R-values Aren't Always Useful

July/August 2005
http://www.motherearthnews.com/diy/r-values-arent-always-useful.aspx




R-values are a measure of a material's resistance to heat; the higher the value, the better the insulative properties. Although they are a convenient and common yardstick to measure thermal performance, the usefulness of them is limited.

In a 2000 study by Brock University in Ontario, two almost identical homes ? one built with a stick frame and the other with Structural Insulated Panels (SIPs) ? were compared for energy efficiency. Thermal analysis revealed the 2-by-6 stick frame house (rated at R-20) performed at an R-4 equivalent, while the 4 1/2-inch-wide SIP home performed at a true R-17 level in 13-degree temperatures.

The reason for the wide discrepancy is because, unlike frame systems, SIPs offer continuous insulation values across large areas of airtight walls, unbroken by studs that would otherwise conduct heat and let cold air into the building. In terms of real-world energy conservation, the SIPs home consumed only a quarter as much energy over one year as its stick-built counterpart, even though R-value ratings were comparable.