R-Values, Explained Without the Snake Oil

R-value is the single most-marketed and most-misunderstood number in residential construction. Spray foam companies brag about R-7 per inch. Fiberglass companies advertise R-38 attic batts. Building codes mandate climate-zone minimums. None of these numbers tell you the full story. Here's what R-value actually measures and what to target without falling for marketing.

What R-value actually is

R-value is a measure of resistance to heat transfer through a material by conduction. Higher R = more resistance = less heat loss. The unit is ft² · °F · hr / BTU — a measure of how much temperature difference (°F) is needed to drive 1 BTU/hr through 1 square foot of material.

Translation: an R-19 wall, with a 30°F temperature difference between inside and outside, loses 30 / 19 = 1.58 BTU per hour per square foot. The same wall with R-38 loses 0.79 BTU/hr/sq ft — half as much.

What R-value doesn't measure

• Air leakage (convection): R-value is a conduction-only measurement. A high-R wall with air leakage performs worse than a moderate-R wall with no air leakage.
• Radiation: matters most in attics, where summertime roof-deck temperatures (160°F+) radiate downward. Radiant barriers help; R-value doesn't measure them.
• Thermal bridging: studs, plates, and headers conduct heat better than insulation. A 2x6 wall with R-19 batt insulation between studs has an effective R-value of only R-14–R-16 because of the wood members.
• Real-world install quality: compressed or gappy batt insulation can lose 30–50% of its rated R-value
• Temperature dependence: some insulations lose R-value at very cold temperatures (fiberglass) or very warm temperatures (polyiso). Marketing R-values are typically measured at 75°F.

Climate zone minimums (Texas)

Most of Texas is in IECC Climate Zone 2 (south Texas and gulf coast) or Climate Zone 3 (DFW, central, west Texas). North Texas mountains and panhandle creep into Zone 4.

Code minimums (IECC 2021) for Climate Zone 3:

• Attic ceiling: R-49 (or R-38 with continuous foam)
• Wood-framed walls: R-20 + R-5 continuous, or R-13 + R-10 continuous, or R-30 cavity
• Floors over crawlspace: R-19
• Basement walls: R-15 continuous or R-19 cavity
• Crawlspace walls (conditioned): R-10 continuous

What we recommend for North Texas custom homes

• Attic ceiling: R-49 to R-60 blown-in cellulose (vented attic) OR R-30 to R-38 closed-cell spray foam at roof deck (unvented attic)
• Walls (2x6 framing): R-21 cavity (high-density batt or open-cell foam) + R-5 continuous exterior foam = R-26 total, or flash-and-batt with 2" closed-cell + R-13 batt
• Floor over crawlspace or unconditioned space: R-30 batt (in joist bay) or closed-cell foam directly to subfloor
• Basement walls: 2" closed-cell spray foam directly to concrete, or 2" rigid foam with framed and batt-insulated 2x4 wall
• Garage-to-house common wall: R-13 batt minimum, ideally sheathed with rigid foam for additional break

Continuous insulation — the upgrade that matters most

Continuous exterior insulation (CI) is rigid foam (or mineral wool board) installed on the exterior of the sheathing, before the siding. It does two crucial things:

1. Stops thermal bridging through studs — the effective R-value of the wall jumps significantly
2. Provides a secondary air barrier and water-resistive barrier

A 2x6 wall with R-21 cavity insulation has an effective R-value of ~R-15. Add 1.5" of polyiso CI (R-9.6) and the wall is effectively R-23–R-25 — a 60% increase from one inch of foam. CI is the single highest-leverage insulation upgrade in modern residential construction.

Comparing materials by R-value per inch

• Closed-cell spray foam: ~R-6.5 per inch
• Polyiso rigid foam (faced): R-6.0 per inch (drops in cold weather)
• XPS rigid foam (pink/blue board): R-5.0 per inch
• EPS rigid foam (white bead board): R-3.8 per inch
• Open-cell spray foam: ~R-3.6 per inch
• Mineral wool batt: R-4.2 per inch
• High-density fiberglass batt: R-3.7 per inch
• Standard fiberglass batt: R-3.1 per inch
• Blown cellulose: R-3.5 per inch
• Wood (stud): R-1.0 per inch

What to ignore

• "Equivalent R-value" claims for radiant barriers: radiant barriers reduce radiant heat transfer but don't have a true R-value. Marketing claims of "R-20 equivalent" for a thin foil are not honest comparisons.
• "Effective R-value" for liquid-applied air barriers: air barriers don't have meaningful R-values. They control air leakage, which is separately important.
• "Aged R-value" vs. "long-term R-value" vs. "initial R-value" for polyiso: polyiso's R-value degrades over time as blowing agents diffuse out. Use the long-term thermal resistance (LTTR) value, not the initial value.

The honest takeaway

Target R-49 (attic) and R-21 cavity + R-5 continuous (walls) for North Texas custom homes. Spend money on air sealing first, continuous insulation second, cavity insulation third. Don't fall for marketing claims about R-value alone — the install quality, air sealing, and thermal bridging matter as much as the rated R.

— Daniel Caro, Construction Manager. Twenty years running jobsites — foundation, framing, mechanicals, and the unglamorous details that decide a great home. Get the free Ultimate Home Building Checklist for the field-tested list we walk every Angel home through.