building façade

The following article aims to provide a basic understanding of the heat transfer mechanisms in building façade system and how the choice of material, design and construction may affect the energy consumption, M&E capital investment and indoor environmental quality of buildings.

Understanding Heat Transfer Through Building Facade System
Heat flows through building façade system by three mechanisms:

• Conduction, the molecule-to-molecule transfer of kinetic energy (one molecule becomes energized and, in turn, energizes adjacent molecules).
• Convection, the transfer of heat by physically moving the molecules from one place to another.
• Radiation, the transfer of heat through space via electromagnetic waves (radiant energy).

In Singapore, due to our geographical position and climatic conditions, our buildings absorb the most heat through glazing during the day. This can result in significant cooling energy requirement. The wall system (thermal storage) stores considerable amount of heat in the day and releases the heat indoors through the night. This time lag effect in thermal mass is usually addressed by pre-cooling or air-purging in the next morning, resulting in higher electrical energy consumption.

Regulations Concerning Building Façade System
The following criteria for the thermal performance of building façade system are applicable in Singapore, under the Building Control Regulations:

a. Envelope Thermal Transfer Value (ETTV) for air-conditioned buildings
The envelope thermal transfer value (ETTV) of the building, as determined in accordance with the formula set out in the "Guidelines on envelope thermal transfer value for buildings" issued by the Commissioner of Building Control, shall not exceed 50 W/m2.

b. Roof Thermal Transfer Value (RTTV) for air-conditioned buildings
In respect of roofs with skylight, the roof thermal transfer value (RTTV) as determined in accordance with the formula set out in the "Guidelines on envelope thermal transfer value for buildings" issued by the Commissioner of Building Control, shall not exceed 50 W/m2.

c. Maximum thermal transmittance for roof
In respect of roofs without skylight, the average thermal transmittance (U-value) for the gross area of the roof shall not exceed the limit prescribed in Table 1 below for the corresponding weight group.

Table 1 Maximum thermal transmittance for roofs of air-conditioned and non air-conditioned buildings, with floor area exceeding 500 m2.

Thermal Performance of Façade Components
Higher thermal performance of a building's façade system will result in lower heat gain by the building and therefore lower load on the air-conditioning system. As the air-conditioning system accounts for a significant portion of a typical building's electricity consumption, the thermal performance of a building's façade system will have a direct impact on its energy consumption.

In order to design buildings that can provide comfort with less air-conditioning energy use, it is important to understand the properties of building materials that are intended for use. Different building materials have different thermal conductivities and specific heat capacities, and in turn influences their thermal storage. However, reliable and accurate information on the "coolness" of construction materials is often lacking.

Opaque wall

The conventional school of thought believes that high-mass building materials can offer significant energy benefits in exterior walls. The benefit envisaged is primarily in the shifting of peak load conditions or in an actual reduction in overall heat gain or loss. These benefits are highly dependent upon where the building is located, how it is designed, and how it is operated.

In terms of energy performance, high-mass building materials are still very much open for debate. Some studies have shown that significant heat transfer only occur in the first 300 mm of conventional building materials. In the areas of building thermal engineering and building physics (thermodynamics), more recent innovations such as dynamic insulation (porous cooling or heating) and phase change material (PCM) have defiled the conventional wisdom. Improved wall thermal insulation can help to reduce the U-value significantly. The thermal admittance and time lag in thermal mass (storage) are important considerations in the evaluation of heat transfer mechanism.

Roofing

Most common roofing materials absorb solar radiation, reflecting only a small portion of the incident energy. Dark roofs can reach temperatures of at least 60°C on sunny days. Such high temperatures lead to significant heat conduction into the building through the roof insulation.

Roof temperatures are determined by the heat balance at the outer roof surface. High solar reflectance is desirable and the aim is to enhance heat transfer from the hot roof to the environment, which occurs by thermal emission of infrared radiation and by heat convection. At least half of the energy content of sunlight is in the invisible infrared and ultraviolet portions of the spectrum. Bare metal surfaces and aluminum-pigmented coatings are known to have lower emittances, which also vary with wavelength in the thermal infrared range.

Solar reflectance values can be found in the literature for building materials, but the available information is quite limited to date.

Glazing system

This is an area where research and development are fervently supported by industrial patronage. The building performances of glazing system are continuously improved as more new materials and systems are invented. A list of glazing with high thermal performance is available here.

Assessing Thermal Performance of Façade system
There are a number of parameters to be considered in the thermal performance assessment of façade system. In particular, the U-value, shading coefficient of glazing, and cooling index of glazing are perhaps the most important indicators towards assessing façade system in tropical hot-and-humid climate for energy performance. Other innovative design approaches such as thermal break can help in the reduction of heat transfer into indoors.

U-Value

Refers to the amount of heat transmitted by the building material. The lower the number, the more efficient the façade system in reducing cooling cost.

Window is the weakest element that admits the most heat transfer into buildings through the mechanism of radiation, which is determined by the shading coefficient.
The ETTV and RTTV used in Singapore comprise U-values for the opaque wall and roof respectively, as well as shading coefficient of windows.

Convective heat transfer can be almost eliminated through use of glazing system that has inert gas filling. Conduction heat gain into building can be reduced significantly by a low U-value in opaque wall and glazing.

R-Value

Designates a building material's resistance to heat flow. The number is the inverse of the U-Value, the higher the number, the more efficient the façade system in reducing cooling costs.

Shading Coefficient of glazing

The ratio of solar heat gain through a glazing to the solar heat gain through a single 1/8" glass. The smaller the number, the better the glazing is at preventing solar gain. The shading coefficient is specific to normal incidence angle and is being phased out in favor of solar heat gain coefficient. The lower the number, the more efficient the window is at reducing cooling costs.

Solar Heat Gain Coefficient of glazing

This is the fraction of incident solar radiation entering the building through the windows. The lower the number, the better the window is at preventing solar gain - critical at reducing cooling costs.

Cooling Index of glazing

This is the Visible Light Transmittance divided by the Shading Coefficient. It relates to the ability of glass to let light in but keep solar heat out, which helps reduce energy costs for both air conditioning and lighting.

Visible Light Transmission of glazing

For glazing that allows 30 to 45 per cent of visible light transmission, more artificial lighting is usually required. For glazing that allows approximately 50 per cent of visible light transmission, it is a good balance between artificial lighting and natural lighting. For glazing that exceeds 60 per cent of visible light transmission, more air-conditioning is necessary.

Ultraviolet Light Transmission of glazing

The amount of ultraviolet light allowed to pass through the glazing.

Conclusion
Proper selection of façade materials will lead to significantly lower cost in building energy consumption for both lighting and air-conditioning while maintaining the thermal comfort and visual comfort requirements of the space.