What is Thermal Resistance & U-Value?
Physical Model & Key Equations
The total thermal resistance of a multi-layer wall is the sum of the conductive resistance of each solid layer and the convective resistances of the air films on both sides.
$$R_{total}= R_{conv,in}+ \sum_{i=1}^{n}R_{cond,i}+ R_{conv,out}= \frac{1}{h_{in}}+ \sum_{i=1}^{n}\frac{t_i}{k_i}+ \frac{1}{h_{out}}$$Where $R_{total}$ is the total thermal resistance [$m^2K/W$], $h$ is the convection heat transfer coefficient [$W/m^2K$], $t_i$ is the thickness of layer $i$ [m], and $k_i$ is its thermal conductivity [$W/mK$].
The overall heat transfer coefficient (U-value) is the inverse of the total resistance. The steady-state heat flux and interface temperatures are then calculated from it.
$$U = \frac{1}{R_{total}}, \quad q = U (T_{in}- T_{out}), \quad T_{j}= T_{in}- q \left( \frac{1}{h_{in}}+ \sum_{i=1}^{j-1}\frac{t_i}{k_i} \right)$$Where $U$ is the U-value [$W/m^2K$], $q$ is the heat flux [$W/m^2$], and $T_j$ is the temperature at the interface after layer $j-1$. This shows how temperature drops linearly through each resistive layer.
Frequently Asked Questions
Real-World Applications
Building Energy Code Compliance: Architects and engineers use U-value calculations to ensure wall assemblies meet strict building energy codes. For instance, designing an exterior wall with brick, insulation, and drywall requires proving the composite U-value is below a legal limit, which is easily verified with a tool like this simulator.
HVAC System Sizing: Heating, Ventilation, and Air Conditioning (HVAC) engineers need to know the total heat loss through a building's envelope to correctly size boilers, chillers, and air handlers. An accurate multi-layer U-value calculation is the first critical step in determining the building's peak heating and cooling load.
Condensation & Mold Risk Assessment: By calculating the temperature at each interface within a wall, engineers can predict if and where moisture in the warm indoor air will condense inside the wall. This is vital for preventing structural damage and mold growth, especially in cold climates.
Retrofit & Insulation Planning: When upgrading an old building, you need to know how much new insulation to add. This simulator lets you model the existing wall, then add a virtual insulation layer to see its impact on the U-value and interface temperatures, helping to choose the most cost-effective retrofit strategy.
Common Misconceptions and Points to Note
When you start using this simulator, there are a few points beginners often stumble on. First is that thermal conductivity values are not absolute. For example, the preset "Glass Wool" value is a representative one, but actual products vary based on density and moisture content. Always check the manufacturer's catalog values for design. The second point is the misconception that "a lower U-value means everything is OK". While the U-value is certainly important, it does not account for the overall heat capacity (thermal mass) of the wall. Effects like concrete storing cool night air in summer cannot be evaluated with this simple steady-state calculation. The third point is overlooking contact thermal resistance at interfaces. The simulator assumes layers are perfectly bonded, but in reality, gaps or air layers can make heat flow more easily than assumed (worsening the U-value). This is especially critical as on-site construction quality directly impacts performance.