You can insulate the cavities of a wall perfectly, fill every stud bay with high-quality material, leave no gaps, and still end up with a wall that performs well below its rated R-value. The reason is thermal bridging, one of the most underappreciated weaknesses in ordinary construction. Heat is opportunistic; it flows through whatever path offers the least resistance, and in a conventionally framed wall that path runs straight through the wood or steel framing, bypassing the insulation entirely. Understanding thermal bridges, and how continuous insulation addresses them, separates a wall that merely looks well insulated from one that actually performs.
What a thermal bridge is
A thermal bridge is any part of the building envelope where a more conductive material creates a shortcut for heat across an otherwise insulated assembly. The most common example is the wood stud. Cavity insulation such as fiberglass or cellulose might provide around R-3.5 to R-4 per inch, but a wood stud provides only about R-1.25 per inch. Every stud, plate, and header in the wall is therefore a lane where heat crosses far more easily than it does through the insulated cavities beside it. Steel framing is dramatically worse, because steel conducts heat hundreds of times faster than wood, which is why steel-framed walls can lose more than half of their nominal cavity R-value to bridging.
The framing factor and effective R-value
People are often surprised by how much of a wall is actually solid framing rather than insulation. Between studs spaced every sixteen inches, top and bottom plates, corners