R-value measures resistance to heat flow, and the recommended r values by region exist because the same house needs very different resistance in Duluth and Dallas: the national climate-zone map (zones 1 through 8, hot to cold) keys every target, and the assembly-by-assembly numbers follow a logic worth understanding, big numbers where inches are cheap, hard ceilings where framing limits them, and diminishing returns everywhere. This reference covers the measure, the map, the tables, and the priorities.
What R actually measures, briefly
R-value is resistance to conductive heat flow, additive with thickness: two inches of R-3.5-per-inch material is R-7, and an assembly’s real performance is its weakest paths averaged in, framing (wood is about R-1 per inch, a thermal short circuit through every stud), gaps, and compression discounting the label per the installation sermon. Two honest footnotes frame every table below: R measures conduction only, so air leakage bypasses it entirely, sealing being the prerequisite everywhere, and returns diminish, the step from R-0 to R-19 doing enormous work, R-38 to R-49 modest work, and R-49 to R-60 paying only where winters are long and the filling is cheap.
The map and the attic table
Find the county’s climate zone (a one-search lookup on the national map): zones 1-2, the hot south, attic target R-38; zones 3-4, the broad middle, R-49; zones 5-8, the cold half, R-49 minimum with R-60 the current cold-climate standard. Translated to the tape measure, per the depth-reading reference, that is roughly 13 to 18 inches of blown fiberglass or cellulose, and the attic gets the big numbers for the practical reason that open, blowable space makes each added R the cheapest in the house. Existing-house strategy follows: measure what is there, top up to the zone’s target over a sealed ceiling with baffled eaves, and let the diminishing-returns curve, not enthusiasm, set the stopping point.
Walls, floors, and the below-grade set
Walls carry a hard ceiling: a 2x4 cavity maxes near R-15, a 2x6 near R-21, and the zone tables ask exactly that (R-13 to 15 south, R-20 to 21 north), with cold zones increasingly adding continuous exterior rigid foam (R-5 and up) over the framing, the layer that blankets the studs’ short circuits and helpfully rewrites the vapor rules by warming the sheathing; retrofit walls fill by dense-pack at residing or remodel moments rather than on their own economics. Floors over unconditioned space (vented crawls, garages below rooms): R-19 in the south to R-30 in the north, with the honest note that encapsulating the crawl and insulating its walls instead often beats floor batts that sag and gap. Basement and crawl walls: R-10 continuous rising to R-15 in cold zones, as rigid foam or foam-faced assemblies against the concrete, never bare fibrous batts on masonry, per the moldy-batt findings the moisture references catalog. Rim joists take the wall target in foam form per their own reference, and cathedral ceilings chase the attic number in cavity-plus-foam assemblies with their own moisture rules. Windows sit outside the R tables in U-factor land, the storm-versus-replacement math covering that ledger.
Priorities: where the next dollar goes
The retrofit order the numbers imply, and the energy audit confirms house-by-house: air sealing first, always, the attic plane and rim joist leading; attic to zone target second, the cheap big win; basement and crawl walls third in cold zones, comfort-dense for their cost; floors and walls as their access moments arrive (residing, remodels); and windows last on pure energy math, earlier on comfort and condensation grounds. The pairing rule runs throughout: insulation added over unsealed leaks, unbaffled eaves, or unresolved moisture buys a fraction of its label, which is why the zone tables are the destination and the sealing-and-moisture work is the road.
The bottom line
Look up the zone, then hit the numbers: R-38 to 60 in the attic by climate, R-13 to 21 in walls plus exterior foam where cold, R-10 to 15 on below-grade walls, R-19 to 30 over unconditioned floors. Seal before filling, baffle the eaves, respect the moisture rules below grade, and stop where the curve flattens, because past the zone target the cheapest insulation in the house is the leak you sealed instead.