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Insulation R-Value Calculator: Layers, RSI and Heat Loss
Add material layers with editable R per inch or thermal conductivity. Calculate total R and RSI, U-value and heat loss, with metric and imperial measurements and clearly scoped comparisons.
Insulation R-Value Calculator: Layers, RSI and Heat Loss: Sums layer resistances in series. For each layer, R is thickness in inches times R per inch, or RSI is thickness in metres divided by thermal conductivity. One US R equals 0.1761101838 RSI. U is the reciprocal of total RSI; heat loss is U times area times temperature difference. Use the [drywall calculator](/drywall-calculator) to plan sheets after measuring the wall. Runs 100% locally in your browser with zero server file uploads.
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Typical R per inch is editable. Switching to λ converts the same thermal property. Replace these examples with the product's declared value.
Formula: layer R = thickness in inches × R per inch; layer RSI = thickness in metres ÷ λ. RSI = sum of layers + surface RSI; 1 US R = 0.1761101838 RSI; U = 1 ÷ RSI; heat loss = U × area × ΔT.
Material RSI: 2.219. Total R including surface resistance: 12.6. Area 20 m² / 215.28 ft²; ΔT 20 K / 36 °F.
DOE uninsulated attic ceiling recommendation: R-60. Additional material R to reach this: 47.4. This attic guidance does not apply to every wall or floor.
Part L new-dwelling limiting U: 0.26 W/m²·K; calculated U: 0.451. Difference from limit: 0.191 W/m²·K (positive means above limit). This is an illustrative comparison, not a compliance determination.
Assumes uniform layers in series and steady heat flow. Excludes framing bridges, air leakage, moisture and compression. Surface resistance starts at zero; a complete wall or roof calculation needs appropriate surface films and all layers. Floor U-values also depend on ground geometry.
| Material | R per inch | Derived λ (W/m·K) |
|---|---|---|
| Fibreglass batt | 3.2 | 0.0451 |
| Loose cellulose | 3.7 | 0.039 |
| Mineral wool | 3.1 | 0.0465 |
| Expanded polystyrene | 4 | 0.0361 |
| Extruded polystyrene | 5 | 0.0288 |
| Polyurethane | 6 | 0.024 |
Material table: University of Maryland Extension
US DOE climate-zone recommendations
Part L here means England, Volume 1, 2021 edition with 2023 amendments, Table 4.1: new-dwelling limiting values, not tighter notional design targets. The 2026 edition takes effect in March 2027, or September 2027 for higher-risk work, subject to transition rules. Other UK nations have their own requirements. Part L editions and scope
Typical material values
University of Maryland Extension publishes typical US R per inch: fibreglass batt 3.2, cellulose 3.7, mineral wool 3.1, expanded polystyrene 4.0–4.5, extruded polystyrene 5.0–5.5 and polyurethane 6.0–7.5. The tool uses the lower value of each stated range; derived conductivity is 0.0254 divided by (R per inch times 0.1761101838). Replace typical values with product data. Source: https://www.extension.umd.edu/resource/home-energy-insulation
Climate and regulatory comparisons
US DOE attic ceiling guidance for uninsulated attics: zone 1 R30, zones 2–3 R49, zones 4–8 R60. Source: https://www.energy.gov/cmei/buildings/articles/energy-efficient-home-improvement-credit-insulation-and-air-sealing
England Approved Document L Volume 1, 2021 edition with 2023 amendments, Table 4.1: new-dwelling roof 0.16, wall 0.26 and floor 0.18 W/m²·K limiting values. Other UK nations and existing buildings have different requirements. Source: https://www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l
The 2026 edition takes effect on 24 March 2027 for ordinary work and 24 September 2027 for higher-risk work, subject to transition provisions. It retains those three new-dwelling limits in Table 3.1. Source: https://www.gov.uk/government/publications/approved-document-l-2026
How to use it
- Choose units and add layers with material, thickness and thermal value.
- Enter area, temperature difference and optional surface resistance.
- Read R, RSI, U and heat loss; choose a DOE zone and Part L element for comparison.
Privacy & limitations
Layers and building measurements are calculated in your browser.
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Frequently asked questions
Is this a whole-wall U-value?
Only if the entered layers and surface resistance represent the assembly. Framing bridges, air leakage, moisture and compression are excluded. Ground-floor U-values need geometry and ground heat-transfer methods.
Which recommendations are compared?
The DOE comparison is for a previously uninsulated attic ceiling, using its 2021 IECC guidance. The Part L comparison uses England's new-dwelling limiting U-values from Volume 1, 2021 with 2023 amendments. These are not notional design targets or a compliance certificate.
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