Methodology

Thermal Resistance in Series

Why R-values add but U-values do not, and why the assembly you build never performs like the assembly you specified.

Resistances add; conductances do not

Heat passing through a wall crosses each layer in turn, and each layer resists it. Because they are in series, their resistances add directly — plasterboard plus insulation plus sheathing plus cladding, plus the thin but real resistance of the still air films on each face.

The U-value is the reciprocal of that total. It is a conductance, and conductances in series emphatically do not add: two layers of U = 0.5 do not make U = 1.0, they make U = 0.25. Adding U-values is a common and large error, and it always errs toward claiming better performance than exists.

U = \frac{1}{\sum_{i}^{n} R_{i}}

U equals one divided by the sum of the layer resistances.

U: thermal transmittance (W/m²·K)
R: thermal resistance of layer i (m²·K/W)

Thermal bridging: the parallel path

The series model assumes heat crosses uniform layers. A framed wall is not uniform: at every stud there is a parallel path of much lower resistance, and heat preferentially takes it. A wall with insulation rated R-19 between studs does not perform at R-19, because the studs occupy a meaningful fraction of the area and conduct several times better.

This is why continuous insulation outboard of the framing is disproportionately effective. It is the only layer with no bridge through it, so it raises the whole assembly rather than only the cavity.

And then there is air

R-value describes conduction. It says nothing about air moving through or around the insulation, and air movement can dominate. A well-insulated wall with an interrupted air barrier can underperform a modest wall that is properly sealed, because convection carries heat past the insulation rather than through it.

The practical consequence is that measured performance is found with a blower door, not with a specification. Several calculators on this site size the air-sealing side of that pair for exactly this reason.

Calculators that use this method

R-Value CalculatorAdd up the R-values of layered materials in a wall, floor, or ceiling assembly to get the total insulating value.
Insulation Batt CalculatorEstimate how many packages of batt insulation you need to cover an area, using your product's stated coverage.
Attic Insulation Upgrade CalculatorFind how many more inches of blown-in insulation to add to reach your target attic R-value.

Basis

Layers in series: total resistance is the sum of layer resistances plus internal and external surface resistances.