Calculator
Example data table
| Scenario | Unit | Target U | k | Fixed R (existing + surfaces) | Safety | Recommended thickness |
|---|---|---|---|---|---|---|
| Mineral wool wall | Metric | 0.30 | 0.037 | 0.67 | 5% | ≈ 120 mm |
| Rigid foam retrofit | Metric | 0.25 | 0.030 | 0.55 | 10% | ≈ 140 mm |
| Attic insulation top-up | Imperial | 0.045 | 0.25 | 7.5 | 5% | ≈ 46 in |
Formula used
- U = 1 / Rtotal
- Rtotal = Rfixed + Rinsulation
- Rinsulation = t / k (metric, t in meters)
- t = Rinsulation × k (imperial, t in inches when k is BTU·in/(h·ft²·°F))
Rfixed includes any existing layer resistances plus inside and outside surface resistances. Adding these makes the thickness estimate more realistic for full assemblies.
Thickness is increased by a safety percentage, then rounded up to your chosen increment to match common product sizes and installation tolerances.
How to use this calculator
- Select Metric or Imperial based on your specification sheet.
- Choose whether you want to design to a target U or target R.
- Enter the material’s thermal conductivity (k) from the certificate or data sheet.
- Add existing R for other layers (boards, cavities, cladding, air films).
- Set surface resistances if you want assembly-level accuracy.
- Apply a safety margin and a practical rounding increment.
- Press Calculate thickness. The result appears above the form, with CSV and PDF download buttons.
For high-stakes design, confirm local code requirements, moisture risk, and thermal bridge impacts. This tool is a fast estimator for the insulation layer thickness requirement.
Performance targets and insulation intent
Insulation thickness is usually selected to meet a target U-value or an overall R-value for a wall, roof, or floor. This calculator converts a performance target into a required insulation layer, while letting you include other layers and surface films. The result supports early-stage design decisions and quick option comparisons.
Assembly inputs that change the answer
A building element is more than insulation alone. Sheathing, masonry, linings, cavities, and air films all add resistance. By entering existing R plus inside and outside surface resistances, you estimate the whole assembly rather than a brochure-only layer. This helps you avoid over-buying insulation when other layers already contribute meaningful resistance.
Material conductivity and specification quality
Thermal conductivity (k) varies with product type, density, temperature, and moisture. For professional specifications, use the declared or design conductivity from the manufacturer certificate and keep units consistent with your selected system. Even a small change in k can shift required thickness on low-U assemblies, so verify the value before issuing drawings.
Safety margins, rounding, and buildability
Real installations include joints, fixings, compression, and tolerances that can reduce effective performance. A modest safety margin helps protect your target when workmanship varies or when insulation ages. Rounding up to a practical increment aligns the output with available product thicknesses, simplifies procurement, and keeps the design conservative without adding unnecessary complexity.
Example data and quick validation
Example (metric): target U = 0.30 W/m2K, k = 0.037 W/mK, existing R plus surfaces = 0.67 m2K/W, safety = 5%, rounding = 5 mm. The required insulation R is about 2.66 m2K/W, giving an initial thickness near 0.103 m. After rounding, a practical recommendation is 105 mm. Use the achieved U-value shown above to confirm compliance after rounding. Document assumptions in notes so reviewers can reproduce your calculations.
FAQs
1) What is the difference between U-value and R-value?
U-value is the overall heat transfer rate; lower is better. R-value is thermal resistance; higher is better. For a complete assembly, they are inverses: U = 1 / R.
2) Which conductivity value should I use for k?
Use the declared or design k from the product certificate, not a marketing figure. Confirm the units match your selection and consider moisture or temperature corrections if your standard requires them.
3) Should I include surface resistances?
Include them when your target U or R is defined for the full assembly with air films. If your target excludes air films, set Rsi and Rse to zero so you do not double-count.
4) Why does rounding change the achieved U-value?
Rounding changes thickness, which changes the insulation resistance. The calculator recomputes achieved total R and achieved U after rounding so you can confirm the practical thickness still meets the target.
5) What safety margin is typical?
Many projects use 3% to 10% depending on installation quality and product type. Increase it for uneven substrates, loose-fill products, or when you are close to a compliance threshold.
6) Does this account for thermal bridges?
It is a one-dimensional layer estimate. Studs, anchors, and junctions can reduce real performance. For bridge-sensitive assemblies, apply a correction factor or use detailed thermal modeling.
7) Can I use this for roofs, floors, and pipes?
Yes, for slab-like insulation layers where heat flow is mostly one-directional. For curved surfaces or insulated pipes, treat the result as a first estimate and verify with a geometry-specific method.