Insulation
Keep insulation coverage separate from thermal performance
A roll count or nominal R-value does not describe the complete installed assembly.
Coverage calculations answer how much material is required. Thermal calculations answer a different question about resistance to heat flow. Keep the two outputs separate so buying enough area is not mistaken for achieving a specified whole-building performance.
Work out roll coverage
For an illustrative 1.2 m × 20 m roll, the gross area is 24 m². If the selected application requires a 0.1 m side lap, a simplified repeating-width calculation gives (1.2 − 0.1) × 20 = 22 m² before end laps and cuts. Do not use a lap on a product that is meant to be installed without one; follow the actual system.
Understand what R-value addition assumes
For uniform layers in series along the same heat-flow path, illustrative resistances of 1.5 and 2.0 m²·K/W sum to 3.5 m²·K/W. This simple sum does not account for framing bridges, gaps or alternative parallel paths. It is not automatically a total wall or roof R-value.
Treat membranes as selected system components
A vapour barrier, vapour-permeable wrap and bulk insulation are not interchangeable. Their location and suitability depend on the assembly and climate. A roll calculator supplies quantity after those decisions; it does not choose the moisture-control design.
Carry the basis into your estimate
Record the product, coverage, lap assumptions and whether an R-value refers to material or assembly. For uncertain products or layouts, leave the compliance conclusion unresolved and obtain the relevant design or manufacturer advice.
Further reference
The worked numbers in this guide are illustrative unless a project record is identified.
Found a measurement or calculation issue? Send the example and your correction.