Featured Developer Sponsor • Zero-Token Protection
Building Science & Thermodynamics
Fourier's Law of Conduction
R-Value & U-Factor Engine
Thermal Conductivity & Building Heat Loss Calculator
Compute conductive heat transfer rates (Watts and BTU/hr), composite assembly thermal resistance ($R_{\text{total}}$), overall heat transfer coefficients ($U$-factor), 24-hour kilowatt-hour losses, and heating fuel costs using Fourier's Law across single materials and multi-layer building envelopes.
50.0°F (27.8°C)
Heat Loss Rate (Q̇)
1,250 BTU/hr
366 Watts continuous loss
Thermal Resistance (R-Value)
R-20.0
RSI 3.52 m²·K/W
Overall Heat Transfer (U-Factor)
U-0.050
0.284 W/(m²·K)
Daily Heating Cost
$1.58 / day
8.79 kWh/day (0.30 Therms)
🌡️ Multi-Layer Building Wall Temperature Gradient Profile
Cross-section showing heat conduction through wall materials (Drywall → Cavity Insulation → OSB Sheathing → Siding). The temperature curve illustrates thermal drop across each layer and identifies the interstitial condensation boundary.
📐 Step-by-Step Thermal Conduction Derivations
Calculating thermal flux metrics...
⚠️ 5 Fatal Traps & Costly Errors in Building Thermal Insulation
1. Framing Thermal Bridging ("The R-19 Batt Myth")
Homeowners often assume a 2x6 wall insulated with R-19 fiberglass batts provides an R-19 wall. In reality, solid wood studs (R-1.25 per inch) comprise 23% to 27% of total wall area. Heat bypasses the fiberglass and conducts directly through the solid lumber framing studs. This "thermal bridging" degrades the effective overall wall performance from R-19 down to only R-13.8 (a 27% efficiency penalty).
2. Inverted Vapor Barrier Placement (Rotting Wall Cavities)
In cold northern climates (ASHRAE Zones 5–8), the interior air is warm and moist; vapor barriers must be placed on the warm interior side of the insulation. Installing polyethylene sheeting on the exterior cold side traps escaping moisture within the stud cavity, saturating wooden OSB sheathing and breeding toxic mold. In hot humid climates (Zone 1–2), the reverse applies.
3. Compressing Fiberglass Batts into Narrow Cavities
Fiberglass insulation derives its thermal resistance from billions of trapped, stagnant microscopic air pockets, NOT the glass fibers themselves. Forcing an R-30 batt (9.5" thick) into a 2x6 stud cavity (5.5" deep) crushes the air pockets, reducing the total R-value to only R-18. Never compress fibrous insulation to fit a undersized framing cavity.
4. Convective Air Leakage vs. Conductive Heat Loss Blindness
Conductive insulation (fiberglass or cellulose) does not stop air movement. A 1-square-inch unsealed gap around an electrical outlet or top plate can allow warm air leakage that carries 100 times more thermal energy and moisture through a wall assembly than conductive diffusion through solid materials. Always air-seal before insulating.
5. Overlooking Air Film Boundary Resistances ($R_{si}$ and $R_{se}$)
On thin, low-resistance assemblies like single-pane glass windows or uninsulated sheet metal doors, the stagnant thin air film adhering to the indoor surface ($R_{si} approx 0.68$) and outdoor wind film ($R_{se} approx 0.17$) contribute over 70% of the assembly's total thermal resistance. High winds stripping the exterior air film cause dramatic spikes in heating loss.
Frequently Asked Questions
What is Fourier's Law of Thermal Conduction?
What is the relationship between R-value and U-factor?
Why does thermal bridging reduce effective wall R-value by 25%?
How does continuous exterior insulation eliminate thermal bridging?
What is the difference between sensible heat conduction and convective air leakage?
Sponsored Utility
While You're Here
Sponsored Recommendations
Advertisement