The number on the bag is the cavity. Roughly a quarter of a framed wall is wood, and wood is a fraction of that resistance — which is where a lot of the insulation you paid for goes.
Openings deducted in full — unlike board, insulation genuinely does not go in a window.
Continuous insulation goes outside the framing, so it covers the studs too — that is why it counts in full.
Batts fill the cavities. They do not fill the studs, and the studs are a significant fraction of the wall — typically about a quarter once you count plates, corners, headers and window bucks.
Wood is roughly R-1.25 per inch, so a 2×6 stud is about R-6.9 sitting right beside R-21 of batt. Heat takes the easy path, and the whole-wall figure lands well below what the packaging suggests.
That is thermal bridging, and it is why the performance figure above is lower than the R-value you selected.
Each path gets its own U-value, and they are averaged by area:
U = ff × Ustud + (1 − ff) × Ucavity
Because U is a reciprocal, averaging happens in the wrong direction for the wall — the poor path pulls the result down harder than the good path pulls it up. That is exactly why a small percentage of wood costs a large percentage of performance.
Less wood. Advanced framing — 24″ centres, two-stud corners, insulated headers, single top plates where the structure allows — takes the framing factor from about 25% to about 16%. That is often worth more than moving up a batt grade, and it uses less lumber.
Cover the wood. Continuous exterior insulation goes outside the studs, so it applies to every square foot of wall equally. R-5 of foam adds a genuine R-5 to the whole assembly, where R-5 more in the cavity adds only the three quarters that is not stud.
Continuous insulation also warms the sheathing, which changes where condensation forms. That is a benefit when it is designed and a problem when it is not, so the vapour strategy has to suit the climate.
A batt compressed into a cavity that is too narrow loses R-value. A batt cut short leaves a gap, and a gap is not a small local loss — convection in the void moves heat around the insulation entirely.
Buy batts sized for the framing you actually have. Cutting 16″ batts to fit 24″ bays wastes both material and performance.
Marginally. Both are 2×6 batts and the whole-wall difference is under a point. Reducing the framing factor or adding continuous insulation moves the number far more.
No. Foam fills cavities better and air seals, which is a real advantage, but the studs are still there and still bridging. Only exterior continuous insulation covers them.
They add a couple of points to both paths and are included in the figures above along with air films.