Insulation R-Value Calculator
Calculate total R-value for a wall assembly by adding R-values of individual layers (drywall, insulation, sheathing, exterior finish).
Formula & Explanation
This calculator sums the R-values of individual layers to estimate total assembly R-value. **Total R-Value** = R_drywall + R_insulation + R_sheathing + R_exterior + R_additional In a series assembly (layers stacked), thermal resistances add directly. For example, a typical wall might include: - Interior drywall (5/8"): R-0.56 - Fiberglass batt (3.5"): R-11 to R-13 - Foam sheathing (1"): R-4 to R-6 - Vinyl siding: R-0.82 - **Total nominal: ~R-17 to R-20** **Important caveat**: This is the nominal R-value sum. Actual effective R-value is lower because thermal bridging through wood studs, air gaps, and gaps at edges reduces performance. Whole-wall effective R-values are typically 15-40% lower than the nominal sum.
Assumptions
- •R-values are nominal laboratory values; actual performance varies by product, installation quality, temperature, and humidity
- •Fiberglass batts: R-3.2 per inch (R-11 for 3.5", R-19 for 6", R-21 for 6.5")
- •Mineral wool: R-3.6 per inch
- •Extruded polystyrene (XPS) foam: R-5 per inch
- •Polyisocyanurate foam: R-6 to R-7 per inch (with foil facing)
- •Drywall (5/8"): R-0.56
- •Standard vinyl siding: R-0.82; fiber-cement: R-0.71; wood: R-0.81
- •This calculation IGNORES thermal bridging through studs, convection, and radiant effects, which significantly reduce effective R-value
- •For accurate whole-wall performance, consult ASHRAE standards or use specialized whole-assembly calculation tools
Example Calculation
A wall assembly with 5/8" drywall (R-0.56), R-13 insulation, foam sheathing (R-1), and vinyl siding (R-0.82) has a nominal R-value of 15.38. Actual effective R-value is likely 12-14 due to thermal bridging.
Inputs
Results
Typical R-Values by Material and Thickness
| material | r_value |
|---|---|
| Drywall (5/8") | 0.56 |
| Fiberglass batt (3.5") | 11-13 |
| Fiberglass batt (5.5") | 19-21 |
| Mineral wool batt (3.5") | 13-15 |
| Extruded polystyrene (1") | 5-5.5 |
| Polyisocyanurate (1") | 6-7 |
| Expanded polystyrene (1") | 3.8-4.2 |
| Vinyl siding | 0.82 |
| Fiber-cement siding | 0.71 |
| Wood siding | 0.81 |
Frequently Asked Questions
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