How to convert thermal conductivity to r value

Ever-increasing energy consumption expenses and environmental impact demand cost-saving, eco-friendly materials that achieve energy efficiency applications. The universal construction material is "building bulk insulation" that establishes the "thermal envelope" for living environments. Transient fluxes of thermal bridges, called leakage in structural construction, have an impact on the effectiveness of thermal conductivity. To troubleshoot thermal envelope insulation effectiveness, one must first convert thermal conductivity to the R-value.

The formula for converting thermal conductivity "k" (the coefficient of the material) for R-value measurement is expressed as:

Thermal resistance (R-value) = t/k, where "t" is the thickness of the material (meters). R-value is measured in Kelvin square meters per watt (K*m2/W).

Find the thermal conductivity of your material ("k") on a thermal conductivity table. See the Kelvin square meters per watt (K*m2/W) table link ("The Engineering ToolBox") in the References below. Measure and record the thickness of the material ("t").

Divide the material's thickness "t" by the "k" number. If "t" is expressed in inches, convert to meters:

1 inch = 25.4mm or 0.0254m

Calculating the thermal conductivity to R-value. For example:

Fibreglass insulating board thermal conductivity "k" is 0.048 Thickness of the board "t" is one inch or 0.0254m R-value = 0.0254/0.048 R-value is 0.529 per Kelvin square meters

The whole wall R-value over the clear-wall R-value is calculated for the true R-value. This substantiated three--step approach comes from the "Insulation Handbook" by Richard T. Bynum Jr.

Clear Wall R-value: This is the R-value of a wall with just studs and does not include the framing included in windows, doors, and exterior corners.

Center of Cavity R-value: This is the R-value estimate of the area of the cavity space between studs that contains the most insulation.

Whole Wall R-value: This is an R-value estimation that includes both the clear wall estimate of R-value and takes into account additional framing such as windows, doors, and exterior corners."

According to Bynum, "the whole wall R-value is as much as 40 per cent less than the clear wall R-value."


Today, there are eco-friendly insulation materials that remedy clear-wall leakage and improve thermal envelope design. Innovative "green insulation" building products reduce thermal bridges while meeting or exceeding insulation R-values. Formaldehyde-free white fibreglass insulation and closed-cell polyurethane foam have become popular, along with non-toxic cemetitious Air Krete, Icynene, and soy-based Heatlok Soya made from recycled products.

Things You'll Need

  • Calculator
  • Thermal conductivity table
  • Tape measure/ruler (either imperial or metric)
  • Pen/pencil
  • Paper
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