Understanding Insulation R-Values
The R-value measures insulation's resistance to heat flow. A higher R-value means greater insulating power. The R-value you need depends on your climate zone, the area being insulated (attic, walls, or floor), and local building codes.
The U.S. Department of Energy divides the country into 7 climate zones, each with recommended minimum R-values for different parts of the building envelope. Proper insulation reduces energy costs, improves comfort, and helps control moisture.
Insulation Calculation
Recommended R-Values by Climate Zone
| Zone | Attic | Wall (2x4) | Wall (2x6) | Floor | Basement |
|---|---|---|---|---|---|
| Zone 1 | R-30 | R-13 | R-13 | R-13 | R-11 |
| Zone 2 | R-38 | R-13 | R-13 | R-13 | R-11 |
| Zone 3 | R-38 | R-13 | R-19 | R-19 | R-11 |
| Zone 4 | R-49 | R-13 | R-19 | R-25 | R-13 |
| Zone 5 | R-49 | R-15 | R-21 | R-25 | R-15 |
| Zone 6 | R-60 | R-15 | R-21 | R-30 | R-15 |
| Zone 7 | R-60 | R-15 | R-21 | R-30 | R-19 |
Insulation Type Comparison
| Type | R per Inch | Cost (per sq ft) | Best Application |
|---|---|---|---|
| Fiberglass Batts | R-3.2 | $0.50-$1.00 | Walls, attics (open framing) |
| Blown-In Fiberglass | R-2.5 | $0.75-$1.25 | Attics, enclosed walls |
| Blown-In Cellulose | R-3.5 | $0.60-$1.00 | Attics, enclosed walls |
| Spray Foam (Open Cell) | R-3.7 | $1.00-$1.50 | Walls, crawl spaces |
| Spray Foam (Closed Cell) | R-6.5 | $1.50-$3.00 | Basement, rim joist, moisture barriers |
| Rigid Board (XPS) | R-5.0 | $0.75-$1.50 | Basement, exterior walls, under slab |
Frequently Asked Questions
Can I add insulation on top of existing insulation?
Yes, in most cases you can add new insulation on top of existing insulation in attics. Use unfaced batts or blown-in insulation on top of existing material. Do not add a second vapor barrier -- only the bottom layer closest to the heated space should have one.
What is the most cost-effective insulation?
Blown-in cellulose and fiberglass batts offer the best value for most applications. Cellulose is particularly cost-effective for attics and provides excellent coverage. Spray foam costs more upfront but provides superior air sealing, which can justify the investment in specific applications.
How do I know my climate zone?
The IECC (International Energy Conservation Code) climate zone map divides the U.S. into zones 1 through 7 based on heating and cooling degree days. Zone 1 is the hottest (southern tip of Florida, Hawaii), and Zone 7 is the coldest (northern Minnesota, Alaska). Most local building departments can tell you your zone, or use the DOE climate zone map.