Bello Haus — insulation R-values | bellohaus.net
How R-values add across the layers of an assembly, and how the U-factor relates to the sum, with a worked wall example.
R-values are additive for multiple layers of materials placed in series. A wall built from exterior sheathing, an insulation cavity and an interior finish has one assembly R equal to the sum of the individual layer R-values; no single layer can be quoted as the wall's performance.
Worked example: take a wall whose sheathing contributes R-4, whose cavity insulation contributes R-13 and whose interior finish contributes R-1. The assembly R is 4 + 13 + 1 = R-18, and the U-factor of that assembly is the reciprocal of 18. The same wall described in U-factor and the same wall described in R are the same assembly from opposite ends.
The U-factor of an assembly is the reciprocal relationship to its R-value and is covered as its own section of the reference. When a plan line is written in U-factor, take the reciprocal to obtain the equivalent R before you run the thickness procedure; when it is written in R, take the reciprocal to obtain U for a conductance check.
On narrow screens, swipe or scroll the plate sideways.
Because the relationship is reciprocal, the gain from one more inch depends on where you start. At a low starting R, an added inch moves the U-factor noticeably; at a high starting R, the same inch changes U very little, which is why deep already-insulated assemblies show little U improvement from topping up.
Series addition also means a weak layer caps the sum. A wall with a large glazing area or a thin sheathing carries that layer's low R into the total, so raising the cavity insulation alone cannot lift the assembly R past what the other layers allow. Calculate every layer, then decide which one to change.
When comparing two assemblies, compare either both in R or both in U — never one in R against the other in U. The 5.68 factor between US R and metric RSI applies at the R stage, before the reciprocal is taken.
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