Thermal Calculator Glossary
Thermal Calculator Glossary
Plain-language and technical definitions for lambda, conductivity, resistance, R-value, U-value, U-factor, BTU, and thermal bridges.
Reference
Glossary for insulation, R-value, U-value, and lambda
Use this glossary when a calculator label is unfamiliar. Each term includes a plain-language definition, a technical definition, units where they matter, an example, and a related calculator link so the definition can be tested immediately.
Typical values are educational planning references. Always check local code, product documentation, and project-specific constraints.
Glossary
Each term combines a plain definition, technical definition, example, and link.
| Term | Plain definition | Technical definition | Units and example | Link |
|---|---|---|---|---|
| lambda value | how easily a material conducts heat | thermal conductivity | W/mK; lower is usually better | Lambda table |
| thermal conductivity | conductivity of the material | property described by lambda | W/mK; EPS around 0.036 | Materials |
| thermal resistance | resistance to heat flow | resistance of a layer or assembly | m²K/W or R-value | R-value |
| R-value | US measure of resistance | h·ft²·°F/Btu for a layer or assembly | R-19 as common insulation reference | R-value |
| RSI | metric resistance | R in m²K/W | RSI 3.0 gives U 0.333 | U to R |
| U-value | heat crossing an assembly | SI thermal transmittance | W/m²K; lower is better | Wall U |
| U-factor | US thermal transmittance | Btu/(h·ft²·°F) | window 0.30 equals about R-3.33 | Window |
| BTU | US energy unit | British thermal unit | used in U-factor and heat loss | Formulas |
| W/mK | lambda unit | watt per metre and kelvin | 0.035 W/mK for insulation | Lambda |
| W/m²K | U-value unit | watt per square metre and kelvin | 0.25 W/m²K for an assembly | U-value |
| thermal bridge | path bypassing insulation | local increase in heat transfer | studs and fasteners reduce performance | Layers |
| insulation thickness | depth of insulation layer | variable d in R = d/lambda | 100 mm at lambda 0.035 gives RSI 2.86 | Thickness |
| heat transfer coefficient | another name for transmittance | U-value or U-factor depending on units | lower means less loss | R vs U |
| surface resistance | resistance at the surface | Rsi and Rse added to an assembly | small but relevant additions | Layers |
| air layer | controlled air space | resistance depends on orientation and air movement | not every void helps | Layers |
| thermal transmittance | heat transfer through an assembly | technical name for U | W/m²K or U-factor | Formulas |
Glossary for insulation, R-value, U-value, and lambda
Example: thermal conductivity describes the material; thermal resistance describes a thickness; U-value describes the assembly.
Typical values are educational planning references. Always check local code, product documentation, and project-specific constraints.
Practical note
How to interpret the calculator result
Glossary for insulation, R-value, U-value, and lambda is intended for quick option checks and technical discussion before detailed execution. The result depends on the selected units, declared material values, and chosen surface resistances, so each change in layer or thickness should be treated as a separate variant.
The calculator does not automatically verify every local rule, thermal bridge, moisture condition, structural connection, or installation tolerance. If the result is close to a requirement, treat it as a reason for deeper verification rather than a final decision.
For better comparisons, test several realistic thicknesses, check current product data sheets, and review the complete assembly. A calculated value is most useful when the assumptions are clear: material, thickness, layer order, units, and data source.
For insulation or U-value tools, layer order and correct units are especially important. For concrete, electrical, plumbing, or heating tools, the result should be read as a quick quantity or plausibility check before standards and execution conditions are reviewed.
Save the result with the date, material name, and assumptions. If the product, diameter, cable section, or thickness changes later, do not compare the numbers alone without checking which inputs changed.
For calculator pages, clear separation between inputs and result is essential. If a value looks surprising, check units and default fields first, then review the project assumptions.
Assembly thickness
Assembly thickness
Use the tables and formulas to choose a sensible starting thickness before checking the exact assembly.
U-value
U-value
Keep R-value, U-factor, U-value, lambda, k-value, and unit systems separate before comparing results.
CategoryLambdaLayerMaterialThickness
How it works
Use with care
Definitions are intentionally practical and aligned with the site calculators.
Next step
Use the linked pages when a term needs a numerical example.
Thermal Calculator Glossary
Most terms connect through R = d / lambda and U = 1 / R_total.
Use this glossary when a calculator label is unfamiliar. Each term includes a plain-language definition, a technical definition, units where they matter, an example, and a related calculator link so the definition can be tested immediately. Example: thermal conductivity describes the material; thermal resistance describes a thickness; U-value describes the assembly.
Add layerRestore default setupRemove
Assembly nameThickness unitInternal Rsi (m2K/W)External Rse (m2K/W)
Calculation assumptions
The calculators use visible formulas and explicit unit conversions. Treat the result as a preliminary check, not a complete building design.
- SI and US units are converted separately; R, RSI, U-value, and U-factor are not mixed without the unit factor.
- Enter positive values and compare the result with the selected product datasheet.
- Local codes, thermal bridges, fasteners, and installation quality can change the requirement.
- Last formula review: 2026-04-27.
See how formulas, unit conversions, rounding, and limitations are handled. Methodology details.
Reference FAQ
No. They are educational references for planning, comparison, and calculator input checks.
Use typical values only for early estimates. Product labels, declarations, and local rules should override them.
R-value and U-factor are common in US practice, while lambda and U-value are common in SI reporting. The pages keep unit systems explicit.
Yes. You can print the result or export it to CSV, Excel, or PDF for reports and documentation.
Yes. It is designed for layered assemblies such as external walls, flat roofs, pitched roofs, floors, and slabs. For unusual assemblies, add every relevant layer and treat the result as a planning check before formal verification.
Yes. It is intended for fast concept-stage calculations, insulation comparison, and envelope optimisation before detailed design. It is best used to narrow choices, not to replace a code check or project-specific thermal bridge assessment.
Yes. You can switch between millimeters, centimeters, and inches, and the calculator keeps the values consistent. For fewer mistakes, choose one unit system at the start of a project and review converted thicknesses before export.
Use Glossary for insulation, R-value, U-value, and lambda as a first-pass reference. Before specifying anything, compare the result with the project note, actual project dimensions, product data sheet, and local requirements.