R-value and U-value calculations

Insulation value depends on the thermal resistance of construction materials making up a building's envelope. It is expressed as Resistance-value or R-value and U-value and can be found in tables or calculated for any single component or component group. For example, the insulation value of SIPs panels manufactured from two 7/16 inch (11 mm) thicknesses of oriented strandboard (OSB) laminated over 5 1/2 inches (14 cm) of extruded polystyrene foam into a sandwich, is derived by simply adding together the known values of the separate components. The value for an entire wall would be derived in a similar way be including the values of siding, house wrap, drywall or roofing materials.

R-value is the inverse of U-value. For example if an Imperial (American Standard) R-value of 26 is given for a SIPs panel, the Imperial U-value is derived simply by dividing 1 by 26, or, .038462. R-value is preferred over U-value only because whole numbers like 26 are easier to work with than decimal fractions like .038462.

However, European insulation values are given in
metric U-factors (Resistance Systeme Internationale, ie, RSI). As such, the calculations for thermal transfer and building envelope volumes use metric instead of Imperial units for thickness (centimeters instead of inches), weight (kilos instead of pounds), volume (cubic meters instead of cubic feet) and temperature (centigrade or Celsius, instead of Fahrenheit).

Metric U-factors are defined as
Watts per square meter per degree Celsius. To convert inch-pound Imperial U-values to metric U-factors, multiply by 5.678. To convert metric U-factors to Imperial inch-pound U-factors, divide by 5.678. To convert Imperial inch-pound R-values to metric R-values, multiply by 0.1761. One inch = 2.54 cms. One (RSI) U-factor coefficient = 5.678; one R-value = .1761 RSI.

(There are many on-line metric conversion sites to help, ie, or Just remember, if you are going from metric to American measurement units using material U-values, you must convert all units to metric.