How To Convert Thermal Conductivity To R Value
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Table of thermal conductivities
To design comfortable and ecologically sound buildings, architects and engineers make use of the thermal properties of materials. Different building materials have different conductivities, meaning that they let heat flow through them differently. High conductivity, K, indicates rapid heat flow, and low conductivity indicates low heat flow. Thicker pieces of the same material cause heat to flow more slowly than thin pieces. So actual thermal performance depends on both the conductivity and the thickness of the material. A piece of material with a conductivity K and thickness L has a conductance of C = K/L, which indicates how well heat can flow through a specific piece of that material such as a wall. In buildings, you are often concerned with stopping heat flow, or insulating. The insulating value of an object is called the Rvalue and can be calculated as R = 1/C = L/K.
Calculating the RValue of a Concrete Slab
Step 1
Look up the conductivity, K, of concrete in a table such as that in resource 1. Light concrete can have a conductivity as low as 0.1 watts per meterkelvin, making it a good insulator. The denser the concrete gets, the more conductive it is to heat. Dense concrete can have a conductance as high as 1.8 watts per meterkelvin. Assume the slab is made of dense concrete with a conductance of 1.2 watts per meterkelvin.
Step 2
Determine the thickness of the material and convert to meters. Assume a 12inchthick slab, which is equivalent to 0.305 meters. Heat would have a harder time flowing through a thick slab than a thin slab. Label the thickness L.
Step 3
Calculate the conductivity. The conductivity of the slab increases with conductance and decreases with thickness. The conductivity for the slab is C = K/L = 1.2/0.305 = 3.93 watts per meter^2kelvin.
Step 4
Calculate the Rvalue by taking the reciprocal of the conductivity C to get R = 1/3.93 = 0.254 kelvinmeter^2/watt. You can also calculate the Rvalue directly as R = L/K = 0.305/1.2 = 0.254 kelvinmeter^2/watt. Notice that the thicker the material is, the higher the Rvalue is. More of a given material makes a better insulator.
Warning
Be sure not to confuse conductance with conductivity. Conductivity is a general property of the material. Conductance is a property of a piece of that material.