfer of heat between the building’s interior and the outside environment. A well-installed thermal insulation will be more comfortable for the steel building’s occupants due to the reduction in thermal transfer through the walls and ceiling. Insulation also prevents excess moisture and control sound transmission through the metal building walls. In addition, insulation can help reduce energy costs for heated buildings during winter by minimizing the amount of heated air that’s leaked through the walls.
MEASURES OF PERFORMANCE OF INSULATION
K-Values, C-Values and R-Values are all factors that measure an insulation system’s thermal performance. Each factor pertains to a different thermal performance characteristic.
K-Value is a measure of an insulation material’s effective thermal conductivity, which is the time rate of steady flow of heat per one inch thick of homogenous material induced by a unit temperature gradient perpendicular to that unit area. It is basically the measure of the amount of heat that will be transferred through a one inch (1") thick piece of homogenous material that is one square foot (1 ft2) in size, in one (1) hour, when there is one degree Fahrenheit (1°F) temperature change. The lower the K-Value, the better the insulator.
R-Value measures the resistance to the transfer of heat through the insulation material. As a rule, a high R-Value means an efficient insulator. It is expressed as
C-Value is simply a measure of the thermal conductance of a particular insulation material and is the reciprocal of the R-Value, or
R-VALUES FOR WINTER CONDITIONS (MEAN 40°F)
During cold weather, it is ideal for the insulation system to be a poor conductor so that the heat is retained inside the steel building. The product’s R-Value is the most important criteria in the selection of insulation materials specifically for the winter season. The table below shows the R-Values of insulation material typically used for steel building insulation systems.
|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|
TYPICAL MATERIALS USED IN METAL BUILDING CONSTRUCTION
In commercial construction, U-Values are commonly used. The U-Value is a factor used to express heat passage through a complete building section, including air films. It can be derived from "K" or "C" or "R" factors. A calculated U-Value is equal to the reciprocal of the total R-Values of the construction assembly.
To help better understand the concepts of R-Values and their importance in determining the performance of an insulation assembly, an example is necessary.
|Building Section:||Wall cross section|
|Composition:||An exterior metal panel|
|3" thick, 0.6 lbs density fiberglass insulation|
|dead air space|
|3/8" Gypsum board interior finish|
Using the table above, the corresponding R-Values are:
|Air Surface Outside||0.17|
|0.6 lb. Fiberglass||10.0|
|Dead Air Space||1.01|
|Air Surface Inside||0.68|
|Total R 12.18|
|U = 1/R+ = 1/12.18 = 0.082|
The value 0.082 represents the amount of heat transfer, in BTU’s per hour, through a one square foot of wall area for a temperature difference of one degree Fahrenheit. Based the given scenario of the example above, there is a 40° Fahrenheit temperature difference between the outside and the inside of the steel building. The final amount of heat transfer is 3.28 BTU’s per hour per square foot, obtained by multiplying 0.082 by 40.