What thermal bridging actually is
Insulation slows heat flow. Wood framing does too, but far less effectively. Steel framing or metal fasteners barely slow it at all. Thermal bridging is what happens when heat finds a path through the more conductive material instead of through the insulation around it: a stud, a top plate, a rim joist, a window buck, a corner. The insulation between the studs might be doing its job perfectly, and the wall will still lose more heat than the cavity R-value alone suggests, because roughly one-sixth to one-fifth of a typical framed wall's area is actually wood, not insulation.
Building scientists distinguish clear-wall R-value (the insulation plus the necessary framing in a repeating section, no corners or windows) from whole-wall R-value (every interface included: corners, headers, the wall-to-roof and wall-to-floor connections). Whole-wall is always lower, because those interfaces are where bridging concentrates. Marketing a wall by its cavity R-value alone routinely overstates what a homeowner will actually get.
Where it happens in a Prairie stick-framed wall
In a typical Prairie house frame, the specific bridge points are: top and bottom plates (running the full length of every wall), every regular stud, doubled studs at corners and partition intersections, jamb studs and headers around every window and door, and rim joists at each floor line. None of these are defects; they're structurally necessary, but each one is a place where heat moves faster than it does through the insulation beside it.
The worked numbers
| Wall configuration | Nominal cavity R-value | Effective whole-wall R-value | Loss to bridging |
|---|---|---|---|
| 2x6, 24″ o.c., R-22 batt, no exterior insulation | R-22 | R-17.6 (RSI 3.10) | ~20% |
| 2x6, 16″ o.c., R-20 batt + 1.5″ exterior EPS | R-20 + R-6 | R-21.0 (RSI 3.69) | ~19% |
| 2x6 advanced-framed, 24″ o.c., cavity only | R-22 (typ.) | ~R-14.4 | ~35% |
| 2x6 advanced-framed, 24″ o.c. + 4″ exterior XPS | R-22 + R-20 | ~R-38 | n/a — bridging largely eliminated |
The pattern is consistent across every example: cavity insulation alone, no matter how high its rating, leaves 15–35% of its stated value on the table once real framing is accounted for. Continuous exterior insulation is the only strategy that closes that gap by design, because it wraps the entire structure in a layer the studs can't interrupt.
Why this is the code-relevant number
Zone 7A requires an effective RSI 3.08 (R-17.5) for above-grade walls without an HRV, not the nominal batt rating. A bare 2x6/R-22 cavity-only wall at 24″ o.c. (effective R-17.6) just barely clears that minimum, with essentially no margin once real-world workmanship variation is factored in. See what code actually requires where you live for the full table.
What actually closes the gap
Two levers work, and they work together:
- Continuous exterior insulation: rigid foam board or an exterior spray-applied layer, uninterrupted by framing except fasteners. This is the only way to directly address bridging through the studs themselves, because it wraps the whole structure in a layer the wood can't cross.
- Full-cavity spray foam fill: doesn't remove the stud as a thermal bridge, but eliminates the convective air movement inside the cavity that causes batt insulation to underperform its own rating in the field. This closes the gap between a wall's theoretical effective R-value and what it actually delivers once built, which matters because workmanship variance (gaps, compression, settling) is a second, separate source of real-world underperformance beyond framing geometry alone.
The National Building Code recognizes this directly: Section 9.36.2.5 requires that penetrations, headers, rim joists, and other insulation-plane interruptions either be insulated to a minimum ratio of the surrounding assembly's R-value, or that the effective R-value calculation account for the reduction. In other words, the code no longer lets anyone claim a wall's nominal number without doing this math.
Frequently asked questions
What is thermal bridging in a wall?
Thermal bridging is heat flow that bypasses your insulation through a more conductive path, typically wood or steel framing, but also headers, rim joists, and window bucks. Because wood conducts heat faster than the batt or foam beside it, every stud is a small highway for heat to escape, even though the cavity itself is fully insulated.
How much R-value does a wall actually lose to thermal bridging?
In a standard 2x6 wall at 24 inches on-centre with R-22 batt insulation, accounting for framing, air films, sheathing, and cladding brings the effective whole-wall value down to about R-17.6, a loss of roughly 20 percent, before any exterior insulation is added.
Does spray foam eliminate thermal bridging?
No insulation eliminates thermal bridging through the framing itself: heat still moves faster through a stud than through foam. What spray foam does is eliminate the air movement inside the cavity that makes batt insulation perform worse than its rating in practice, and closed-cell foam's higher R-value per inch reduces (but does not remove) the relative impact of the framing fraction. Eliminating the bridge itself requires continuous exterior insulation that wraps the framing altogether.
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