Insulation R-Value Calculator
Insulation R-Value Calculator
Sum Your Assembly's Total R-Value
Optional: Estimate Heat Loss
Understanding R-Value
R-value measures resistance to heat flow -- higher is better insulation. A wall or ceiling assembly is built from multiple layers (drywall, insulation, sheathing, siding), and their R-values simply add together for the assembly's total.
R-Values Are Additive
Stack any number of layers -- drywall, batt insulation, rigid foam, sheathing -- and add their individual R-values for the total assembly R-value.
Heat Loss Formula
Heat loss (BTU/hr) = area × temperature difference ÷ total R-value -- the standard conductive heat transfer equation.
Where to Find Layer R-Values
Insulation R-values per inch are printed on the product packaging -- fiberglass batts are typically labeled with their total R-value directly (e.g., "R-13").
An Estimate
Real-world heat loss is affected by air leaks, thermal bridging through studs, and moisture -- this calculator models pure conductive loss through the layers only.
Worked Example
Suppose a wall has an R-13 fiberglass batt, R-0.5 drywall, and R-0.5 sheathing, covering 200 sq ft with a 40°F indoor/outdoor difference:
- Total R-value: 13 + 0.5 + 0.5 = R-14.
- Heat loss: (200 × 40) ÷ 14 ≈ 571 BTU/hr.
Result
- Total R-Value: R-14
- Heat Loss: ≈571 BTU/hr
Key Takeaways
- Higher R-Value = Less Heat Loss: Doubling total R-value roughly halves conductive heat loss for the same conditions.
- Check Recommended Levels: Recommended R-values vary by climate zone and by whether you're insulating a wall, attic, or floor.
- Air Sealing Matters Too: Gaps and air leaks can undermine even a high-R-value assembly -- insulation and air sealing work together.