One of the keys to efficient mechanical system performance is using the proper type of insulation and using it in the correct size.

The primary function of insulation is to reduce the heat transfer in a system. In addition to the quality of the insulation material and installation, choosing the correct insulation thickness is crucial in slowing heat transfer to achieve long-term thermal and condensation control, even under extreme conditions. To calculate the insulation thickness it is necessary to know and understand the thermal properties of the insulation. Three of the most important of these properties are thermal conductivity (K-Value), thermal Resistance (R-value), and thermal conductance (C-factor).

Thermal Conductivity:

Thermal conductivity, often referred to as K-value, is the rate of steady heat flow through a unit area of a homogeneous material induced by a unit temperature gradient in a direction perpendicular to that unit area.  Definition aside, the most important thing to know is the lower the K-value, the higher the insulating value.   Thus, most insulation materials have K-values less than one.  Also, it is important to understand that K-value is a material property; meaning it is independent of thickness.

Another important thing to know about the K-value is that it changes depending on the mean temperature (the average of the temperatures on each side of the insulation). As the mean temperature goes up, so does the K-value.  Therefore, one must look at the K-value at the proper mean temperature to determine the actual K-value for a specific application.

Thermal Resistance:

Thermal resistance, better known as R-value, is the insulation's resistance to heat flow. Therefore, it follows that the higher the R-value, the greater the insulating power. R-value is dependent on the K-value and thickness of the insulation and for flat insulation, like duct liner, the R-value is simply the thickness divided by the K-value.  For cylindrical insulation, like for pipe, the calculation is more complicated and relies also on the inner diameter of the insulation with smaller inner diameters having higher R-values for a given insulation thickness.  

All materials having the same R-value, regardless of type; thickness; or weight, are equal in insulating power. As a result, energy standards, building codes, and specifications often require a specific R-value so that all insulation materials can be compared equally.

Thermal Conductance:

Thermal conductance, or C-factor, is the rate of heat flow through a specified thickness of insulation and is the reciprocal of the R-value. It then follows that the lower the C, the better the insulator and the C-factor for flat insulation is the K-value divided by the thickness of the insulation.

Insulation Thickness Calculator:

To help you determine the right insulation thickness, Armacell developed ArmWin, a free professional insulation thickness calculation tool. With this tool, anyone can calculated the proper thickness of Armaflex to meet their design criteria whether that be condensation control, energy conservation, or freeze protection. Please click here to access ArmWin.

Glossary Terms:

R = thermal resistance; a number denoting the resistance a material or system to the flow of heat (hr ft2 °F/Btu)

K = thermal conductivity; heat transferred per unit of time for unit area for a temperature gradient of 1° F per unit length of heat path (Btu in/hr ft2 °F)

C = thermal conductance; a number denoting the rate of heat flow through material or system (Btu/hr ft2 F)