Five Types of R-Value

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We talk about R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor.
all the time. “I’ve got an R-19 wall,” or “Code requires R-38 in my ceiling.” But what are those numbers? As it turns out, when we talk about R-value we usually give the R-value of the insulation material itself. That’s the case with both of those statements above. But what’s the real R-value of the wall or the ceiling? Insulation makes up only a part of each. There’s also wood and drywall and sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen.
and claddingMaterials used on the roof and walls to enclose a house, providing protection against weather.

and…

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[source: http://www.greenbuildingadvisor.com/articles/dept/building-science/five-types-r-value]