Insulation R-Value Guide: What You Need and How to Calculate It
Complete guide to insulation R-values by climate zone, wall assembly, and material type. With comparison tables and installation tips.
R-value is the measure of thermal resistance — how effectively insulation slows the transfer of heat. Higher R-value means better insulation. But knowing which R-value you need, and how much material that requires, depends on where you are and what you’re insulating.
Quick Answer: For most of the US (climate zones 4–5): R-49 attic, R-13 to R-15 walls in a 2×4 cavity, R-19 in a 2×6 cavity. Add rigid foam to the exterior if you need more wall R-value without rebuilding the framing.
What Is R-Value?
R-value is a measure of thermal resistance per inch of material thickness. It’s expressed as R-X where X is the resistance value. An R-19 wall resists heat transfer at a rate 19 times that of R-1.
R-values are additive. A wall assembly with 3.5 inches of fiberglass batt (R-13) plus 1 inch of rigid foam (R-5) has a total R-value of R-18.
R-Value Per Inch by Material Type
| Insulation Type | R-Value per Inch |
|---|---|
| Fiberglass batt | R-3.1–3.8 |
| Mineral wool batt | R-3.7–4.2 |
| Cellulose (blown) | R-3.2–3.8 |
| Open-cell spray foam | R-3.5–3.9 |
| Closed-cell spray foam | R-6.0–7.0 |
| Rigid EPS foam (Type I) | R-3.6 |
| Rigid XPS foam (blue/pink) | R-5.0 |
| Rigid polyiso | R-5.7–8.0 |
Closed-cell spray foam and polyiso deliver the highest R-value per inch, which matters in assemblies with limited space.
Code Minimum R-Values by Climate Zone
The US is divided into 8 climate zones (IECC). Insulation requirements increase with zone number:
Attic Insulation (new construction minimum)
| Climate Zone | R-Value Minimum |
|---|---|
| Zone 1 (Hawaii, South FL) | R-30 |
| Zone 2 (Gulf Coast) | R-38 |
| Zone 3 (Southeast, AZ) | R-38 |
| Zone 4 (Mid-Atlantic, PNW) | R-49 |
| Zone 5 (Great Lakes, CO) | R-49 |
| Zone 6 (Northern states) | R-49–60 |
| Zone 7–8 (Very cold) | R-60 |
Most energy efficiency programs (Energy Star, LEED) recommend exceeding these minimums.
Wall Insulation (cavity only)
| Climate Zone | Wall Cavity R-Value |
|---|---|
| Zone 1–2 | R-13 |
| Zone 3–4 | R-13 to R-15 |
| Zone 5–8 | R-20 or R-13 + R-5 CI |
CI = continuous insulation (rigid foam on the outside of framing). It addresses thermal bridging through studs, which can reduce effective wall R-value by 15–30% compared to cavity-only insulation.
Common Insulation Products and Their R-Values
Fiberglass Batts
The most common residential insulation.
- R-11: 3.5” thick, fits 2×4 wall
- R-13: 3.5” thick (high density), fits 2×4 wall
- R-15: 3.5” thick (HD), fits 2×4 wall
- R-19: 5.5” thick, fits 2×6 wall
- R-21: 5.5” thick (HD), fits 2×6 wall
- R-30: 10” thick, attic batt
- R-38: 12” thick, attic batt
Blown Cellulose (Attics)
Blown cellulose is one of the most cost-effective attic insulation options.
- Typical attic application: 10–14 inches for R-38
- Bags per 1,000 sq ft at R-38: roughly 23–27 bags (varies by brand)
Rigid Foam Board
- 1” XPS = R-5
- 2” XPS = R-10
- 1” polyiso = R-6.5 (effective, derated from nominal in cold)
Spray Foam
Open-cell: R-3.7 per inch, expands to fill cavities, air seals Closed-cell: R-6.5 per inch, structural, vapor barrier
How to Calculate Insulation Quantity
Use our Insulation Calculator for automatic results. Manual calculation:
For batts: Count the number of stud bays. Each batt fits one bay. Add 10% waste.
For blown insulation: Manufacturers publish coverage charts by R-value on each bag.
For rigid foam: Area in square feet ÷ sheet area = sheets needed. Add 10% for cuts.
Frequently Asked Questions
Is more insulation always better? Up to a point. There are diminishing returns — going from R-13 to R-20 in walls makes a large percentage difference; going from R-60 to R-80 in an attic makes a small difference. The optimal point depends on energy costs and climate.
What’s the best insulation for a 2×4 wall? R-13 or R-15 fiberglass or mineral wool batt fills the 3.5-inch cavity. If you need more, adding 1” rigid foam to the exterior gets you to R-18–20 without rebuilding the wall.
Does insulation reduce sound as well as heat? Yes, especially dense-pack cellulose and mineral wool. Sound Transmission Class (STC) ratings are separate from R-value, but most insulated walls perform noticeably better acoustically than uninsulated ones.
Should I remove old insulation before adding new? For attics: usually not necessary. You can blow new insulation over old batts if the old insulation is dry and undamaged. For walls: replacement requires opening the wall. For basement rim joists: remove old fiberglass, replace with spray foam.
See also: Framing Calculation Methodology — IRC R602 references, stud count formulas, and lumber waste factors behind ConstructCalc framing calculators.