Insulation R Value
Calculator
Results
- R-value (SI, m²·K/W)
- 4.285714
- U-value (W/m²·K)
- 0.233333
- R-value (imperial, ft²·°F·h/BTU)
- 24.335414
Construction results
| R-value (SI, m²·K/W) | 4.285714 |
| U-value (W/m²·K) | 0.233333 |
| R-value (imperial, ft²·°F·h/BTU) | 24.335414 |
formula-map diagram
- R-value (SI, m²·K/W)
- 4.285714
- U-value (W/m²·K)
- 0.233333
- R-value (imperial, ft²·°F·h/BTU)
- 24.335414
Formula breakdown
Formula
R = thickness (m) ÷ λ; U = 1 ÷ R= 4.2857142857143
Note
Simplified model: results are quantity estimates only. They ignore openings, overlaps, cuts, site conditions and local building codes. Structural figures are not a substitute for a design by a qualified engineer.
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See all →Frequently asked questions
What does R-value actually measure?+
R-value measures a material's resistance to heat flow — higher R-value means better insulating performance, and it directly correlates with thickness for a given material type, since more material in the path of heat transfer means more resistance.
How does the calculator relate insulation thickness to R-value?+
It uses the material's R-value per inch (which varies by insulation type — fiberglass batts are typically around R-3 to R-3.8 per inch, while rigid foam boards can be R-5 to R-8 per inch) and multiplies by the thickness to get total R-value.
Why do different insulation materials need very different thicknesses to reach the same R-value?+
Materials insulate through different mechanisms — trapped air pockets, reflective properties, or foam cell structure — so their R-value per inch varies significantly; achieving R-30 might take about 8-10 inches of fiberglass batt but only 4-5 inches of certain rigid foam boards.
Does stacking two layers of insulation simply add their R-values together?+
Generally yes, for insulation layers installed without significant compression or gaps — total R-value is approximately the sum of each layer's R-value, which is why adding a layer of rigid foam over existing batt insulation is a common way to boost a wall or roof's total performance.
Why might my achieved R-value be lower than the calculated total?+
Compressed insulation loses R-value per inch (fitting a thicker batt into a shallower cavity than it's rated for reduces its effectiveness), and gaps, thermal bridging through studs, and moisture intrusion all reduce real-world performance below the material's rated value under ideal lab conditions.