Thermal resistance series calculator

A layered wall resists heat flow through every material between its warm and cold faces. This calculator turns layer thickness and thermal conductivity into resistance per unit area, adds the conductive resistances in series, and reports both the assembly total and its reciprocal U-value. Add brick, insulation, sheathing, plaster, or any other homogeneous layer using consistent SI inputs. An optional wall area also converts area-normalized resistance into absolute K/W. The calculation is deterministic, transparent, and suitable for design checks, teaching, estimating, and repeatable software workflows.

Enter each wall layer with consistent physical data

Create the layers array in the order materials occur through the wall, although changing their order does not change a pure series sum. Conductivity is normally taken from a manufacturer data sheet, a tested product declaration, or an engineering reference appropriate to the material condition. Do not enter an advertised insulation R-value in the conductivity field: conductivity and resistance are different quantities.

Convert millimetres to metres before entry, so 100 mm becomes 0.1 m. You may add a name such as Brick, Mineral wool, or Gypsum board to make the output easier to audit. All thicknesses and conductivities must be positive finite values.

A missing, zero, negative, infinite, or nonnumeric value is rejected because it cannot describe a valid homogeneous conductive layer. The calculator accepts from one through 100 layers, which is ample for ordinary building assemblies while keeping execution bounded. Supply the actual heat-transfer area only when you also need absolute thermal resistance in K/W; the assembly resistance per unit area and U-value do not depend on that area input.

Keep all values in the declared SI units rather than mixing inches, feet, or imperial conductivity data into the same request.

Understand the series calculation and returned results

The result has units of square metre-kelvin per watt and represents conductive thermal resistance per unit area. It then adds every R_layer because the same one-dimensional heat flow passes successively through all layers. A larger resistance therefore produces a smaller U-value and, for the same temperature difference, less heat transfer.

The reported formula documents exactly what was applied, with no lookup tables, hidden material assumptions, weather data, or probabilistic model. This calculation includes only layers explicitly entered. Surface film resistance, ventilated cavities, contact resistance, fasteners, framing bridges, moisture effects, and multidimensional edge losses are not silently added.

If a standard requires interior or exterior surface resistances, include those effects through an appropriate separate method rather than pretending they are homogeneous solid layers with invented conductivity.

Use the result responsibly in design and automation

Use the browser calculator for quick comparisons between candidate wall constructions, classroom examples, or a transparent check against a spreadsheet. For example, increasing insulation thickness increases its layer resistance linearly when conductivity remains constant. Be cautious when using nominal conductivity: temperature, density, aging, and moisture can change real product performance, and regulatory calculations may prescribe declared or design conductivity values.

The simple series model is appropriate for planar, homogeneous layers under one-dimensional steady conduction. It is not a substitute for a parallel-path calculation where timber or steel studs bypass insulation, a two-dimensional thermal-bridge model around balconies and corners, or a transient simulation of thermal mass. Record the source and condition of every conductivity value alongside your project results.

If you need heat flow after calculating resistance, combine the appropriate total resistance with a defined temperature difference and boundary conditions in a separate calculation.