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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
R-value measures resistance to heat flow — higher is better. Pick your material and thickness to see the R-value you get, then compare it against the recommended level for your climate zone.
With these inputs:
| Result | Value |
|---|---|
| R-value achieved | R-15 |
| Recommended for your zone | R-60 |
| Shortfall | R-45 short |
| Extra thickness needed | 18 in more |
| U-value | 0.0667 |
R-value = R-per-inch × installed thickness · Shortfall = recommended R − achieved R · Extra inches = shortfall ÷ R-per-inch · U-value = 1 ÷ R-value
Most of the US lands between R-49 and R-60 for attics. Warm southern zones can stop at R-30 to R-49; anything from the Midwest north should target R-60.
For batts and boards, essentially yes. Loose-fill settles, so blown products lose a little R over time — buy to the coverage chart depth, not the bare minimum.
In series through an assembly, yes — a layer of R-13 behind a layer of R-5 gives R-18 through that path. What does not add up is the whole wall, because studs run straight through the insulation and conduct around it. A wall with R-13 batts between wood studs performs closer to R-11 overall once the framing is accounted for.
They are reciprocals of each other. R-value measures resistance to heat flow, so higher is better; U-value measures the rate of heat flow, so lower is better. U equals 1 divided by R. Windows and doors are normally specified in U, and insulation in R, which is why comparing the two directly needs a conversion.
Yes, and for some materials noticeably. Polyiso loses R-value as it gets very cold, which matters in a cold-climate roof where its rated performance is measured at a warmer mean temperature than it will actually see. Fibreglass and cellulose are comparatively stable. This is one reason published R per inch should be treated as a comparison figure rather than a guarantee.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.