# R and K-Values: Evaluating your Insulated Shipper

Posted on: November 25th, 2014 by travis

When trying to determine the effectiveness of an insulated shipping container, there are a few properties that will tell you a lot about the package’s performance.  The most basic of these measures is known as the “K-value”, or the measure of thermal conductivity of a material.  This is measured in Watts per meter-Kelvin (W/(m·K)).  Since thermal conductivity refers to the amount of heat being transferred, it follows that a high-quality insulated shipping container should have as low a K-value as possible.

The most commonly used measurement in insulation calculations is the “R-value”.  The R-value measures the thermal resistance of a material, or group of materials, and is measured in (m²·K)/W.  It is proportional to the thickness of the material.  Since thermal resistance is also inversely proportional to thermal conductivity, a higher R-value is desirable for an insulated shipping box.  This means that the material, expanded polystyrene in this case, has a higher resistance to the transfer of heat from outside temperatures to the product inside.

The most comprehensive measure of a a system’s insulation capabilities is the U-factor, also known as the “Overall Heat Transfer Coeffeicient”, and it relates to both the R and K-values.  The U-factor is equal to (1 / R-value), which is also equal to (K-value / thickness of the material).  Similar to the K-value, a low U-factor is indicative of a high level of insulation.

So what are these values for our insulated shipping boxes?  Our EPS foam coolers have a density of 1.2 pounds per cubic foot, and are a standard 1.5″ (.0381 m) thick.  EPS of this density generally has an R-value of about 3.9 (m²·K)/W per inch of thickness.  This gives our insulated shippers an R-Value of about 5.85 (m²·K)/W.  This translates to a standard K-value of about .26 W/(m·K) per inch and an overall U-factor of of only .171 W/(m²·K)!  By limiting the transfer of heat with a K-Lock™ Insulated Shipping Box, your products remain safe from the fluctuating temperatures and breaks in the cold chain.