Insulation materials: lambda, R-values and comparisons
Compare the actual product, thickness and installation conditions before selecting insulation. Fiberglass, cellulose, EPS, XPS and spray foam do not have one universal R-value. This reference explains the comparisons and lets you convert a declared conductivity into SI and US resistance.
Reviewed:
Calculator
R-value per inch and material-layer resistance
RSI = d / λ; RUS = RSI × 5.678263; RUS per inch = 0.14422788 / λ
Thickness d is in metres and conductivity λ in W/(m·K). For an illustrative λ = 0.040, one inch gives RSI = 0.635 and US R ≈ 3.61. At 100 mm the same example gives RSI = 2.50 and US R ≈ 14.20. These are calculated examples, not a product rating. A material layer is not the whole wall U-value.
Fiberglass versus cellulose
Fiberglass is a mineral fibre available as batts or loose fill; cellulose is usually a treated plant-fibre loose fill. Compare declared resistance at the installed thickness, coverage, density and settling allowance. Gaps, compression and air leakage can change the assembly performance. A loose-fill product's coverage chart matters more than a generic internet R-value.
EPS versus XPS
Both are polystyrene boards, but their production, moisture behaviour and load-bearing grades differ. Choose by declared λ, water absorption, compressive strength and intended application. XPS is not automatically the best option everywhere, and the density or colour of a board does not establish its thermal performance. Read the exact product's declaration.
Spray foam: open-cell versus closed-cell
Open-cell and closed-cell spray foams are different product classes. Their density, vapour permeability and resistance per thickness differ. Use the declared aged thermal resistance, installed thickness and approved substrate. Check curing, ventilation, fire protection and moisture design with the supplier. A foam R-value alone does not establish an air barrier or vapour-control specification.
Attics and basements: different constraints
An attic comparison should include access, roof ventilation, air sealing and continuity around framing. A basement comparison must start with water entry, drainage, moisture exposure and the required mechanical protection. Fibrous cavity insulation and ground-contact board insulation serve different roles. Match the declared use to the assembly before comparing price or lambda.
Recommended R-values depend on location
There is no worldwide R-value requirement for every wall or attic. Climate, building use, existing insulation and the applicable local rules change the target. The linked DOE table is US guidance from 2021, not a universal requirement or a current European compliance table. Distinguish US R from SI RSI and document the source and date of your project's target.
Brick and other building materials
Brick conductivity depends on density, cavities, mortar and moisture; use the product's design value for the actual masonry system. For a purely illustrative brick layer d = 0.24 m and λ = 0.70, RSI = 0.343 m²·K/W. This is not a recommended value for an unidentified brick. The same d/λ method applies to concrete or aerated concrete, but their source values must be checked separately.
EPS and mineral wool: compare declared lambda
At equal thickness, a lower declared conductivity gives greater layer resistance. For illustrative 100 mm layers, λ = 0.038 gives RSI = 2.632, while λ = 0.040 gives RSI = 2.500. These are arithmetic examples, not universal EPS or mineral-wool ratings. Compare the exact products and the complete system, including fire, moisture, acoustic and fixing requirements.
Fiberglass and mineral wool R-values
Glass wool and stone wool are mineral-fibre insulation classes. Their resistance depends on the declared conductivity and installed thickness, not the name alone. At an illustrative λ = 0.040, 100 mm gives RSI = 2.50 and US R = 14.20; one inch gives US R = 3.61. Use the calculator with the exact product value. Compare density, compression, settling, moisture and the declared acoustic and fire properties separately.
Choose insulation for its application
Compare equal installed thickness and the same units. Facades require a compatible complete system; roofs require continuity, ventilation and air control; floors and basements require the appropriate compressive and moisture performance. A lambda ranking does not identify the best product for every application. Use declared product data and installation instructions before comparing layer R-values.
Thermal glossary and units
Lambda (thermal conductivity) describes a material in W/(m·K). Thickness d describes the layer, in metres for RSI = d/λ. RSI is thermal resistance in m²·K/W; US R uses h·ft²·°F/Btu. Thermal transmittance, also called the heat transfer coefficient, is U in W/(m²·K) or US U-factor in Btu/(h·ft²·°F). For a consistently defined complete resistance, U = 1/R in the same unit system.
Rsi and Rse are surface resistances, not extra material thickness. A thermal bridge is a heat-flow path through framing, fasteners or a junction that bypasses insulation. An air layer needs a suitable resistance model based on its ventilation and orientation; a void is not automatically insulating. BTU is energy, BTU/h power. US k with an inch in its numerator is conductivity in Btu·in/(h·ft²·°F), so R per inch = 1/k. Product-declared resistance, nominal layer resistance and effective assembly resistance describe different quantities.
Thermal formula reference
RSI = d/λ; RUS = RSI × 5.678263; US U-factor = USI / 5.678263; USI = US U-factor × 5.678263; λ = d/RSI; kUS ≈ λ × 6.934813; RUS per inch = 1/kUS; d [in] = target RUS / RUS per inch; Rtotal = Rsi + ΣRlayer + Rse; USI = 1/Rtotal; US U-factor = 1/RUS; Q [W] = USI × A [m²] × ΔT [K]
Use one unit system throughout each equation. USI here means SI U-value, not US U-factor. For example, 100 mm at λ = 0.035 gives RSI = 2.857 and US R ≈ 16.223. Total RSI = 4 gives U = 0.25. A 20 m² element at U = 0.25 and ΔT = 30 K loses 150 W. Surface resistances and assembly limits must match the calculation; framing is not a series layer.
Worked examples — illustrative inputs
| λ [W/(m·K)] | RSI: 100 mm [m²·K/W] | US R: 1 in |
|---|---|---|
| 0.025 | 4.000 | 5.77 |
| 0.035 | 2.857 | 4.12 |
| 0.040 | 2.500 | 3.61 |
| 0.045 | 2.222 | 3.21 |