Meaning and mechanism
In normal thermal conduction through a flat wall, the greater the thickness or the smaller the thermal conductivity, the more difficult it is for heat to be transmitted. This is represented as thermal resistance.
For heat passing successively through several layers, add the thermal resistance of each layer. Heat transfer at the indoor and outdoor surfaces can also be included as resistance terms.
Relationships and conditions
The thermal resistance per unit area is d/k, and the rate of heat loss is P=AΔT/R. If the thermal resistance is doubled while maintaining the same temperature difference, the ideal rate of heat loss would be halved.
Actual buildings exhibit windows, thermal bridges, and gaps for ventilation and heat accumulation. However, solely relying on the thermal conductivity of the wall’s 1 dimension to predict the total heating energy consumption is not an accurate method.
What to observe in the simulation
Wall and insulation thermal resistances add in series. At fixed temperature difference, higher resistance reduces heat loss.
Insulation conductivity is 0.04 W/(m·K); the 0.15 m wall is 0.8 W/(m·K). Surface resistances are 0.13 and 0.04 m²K/W. Steady one-dimensional conduction omits windows, bridges, ventilation and heat storage.
Key relationships and symbols
Rtotal = 0.13 + 0.15/0.8 + d/0.04 + 0.04 · P = AΔT/Rtotal
| Symbol | Meaning | Unit |
|---|---|---|
| d | Insulation thickness | m |
| k | Thermal conductivity | W/(m·K) |
| R_total | Total thermal resistance per unit area | m²·K/W |
Worked example
How are wall and insulation resistances combined?
Add them in series · Wall and insulation thermal resistances add in series. At fixed temperature difference, higher resistance reduces heat loss.
Common misconceptions
- Insulation materials do not create heat to warm the interior; instead, they slow down the movement of heat.