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
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
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.