R-value measures how well insulation resists heat flow. The higher the R-value, the better the insulation. Here's how to calculate what you need for your home.
What Is R-Value?
R-value (thermal resistance) is measured in ft²·°F·hr/BTU in the US:
R-value = Thickness (inches) ÷ k-value
Where k-value is the thermal conductivity of the material (BTU·in / hr·ft²·°F).
In simpler terms: higher R-value = better insulation = lower energy bills.
Recommended R-Values by Location and Climate Zone
The US Department of Energy divides the country into 8 climate zones:
Attic / Ceiling Insulation
| Climate Zone | Region Examples | Recommended R-Value |
|---|---|---|
| Zone 1 | South FL, Hawaii | R-30 to R-49 |
| Zone 2 | FL, Gulf Coast | R-30 to R-60 |
| Zone 3 | Mid-South, CA coast | R-38 to R-60 |
| Zone 4 | Mid-Atlantic, PNW | R-38 to R-60 |
| Zone 5 | Great Lakes, New England | R-49 to R-60 |
| Zone 6 | Upper Midwest, Montana | R-49 to R-60 |
| Zone 7 | Minnesota, N. Montana | R-49 to R-60 |
| Zone 8 | Alaska | R-49 to R-60 |
Wall Insulation (Exterior Walls)
| Climate Zone | Existing Home | New Construction |
|---|---|---|
| Zones 1–3 | R-13 | R-13 to R-15 |
| Zones 4–7 | R-13 to R-15 | R-20 or R-13 + R-5 continuous |
| Zone 8 | R-15 | R-21 |
Floor / Crawl Space Insulation
| Climate Zone | Recommended Floor R-Value |
|---|---|
| Zones 1–3 | R-13 |
| Zones 4–7 | R-25 to R-30 |
| Zone 8 | R-25 to R-30 |
R-Values of Common Insulation Materials
| Insulation Type | R-Value per Inch |
|---|---|
| Fiberglass batts | R-2.9 to R-3.8 |
| Blown-in fiberglass | R-2.2 to R-2.7 |
| Blown-in cellulose | R-3.2 to R-3.8 |
| Spray foam (open-cell) | R-3.5 to R-3.6 |
| Spray foam (closed-cell) | R-6.0 to R-6.5 |
| Rigid foam (EPS) | R-3.6 to R-4.2 |
| Rigid foam (XPS, blue/pink board) | R-5.0 |
| Rigid foam (polyiso) | R-6.0 to R-6.5 |
| Mineral wool batts | R-3.7 to R-4.2 |
| Structural insulated panels | R-3.8 to R-8.0 |
Calculating Thickness Needed
Required Thickness = Target R-Value ÷ R-Value per Inch
Example: Achieve R-49 in attic using blown-in cellulose (R-3.5/inch):
- Thickness = 49 ÷ 3.5 = 14 inches
Example: Achieve R-49 using blown-in fiberglass (R-2.5/inch):
- Thickness = 49 ÷ 2.5 = 19.6 inches
Adding to Existing Insulation
If you have existing insulation, you only need to add the difference:
Additional R-Needed = Target R-Value − Existing R-Value
How to estimate existing R-value:
- Measure depth of existing insulation in inches
- Identify the type (batts, blown-in, etc.)
- Multiply depth × R-per-inch from table above
Example: 4 inches of existing blown cellulose (R-3.5/inch), target R-49:
- Existing R = 4 × 3.5 = R-14
- Additional needed: R-49 − R-14 = R-35
- With cellulose (R-3.5/inch): add 35 ÷ 3.5 = 10 more inches
Calculating Insulation Quantity (Bags/Rolls)
Blown-In Insulation
Check the coverage chart on the bag — each bag covers a specific area at a specific depth. Generally:
Bags = (Area sq ft × Target Depth) ÷ Coverage per Bag
Example: 1,200 sq ft attic, need 14 inches of cellulose:
- If each bag covers 40 sq ft at 1 inch depth → bags per inch = 1,200 ÷ 40 = 30 bags per inch
- Total = 30 × 14 = 420 bags
(Actual bag coverage varies by product — always use the manufacturer's coverage chart)
Batt Insulation (Rolls/Batts)
Rolls = Ceiling Area ÷ Coverage per Roll
Standard R-38 batt rolls typically cover 40–50 sq ft each. Check the package.
Energy Savings from Adding Insulation
Adding insulation in an under-insulated attic typically yields:
- 15–25% reduction in heating/cooling costs
- Simple payback period of 1–5 years depending on energy costs and climate
The attic is the highest-priority area because heat rises — inadequate attic insulation is the single biggest source of energy loss in most homes.
Use our electricity usage calculator to model current vs projected energy costs after insulating to estimate your payback period.