Overall Heat Transfer Coefficient and Example for a Roof
The overall heat transfer coefficient (U-value) is a measure of a material’s ability to conduct heat. It indicates the rate at which heat flows through a specific material, from the warmer side to the cooler side. The U-value of a building envelope is an essential factor that determines its energy efficiency. The lower the U-value, the more thermally efficient the building, which results in lower heating and cooling costs.
Roof is one of the most crucial components of a building envelope that contributes to heat transfer. The U-value of a roof assembly depends on several factors, including the type and thickness of insulation, the type of roof membrane, and the presence of ventilation gaps. The overall U-value for a roof assembly can be calculated by adding the thermal resistance (R-value) of each component in the assembly and dividing it by the total thickness of the assembly.
In India, the Energy Conservation Building Code (ECBC) specifies the maximum allowable U-values for various types of roofs, depending on the climate zone and the type of building. For example, Table 4-4 through Table 4-6 of ECBC provides the maximum allowable U-values for roofs in composite hot and dry, warm and humid, temperate, and cold climates, for different types of buildings.
Let’s take an example of calculating the overall U-value for a flat roof in a composite hot and dry climate zone for a business building, complying with ECBC. The roof assembly consists of a 10 cm thick concrete slab with a density of 2400 kg/m³, 5 cm thick extruded polystyrene (XPS) insulation with a density of 32 kg/m³, and a 1.5 mm thick bituminous membrane. The concrete slab has a thermal conductivity of 1.8 W/m.K, while XPS insulation has a thermal conductivity of 0.033 W/m.K, and the bituminous membrane has a thermal conductivity of 0.17 W/m.K.
To calculate the overall U-value, we need to determine the thermal resistance (R-value) of each component in the assembly. The R-value of the concrete slab can be calculated as follows:
R_concrete = thickness/conductivity = 0.1/1.8 = 0.056 m².K/W
Similarly, the R-value of XPS insulation and bituminous membrane can be calculated as:
R_XPS = thickness/conductivity = 0.05/0.033 = 1.515 m².K/W R_membrane = thickness/conductivity = 0.0015/0.17 = 0.0088 m².K/W
The total thermal resistance (R_total) of the roof assembly is the sum of R-values of all components:
R_total = R_concrete + R_XPS + R_membrane = 1.5798 m².K/W
Finally, the overall U-value (U_total) can be calculated by dividing the thickness of the assembly by the total thermal resistance:
U_total = 1/R_total = 0.633 W/m².K
Therefore, the calculated overall U-value for this roof assembly is 0.633 W/m².K, which complies with the maximum allowable U-value of 0.20 W/m².K for business buildings in the composite hot and dry climate zone, as per Table 4-5 of ECBC.
The Energy Conservation Building Code (ECBC) is a code for building energy efficiency in India. The code specifies the maximum permissible values of the overall heat transfer coefficient, or U-factor, for different building elements, including roofs. The U-factor measures the rate of heat transfer through a roof assembly and is expressed in units of W/m2.K. The lower the U-factor, the better the insulation of the roof assembly.
The ECBC specifies different U-factor requirements for different types of buildings and climatic zones. Table 4-4, Table 4-5, and Table 4-6 in the ECBC specify the U-factor requirements for roofs in ECBC Compliant Buildings, ECBC+ Compliant Buildings, and SuperECBC Buildings, respectively.
For ECBC Compliant Buildings, the maximum U-factor for roofs is specified in Table 4-4, as shown below:
| Composite | Hot and dry | Warm and humid | Temperate | Cold |
|---|---|---|---|---|
| All building types, except School <10,000 m2 | 0.33 | 0.33 | 0.33 | 0.33 |
| School <10,000 m2 | AGA 0.47 | AGA 0.47 | AGA 0.47 | AGA 0.47 |
| Hospitality > 10,000 m2 AGA | 0.20 | 0.20 | 0.20 | 0.20 |
For ECBC+ Compliant Buildings, the maximum U-factor for roofs is specified in Table 4-5, as shown below:
| Composite | Hot and dry | Warm and humid | Temperate | Cold |
|---|---|---|---|---|
| All building types, except School <10,000 m2 | 0.28 | 0.28 | 0.28 | 0.28 |
| School <10,000 m2 | AGA 0.37 | AGA 0.37 | AGA 0.37 | AGA 0.37 |
| Hospitality > 10,000 m2 AGA | 0.18 | 0.18 | 0.18 | 0.18 |
For SuperECBC Buildings, the maximum U-factor for roofs is specified in Table 4-6, as shown below:
| Composite | Hot and dry | Warm and humid | Temperate | Cold |
|---|---|---|---|---|
| All building types, except School <10,000 m2 | 0.23 | 0.23 | 0.23 | 0.23 |
| School <10,000 m2 | AGA 0.29 | AGA 0.29 | AGA 0.29 | AGA 0.29 |
| Hospitality, Healthcare, Assembly | 0.15 | 0.15 | 0.15 | 0.15 |
| Business, Educational, Shopping Complex | 0.18 | 0.18 | 0.18 | 0.18 |
It is important to note that all roofs that are not covered by solar photovoltaics, or solar hot water, or any other renewable energy system, or utilities and services that render it unsuitable for the purpose, shall be either cool roofs or vegetated roofs as per Section 4.3.1.1 of the ECBC. For qualifying as a cool roof, roofs with slopes less than 20° shall have an initial solar reflect