Insulation R-Value Calculator

Thermal bridging, and why cavity R is not wall R

Insulation only works where it is. In a framed wall, between fifteen and twenty-five percent of the area is not insulation at all — it is wood, and wood conducts heat roughly three times better than the fill beside it.

A stud is about R-1 per inch. A 2×6 stud is therefore around R-5.5 across its depth, sitting alongside a cavity that might be R-20. Heat takes the easy path, so those studs act as thermal bridges carrying a disproportionate share of the loss.

The consequence is that whole-wall R — the figure that actually determines your heating bill — runs ten to twenty percent below the cavity value. A wall described as R-21 typically performs closer to R-17 in service.

Corners, headers and wall intersections are worse than average because they contain more framing. This is the reason California corners and insulated headers exist: they are not about saving lumber, they are about removing thermal bridges from the parts of the wall that have the most.

Continuous exterior insulation is the only complete answer. A layer of rigid board over the sheathing covers the studs as well as the cavities, and an inch of it can improve whole-wall performance more than upgrading the cavity fill.

Air sealing usually beats adding R

In an existing house, the first dollar spent on insulation is frequently the wrong dollar. Air moving through the envelope carries heat directly, and it bypasses insulation entirely.

Fibrous insulation — batts, blown fibreglass, cellulose — slows conduction but does very little to stop air movement. An attic with R-49 of loose fill and unsealed top plates, wire penetrations and a leaky attic hatch loses heat by convection through the insulation rather than through it.

The usual sequence is to air seal first, then insulate. Sealing penetrations at the ceiling plane, weatherstripping the hatch and closing the gaps around chimneys and ducts costs a fraction of an insulation upgrade and frequently delivers more.

Spray foams are the exception in that they do both. Closed-cell foam is an air barrier and a vapour retarder as well as insulation, which is part of why it commands its price. Open-cell air seals but does not stop vapour.

Where the moisture rules bite

Insulation changes where the dew point sits inside a wall, and getting that wrong causes condensation inside the assembly where nobody can see it.

In a cold climate, warm interior air meeting a cold surface inside the wall will condense. Cavity insulation makes the sheathing colder, because less heat is reaching it, which increases the risk. This is why vapour retarders go on the warm side in heating climates, and why the placement reverses in hot humid ones.

Continuous exterior insulation improves this rather than worsening it: it keeps the sheathing warmer, so the dew point moves outward into the foam where there is no moisture to condense. Building codes specify minimum exterior R by climate zone for exactly this reason, and installing too little is worse than none.

Closed-cell spray foam applied directly to sheathing sidesteps the question by being its own vapour retarder, which is why it is common in roof assemblies. Open-cell in the same position needs careful thought about vapour control.

None of this is modelled by an R-value calculation. If you are changing an assembly rather than topping up an attic, the moisture design deserves as much attention as the R-value.