💡
Quick Answer
For a 2×6 wall in Climate Zone 5, you need R-30 (or R-20+5ci). One bag of blown cellulose covers ~21 sq ft at R-38. Fiberglass batts cost $0.27–$0.75/sq ft material only; spray foam runs $2–$5/sq ft installed.
🌡️ R-Value Requirements by Climate Zone (2021 IECC)
Zone 3 (Atlanta, Memphis, Las Vegas)
Ceiling:R-49
Walls:R-20 or R-13+5ci
Floors:R-19
Slab edge: R-10ci to 2 ft depth
Zone 4 (DC, Baltimore, Seattle)
Ceiling:R-60
Walls:R-30 or R-20+5ci
Floors:R-19
Basement walls: R-10ci or R-13
Zone 5 (Chicago, Denver, NYC)
Ceiling:R-60
Walls:R-30 or R-13+10ci
Floors:R-30
Basement walls: R-15ci or R-19
Zones 6-8 (Vermont, Minnesota, Alaska)
Ceiling:R-60
Walls:R-30 or 0+R-20ci
Floors (Z7-8):R-38
Air sealing: ≤3.0 ACH50 required
Note: "ci" = continuous insulation. R-13+10ci means R-13 cavity + R-10 continuous exterior layer. 2024 IECC reduced ceiling requirements by ~R-11 across all zones.
📊 Insulation Material Comparison
R-Value & Performance by Type
| Material | R-Value/Inch | Air Barrier | Best Use |
|---|---|---|---|
| Fiberglass Batt | R-2.9–3.8 | No | Walls, attics (new construction) |
| Blown Cellulose | R-3.2–3.8 | Yes (dense-pack) | Attics, retrofit walls |
| Open-Cell SPF | R-3.5–3.8 | Yes | Walls, rim joists (above grade) |
| Closed-Cell SPF | R-6.0–7.0 | Yes | Highest R/inch, basements, rim joists |
| XPS Foam Board | R-5.0 | No | Below-grade foundations |
| Mineral Wool | R-3.3–4.3 | No | Fire-rated walls, soundproofing |
Critical: Air sealing is as important as R-value. Fiberglass without air sealing loses 30–40% performance. Grade I installation (no gaps/compression) is mandatory for rated performance.
💰 Material Coverage & Cost (2024-2026)
Fiberglass Batts
R-13 coverage:88 sq ft/pkg
Material cost:$0.27–$0.50/sq ft
Installed cost:$0.80–$1.80/sq ft
Best for: DIY, new construction walls
Blown Cellulose
R-38 coverage:21 sq ft/bag (25 lb)
1,000 sq ft @ R-38:~47 bags
Installed cost:$1.30–$2.10/sq ft
Best for: Attic insulation, retrofit walls
Spray Foam
Open-cell:$2.40–$4.00/sq ft
Closed-cell:$3.00–$5.00+/sq ft
Board foot:1 sq ft @ 1" thick
Best for: Air sealing + insulation combo
Rigid Foam Boards
EPS 1":$0.25–$0.35/sq ft
XPS 1":$0.40–$0.55/sq ft
Polyiso 1":$0.75–$1.10/sq ft
Best for: Continuous exterior, basements
Thickness Required for Common R-Values
| Target R-Value | Fiberglass | Cellulose | Closed-Cell SPF |
|---|---|---|---|
| R-13 | 3.5" | 3.8" | ~2.0" |
| R-19 | 6.25" | 5.3" | ~3.0" |
| R-30 | 9.5–10" | 8.1" | ~4.6" |
| R-38 | 12" | 10.2" | ~5.8" |
| R-49 | 14" | 13.1" | ~7.5" |
👷 Installation Standards & Common Mistakes
✅Grade I Installation (RESNET)
- ✓Uniformly fills cavity with no gaps or voids
- ✓Maximum 2% defects allowed
- ✓No compression below 70% of original thickness
- ✓Required for Energy Star certification
Only ~50% of homes achieve Grade I installation
⚠️Compression Penalties
R-19 in 2×4 wall:R-13 (32% loss)
R-30 in 2×6 wall:R-20 (33% loss)
R-38 in 2×10 ceiling:R-29 (24% loss)
❌Never compress insulation to fit — use correct thickness for cavity depth
Thermal Bridging Through Framing
Standard 2×4 wall with R-13 cavity insulation:
Labeled R-value:R-13
Actual whole-wall R:R-8.7 (33% loss)
Solution: Continuous exterior insulation eliminates thermal bridging by covering studs completely. R-13+5ci = true R-18 whole-wall performance.
💵 Energy Savings & Financial Incentives
Typical Savings & Payback
15%
Average heating/cooling cost reduction
$200–$600
Annual savings per household
4–8 years
Typical payback for attic insulation
Available Rebates (2025-2026)
⚠️
Federal 25C Tax Credit EXPIRED
30% credit up to $1,200 ended December 31, 2025. No replacement announced as of 2026.
✅
IRA HOMES Program (Active)
Up to $2,000 for 20–34% energy reduction or $4,000 for ≥35% whole-home savings. Doubled for households below 80% AMI. State-administered — check availability.
💡
State & Utility Programs
Massachusetts Mass Save (75–100% coverage), Connecticut Energize CT (up to $1.70/sq ft), New York EmPower+ (free for income-eligible). Search DSIRE database for local programs.
🌬️ Air Sealing Requirements (2021 IECC)
≤5.0
ACH50 (Zones 1-2)
Air changes per hour at 50 Pa
≤3.0
ACH50 (Zones 3-8)
Mandatory blower door testing
Critical: Air sealing delivers outsized returns. A Grade I installation of R-13 with proper air sealing outperforms a poorly sealed R-30 installation.
📚 Building Code & Standard References
•2021 IECC Table R402.1.3: Prescriptive R-Value Requirements
•IRC R402.4: Air Leakage & Testing Requirements
•IRC R702.7: Vapor Retarder Requirements
•IRC R316: Foam Plastic Thermal Barrier Requirements
•ASTM C518: Heat Flow Meter R-Value Testing
•ASTM C1363: Hot Box Full-Assembly Testing
•RESNET Standard 802: Grade I, II, III Installation Criteria
•16 CFR §460.5: FTC R-Value Rule (Testing at 75°F)
The 2024 IECC reduced ceiling requirements by ~R-11 but tightened air leakage standards by 20%. Always verify local amendments with your building department.