R-Value measures thermal resistance — how well a material resists heat flow. Higher R-values indicate better insulating capability, keeping heat in during winter and out during summer.
Recommended R-Values by Location
- Attic: R-30 to R-60 (varies by climate zone)
- Exterior Walls: R-13 to R-21
- Floor/Crawlspace: R-13 to R-30
- Basement Walls: R-5 to R-15
Southern climates (Zone 1-3) need lower R-values; northern climates (Zone 5-7) need higher values. The DOE provides specific recommendations by ZIP code.
Types of Insulation
Fiberglass batts (R-3.2/inch), cellulose (R-3.5/inch), spray foam closed-cell (R-6.5/inch), rigid foam board (R-5 to R-8/inch). Spray foam is most expensive but provides the highest R-value per inch and also acts as an air barrier.
Real-World Example
An attic in Climate Zone 5 (Chicago) needs R-49. Using fiberglass batts (R-3.2/inch): 49 ÷ 3.2 = 15.3 inches of insulation. With blown cellulose (R-3.5/inch): 49 ÷ 3.5 = 14 inches. With closed-cell spray foam (R-6.5/inch): only 7.5 inches needed.
Frequently Asked Questions
Is more R-value always better?
There is a point of diminishing returns. Going from R-0 to R-19 makes a dramatic difference. Going from R-38 to R-60 saves much less energy relative to the cost. The DOE recommendations balance energy savings against insulation costs.
Can I add insulation on top of existing insulation?
Yes, for attics this is common and effective. R-values are additive — adding R-19 batts on top of existing R-19 gives you R-38. Ensure there are no moisture issues before adding insulation to walls.
What is the difference between R-value and U-value?
R-value measures resistance to heat flow (higher is better). U-value measures heat transfer (lower is better). They are inverses: U = 1/R. Windows are typically rated in U-value; insulation in R-value.