R-38 is the Oklahoma City target — here is where that number comes from, why R-49 keeps showing up, and how to tell what your attic has today.
Quick answer: Oklahoma City is in IECC Climate Zone 3, and the attic insulation target for Zone 3 is R-38. The R-49 figure you see in national articles and some quotes comes from guidance written for colder climate zones — it is an optional upgrade here, not the Oklahoma benchmark. In spray foam, R-38 means about 10.5 inches of open-cell foam or about 6 inches of closed-cell foam. This guide is the deep dive behind the short version on our attic insulation page: what R-value actually measures, how deep each material has to be, and how to measure what is already in your attic.
R-value is a measure of how well a material resists heat flowing through it. Higher number, slower heat flow. It is not a rating of the material in the abstract — it is a property of the installed thickness. Every insulation material has an R-value per inch, and the total is simple multiplication: per-inch R-value × inches installed = the R-value your ceiling actually has.
That per-inch number is where materials really differ:
Two honest caveats belong with any R-value discussion. First, R-value is measured in a lab with no air movement; it says nothing about air leaks, and air leaks are how a lot of heat actually moves through a real attic. Second, R-value is not linear in benefit: the jump from R-11 to R-19 helps far more than the jump from R-38 to R-49, because each step cuts the remaining heat flow by a smaller slice. Both points matter when we get to when more R stops paying.
The IECC (International Energy Conservation Code) divides the country into climate zones 1 through 8, hot to cold, and sets insulation targets for each. Oklahoma straddles a boundary:
That split, plus decades of national advice written for colder states, is the whole source of the R-38 vs. R-49 confusion. An article written for Minnesota, a manufacturer chart aimed at Zone 4–5, or a contractor quoting a house near the Zone 4 line will all say R-49. For an OKC-metro home, R-38 is the number that fits our climate guidance — hotter summers than winters are our problem, and the code targets reflect that. Nothing is wrong with choosing R-49 as an upgrade; just know you are buying past the local target, and read the diminishing-returns section before paying for it.
Because R-value is bought by the inch, the practical question is depth. These are installed thicknesses for the three attic targets you will see quoted, using the per-inch values above. Blown-in figures are approximate — loose-fill settles after installation, so installers typically blow it deeper than the settled target.
| Target R-value | Open-cell foam (R-3.7/in) | Closed-cell foam (R-6.5/in) | Blown-in cellulose (~R-3.5/in, approx.) | Blown-in fiberglass (~R-2.5/in, approx.) |
|---|---|---|---|---|
| R-30 | ~8 inches | ~4.5–5 inches | ~8.5–9 inches | ~12 inches |
| R-38 — the OKC target | ~10.5 inches | ~6 inches | ~11 inches | ~15 inches |
| R-49 (colder-zone figure) | ~13.5 inches | ~7.5 inches | ~14 inches | ~19.5–20 inches |
Read the table and the foam choice usually makes itself: hitting R-38 with open-cell takes a deep but inexpensive spray, while closed-cell gets there in about 6 inches where headroom, roof geometry, or a roof-deck application leaves no room for depth. The full foam-type comparison lives in open-cell vs. closed-cell, and the attic-specific version is in open-cell vs. closed-cell for an OKC attic.
You do not need a contractor to get a rough read on your attic — a flashlight, a ruler or tape measure, and ten minutes will do it.
Fiberglass and cellulose insulate by trapping air in a fluffy matrix — the loft is the insulation. Compress a batt made for a 2×6 cavity into a smaller space and its R-value drops with its thickness; an R-19 batt squeezed down performs closer to the depth you actually left it. Blown-in materials settle over the years, losing depth they were credited with on install day, and the R-value printed on the bag assumes the full settled depth the manufacturer specified. This is why a 1990s OKC attic "insulated to R-30" on the original paperwork can measure out at R-19 today — and why depth, measured now, beats paperwork from install day.
If the numbers say your attic is thin, the next step is pricing the fix: spray foam cost in Oklahoma City breaks down what attic jobs run locally, and a free inspection confirms the depth readings with the foam options priced against them.
R-value has diminishing returns built into the math. Going from R-11 to R-19 cuts heat flow through the ceiling noticeably; going from R-38 to R-49 trims the remaining flow by a much smaller amount, and in a Zone 3 climate — where the attic battle is mostly summer heat — that last increment buys very little per dollar. For most OKC homes, R-38 is the sweet spot the climate guidance points to, and money past it is usually better spent elsewhere in the attic.
"Elsewhere" almost always means air sealing first. R-value ratings assume still air; a real attic has gaps around plumbing stacks, recessed lights, duct chases, and the attic hatch, and hot air pours through them regardless of what the insulation above is rated. An unsealed attic insulated to R-49 still leaks — the expensive depth sits on top of holes it cannot plug. Spray foam's advantage in this picture is that it seals and insulates in the same pass, which is why a sealed R-38 foam attic outperforms leaky R-49 batts in practice. Sequence matters: seal the leaks, then insulate to the Zone 3 target, then stop.
Attics get the attention, but the same zone logic sets the rest of the envelope. For walls, Zone 3 guidance lands at about R-13 to R-15 for standard framed cavities — which is why a typical 2×4 wall job targets that range. Floors over vented crawl spaces or garages are commonly insulated to a higher level than walls, roughly the R-19 range, because they separate living space from unconditioned air below. These are reference points for judging what you have, not a substitute for a quote: wall and floor work depends heavily on construction type, which is what the free inspection sorts out.
R-38. Oklahoma City is in IECC Climate Zone 3, and R-38 is the prescriptive ceiling target for this zone. The R-49 figure comes from guidance written for colder climates (Zone 4 and up) — northern Oklahoma edges into Zone 4, which is one reason both numbers circulate here. Choosing R-49 in OKC is an optional upgrade, not a requirement, and past R-38 the added savings shrink while air sealing usually matters more.
About 10.5 inches of open-cell spray foam (R-3.7 per inch) or about 6 inches of closed-cell spray foam (R-6.5 per inch). Thickness is how R-value is bought with foam: the installer sprays to a measured depth and the finished R-value is the depth multiplied by the foam's per-inch R-value.
With blown-in insulation, usually yes — new material can be blown over old as long as the old insulation is dry, clean, and not hiding moisture or pest problems. With spray foam, usually no: foam needs to bond to and seal the attic floor or roof deck directly, so old, thin, or damaged insulation typically comes out first. The free inspection settles which situation your attic is in.
The R-38 figure is the prescriptive ceiling target for new construction and major work in Climate Zone 3. Existing homes are generally not forced to retrofit up to it just because the house changed hands, but R-38 is still the right benchmark for judging whether your attic is under-insulated — and meeting it is what rebate programs and most buyers' expectations are built around.
Measure the depth of the insulation with a ruler or tape at several spots — near the hatch, mid-attic, and out toward the eaves — and multiply the average depth by the material's per-inch R-value (about R-3.5 for cellulose, R-2.5 for blown fiberglass, about R-3.2 for fiberglass batts). Six inches of blown fiberglass works out to roughly R-15, well under the R-38 target. Watch for compressed or settled spots, which perform below their label.