Loading…
Calculate heat transfer rate through walls, roofs, and insulation using Fourier Law of conduction. Find thermal resistance R-value, U-value, and heat flux for any building material or industrial insulation application.
Heat transfer by conduction through a flat wall: Q = kA(T1-T2)/L. The thermal resistance R = L/k is the building insulation industry standard. Higher R means lower heat transfer. Total R for multiple layers adds arithmetically: R_total = R1 + R2 + R3 + surface resistances.
Heat transfer occurs through three mechanisms: conduction (through solids, Q = kA dT/L), convection (fluid movement, Q = hA dT), and radiation (electromagnetic waves, Q = εσA(T14-T24)). In building thermal design, conduction through the wall assembly is typically the dominant mechanism, with surface convection resistances added as boundary conditions.
| Material | k (W/m·K) | Notes |
|---|---|---|
| Still air | 0.025 | Excellent insulator if sealed |
| Glass wool / mineral wool | 0.032-0.045 | Common building insulation |
| Expanded polystyrene (EPS) | 0.033-0.040 | Rigid foam board |
| Polyisocyanurate (PIR) | 0.022-0.028 | Best common rigid insulation |
| Plywood / OSB | 0.12-0.13 | Structural sheathing |
| Brick (common) | 0.60-0.80 | Masonry wall |
| Concrete (normal weight) | 1.4-1.7 | Structural concrete |
| Glass (window) | 0.96-1.05 | Single pane window |
| Steel | 45-50 | Thermal bridge element |
| Aluminum | 200-210 | Major thermal bridge |
| Region | Wall R-value | Attic R-value | Floor R-value |
|---|---|---|---|
| Climate Zone 1-2 (hot) | R-13 wall | R-30 attic | R-15 floor |
| Climate Zone 3-4 (mixed) | R-20 wall | R-38 attic | R-19 floor |
| Climate Zone 5-6 (cold) | R-20+5 wall | R-49 attic | R-30 floor |
| Climate Zone 7-8 (very cold) | R-20+10 wall | R-60 attic | R-38 floor |
| Passive House standard | R-40+ wall | R-60+ roof | R-40+ floor |
Thermal bridges are conductive paths that bypass insulation. Steel studs in a wall have k=50 W/m·K vs glass wool k=0.04 W/m·K — 1250x more conductive. Even though studs are small, they can reduce effective R-value by 30-50% in a steel-stud wall. Solutions: continuous insulation outside the structure (breaks thermal bridge path), thermally broken framing, or structural insulated panels (SIPs). Thermal imaging cameras reveal bridge locations in completed buildings.
Engineering design always includes safety factors to account for material variability, load uncertainty, construction tolerances, and progressive deterioration. Typical safety factors: structural steel bending 1.67 (ASD), concrete compression 1.5-2.0, bolts in tension 2.0-3.0, pressure vessels 3.5-4.0. Always verify that your design complies with applicable local codes, national standards, and industry regulations before construction or installation.
Professional engineering calculations should always be documented with: the calculation title and date, applicable codes and standards, all assumptions clearly stated, all input values with sources, step-by-step calculation showing intermediate results, final answer with units, comparison to allowable limits, and conclusion (pass or fail). This documentation protects the engineer professionally and allows others to check, correct, or update the calculation as conditions change.