Building Envelope Heat Gain: Understanding U-Value and How to Reduce Heat Gain

Energy consumption in buildings is a major concern as it contributes to greenhouse gas emissions and affects the environment. One of the ways to reduce energy consumption in buildings is by minimizing heat gain through the building envelope. The building envelope is the physical separator between the interior and exterior environment of a building, which includes the walls, roofs, windows, and doors. In this article, we will discuss U-value and how it plays a crucial role in building heat gain, as well as guidelines for reducing heat gain through buildings.

U-Value: Understanding Thermal Transmittance

U-value, also known as thermal transmittance, is the rate of transfer of heat energy through a structure, divided by the difference in temperature across that structure. The units of measurement are W/m²K. The better insulated a structure or material is, the lower the U-value will be. Thermal transmittance takes heat loss due to conduction, convection, and radiation into account. U-value plays a critical role in building heat gain, which helps in energy simulation of buildings.

The U-value is the reciprocal of the R-value, which can be calculated using the formula U = 1/R. For example, if we consider a single sheet of standard glass with a U-value of 2.0W/m2K, it means that for every degree of temperature difference between the outside and the inside, a square meter of the glazing would lose 2 watts. So, for instance, if the temperature difference on a typical cold day was 15 degrees, then the amount of heat loss would be 15×2 = 30 watts per square meter. Therefore, we can adopt double or triple glazed windows as they have comparatively lower U-value. The U-value of a double and triple glazed window is about 3.5-1.5 and 0.7W/m2K, respectively, which is lower than that of a single glazed window. That means these will save more energy consumption as compared to single glazed glass.

U-Value of Various Building Materials

The U-value of a building element depends on the thermal resistance of the material, which is the ability of the material to resist the transfer of heat. The general formula for calculating the U-value is U = 1/Rt, where Rt is the total thermal resistance of the element composed of layers (m²·K/W). The thermal resistance of each layer can be obtained according to the formula R = D / λ, where D is the material thickness (m), and λ is the thermal conductivity of the material (W/K·m) (according to each material).

ASHRAE standards provide guidelines for U-value of various building materials. For example, the U-value of an uninsulated concrete wall is 1.14W/m2K, while the U-value of a 100mm thick insulation with a thermal conductivity of 0.038W/mK is 0.26W/m2K.

Guidelines for Reducing Heat Gain through Buildings

The reduction of heat gain through buildings could be accomplished by reducing the shading coefficient (SC) and using insulation in combination with walls and roofs. The use of external shading could be performed through various methods, such as employing horizontal overhangs, vertical fins, or combination fins. For example, a 10cm concrete block with 2.5cm polyurethane foam has an annual energy saving of 51.61 kW-hrs/m2 construction U in the