Building Science Fundamentals and Heat Transfer
Building Science Fundamentals and Heat Transfer
This domain tests how heat, air, and moisture move through a house and whether you can turn those principles into defensible numbers. Expect questions on conduction, convection, radiation, R-value versus U-factor, thermal bridging, pressure drivers, and dew point.
Core rules
- Conduction is calculated as Q = U × A × ΔT. U = 1/R. Lower U is better; higher R is better.
- Cavity R-value is not assembly R-value. Framing, headers, plates, and corners conduct heat around insulation. Use area-weighted U-factors.
- Air leakage moves sensible heat and water vapor. In most houses, air transport carries far more moisture than vapor diffusion.
- Pressure drives leakage: stack effect, wind, and mechanical fans. The pressure boundary should align with the thermal boundary.
- Moisture moves by air, diffusion, capillary action, and gravity. Condensation occurs on surfaces below the dew point.
Worked example
A 100 ft² wall has 25% framing at R-4 and 75% cavity at R-13, with ΔT = 30°F. Area-weighted U = 0.75/13 + 0.25/4 = 0.0577 + 0.0625 = 0.1202. Assembly R = 1/0.1202 = 8.3. Heat loss = U × A × ΔT = 0.1202 × 100 × 30 = 361 BTU/h. Using cavity R only gives 231 BTU/h and underestimates conduction by about 36%.
Traps
- Reversing R and U, or averaging R-values instead of U-factors.
- Using nominal batt R-value on a compressed or gapped installation.
- Treating air sealing and insulation as interchangeable; seal first, then insulate.
- Ignoring thermal bridging at rim joists, slab edges, and penetrations.
- Forgetting that air transport dominates many moisture problems.