Insulation materials: R-values and choosing an application
Compare documented resistance at the intended thickness and conditions, then check the product's permitted application. A generic material name cannot determine its lambda, R-value, moisture suitability or installation requirements.
Fiberglass versus mineral wool
Fiberglass and rock or slag wool are fibrous insulation families. Compare the exact product's thermal resistance, declared thickness, density, installation method and relevant fire, moisture and acoustic declarations. Compression, gaps and framing affect installed performance. The word wool alone is not a thermal rating; avoid assigning every product the same R per inch.
Polyiso versus XPS
Polyisocyanurate and XPS are different rigid-foam product families. Check resistance at the specified thickness and relevant service temperature, ageing basis, facers, compressive strength and water exposure. A high advertised R per inch does not establish suitability for below-grade use or an entire roof system. Obtain the current declaration for the exact board and its intended market before comparing cost per achieved resistance.
Choose by the assembly
For attics, identify the insulated boundary and check ventilation, air sealing and continuity. For basements and floors, resolve water exposure, drainage, mechanical loading and required protection first. For framed walls and roofs, account for repeating bridges and continuous insulation. There is no single best insulation for every application; compare permitted uses and documented performance under the same conditions.
Building-material R-values and units
RSI = d [m] / λ [W/(m·K)]; RUS = 5.678263341 × RSI
At illustrative lambda 0.040, one inch gives RSI 0.635 and US R about 3.606. A 100 mm layer of the same hypothetical material gives RSI 2.5. A conductivity table is not an assembly-R table: thickness, surface resistance, moisture and parallel heat-flow paths change the quantity being compared. U is the reciprocal of the matching complete resistance, with consistent units.