R-Value
Definition
R-value is a measure of a material's resistance to heat flow (thermal resistance). Higher R-values indicate better insulating performance. R-value is additive โ multiple layers of insulation sum their R-values. Recommended R-values vary by climate zone: attics R-38 to R-60, walls R-13 to R-21, floors R-25 to R-30 per US Department of Energy guidelines. Common insulation R-values per inch: fiberglass batts R-3.1โR-3.4, spray foam (closed cell) R-6.0โR-7.0, cellulose R-3.2โR-3.8, rigid foam board R-3.8โR-6.5.
Why is R-Value Important?
For homeowners, contractors, and DIY builders across the United States, understanding R-Value is essential to accurate material estimation and cost planning. Whether you are pouring a concrete driveway, framing a deck, or calculating roofing materials, mastering this concept helps prevent costly over-ordering or project delays from material shortages.
Our free construction calculators leverage this concept to provide instant, accurate estimates โ saving hours of manual measurement and arithmetic while ensuring your project stays on budget.
What is R-Value?
R-value is the measurement of a material's resistance to heat flow (thermal resistance). The "R" stands for resistance โ higher R-values mean better insulation performance and lower energy costs. R-value is the most important specification when selecting insulation for walls, attics, floors, and crawl spaces.
R-values are additive: if you layer R-13 batt insulation + R-5 rigid foam board, the total R-value is R-18.
Recommended R-Values by Climate Zone (US DOE)
| Area | Zone 1โ2 (Hot) | Zone 3 (Warm) | Zone 4 (Mixed) | Zone 5โ8 (Cold) |
|---|---|---|---|---|
| Attic | R-30 to R-49 | R-30 to R-60 | R-38 to R-60 | R-49 to R-60 |
| Walls (2ร4) | R-13 | R-13 | R-13 to R-15 | R-13 to R-15 |
| Walls (2ร6) | R-19 to R-21 | R-19 to R-21 | R-19 to R-21 | R-19 to R-21 |
| Floor | R-13 | R-19 to R-25 | R-25 to R-30 | R-25 to R-30 |
| Crawl Space | R-13 | R-19 to R-25 | R-25 | R-25 |
R-Value per Inch by Insulation Type
| Insulation Type | R-Value/Inch | Best For | Cost ($/sq ft at R-13) |
|---|---|---|---|
| Fiberglass Batts | R-3.1 โ R-3.4 | Walls, attics, floors (standard) | $0.40โ$0.80 |
| Blown-In Fiberglass | R-2.2 โ R-2.7 | Attics, retrofit walls | $0.50โ$1.00 |
| Cellulose (Blown) | R-3.2 โ R-3.8 | Attics, dense-pack walls | $0.50โ$0.90 |
| Spray Foam (Open Cell) | R-3.5 โ R-3.7 | Walls, rooflines, air sealing | $0.80โ$1.50 |
| Spray Foam (Closed Cell) | R-6.0 โ R-7.0 | Basements, rim joists, moisture barriers | $1.50โ$3.00 |
| XPS Rigid Foam | R-5.0 | Exterior sheathing, below-grade | $0.75โ$1.25 |
| Polyiso Rigid Foam | R-5.7 โ R-6.5 | Roof decking, exterior walls | $0.80โ$1.50 |
| EPS Rigid Foam | R-3.8 โ R-4.4 | ICFs, exterior sheathing | $0.50โ$0.90 |
| Mineral Wool (Rockwool) | R-3.3 โ R-4.2 | Fire-rated walls, soundproofing | $0.90โ$1.60 |
R-Value vs. Energy Savings
The relationship between R-value and energy savings is not linear โ you get diminishing returns as you add more insulation. The biggest savings come from going from zero insulation to moderate insulation:
- R-0 to R-11: ~70% reduction in heat loss through that assembly
- R-11 to R-19: Additional ~15% reduction
- R-19 to R-30: Additional ~8% reduction
- R-30 to R-49: Additional ~4% reduction
This means upgrading an uninsulated attic to R-38 captures roughly 90%+ of possible savings. Going beyond R-49 has minimal payback in most climates.