Question:

• What is thermal insulation ?

   

Answer:

• Heat flow transfers from higher to lower temperature bodies. Heat transfer is realised via various forms such as i) thermal conduction which occurs in solid, liquid and gaseous substances; ii) thermal convection which occurs in liquid and gaseous substances; iii) thermal radiation which is independent of medium (for instance, electromagnetic waves consisting of visible rays and infrared rays from the Sun travel through space to the Earth); and iv) the combination of these. Thermal conduction is an independent medium to transport heat flow, so is thermal radiation. Meanwhile, thermal convection is not an independent vehicle to transport heat flow.

   

• The following example illustrates the combined effect of conduction, convection, and radiation. A typical house in North America has its external walls built up from timber or steel frame with its exterior side lined with weather resistant calcium silicate boards (or autoclaved cement boards) and interior side lined with wallboards. An air gap is sandwiched between the inner and outer wall surfaces. Suppose the temperature inside the house is lower than the outdoor temperature. When sun lights strike directly the exterior surface of the walls, due to thermal radiation, infrared rays, always part of sun rays, warm up the exterior face of weather boards. Due to thermal conduction, heat flow transfers horizontally through weather boards and reach the air film thats is adjacent to the inner face of weatherboards, keeps going through the air gap by means of thermal convection. When touching the external side of wallboards, heat flow continues its journey to reach the internal side of wallboards (again, it is thermal conduction). By then, two phenomena simultaneously occur: thermal convection due to the existence of air inside the house and secondary thermal radiation as the walls are previously heated by primary electromagnetic radiation from the Sun, both warm up the indoor air.

   

• Thermal insulation or thermal resistance is the ability of a system to slown down the travelling speed of heat flow. For instance, a thermally insulated external wall will impede the heat flux from transferring horizontally from outdoor to indoor in summer, or from inside to outside in winter; an insulated roof/ceiling system will hamper the heat flow from going vertically from outdoor to indoor in summer, or from inside to outside in winter. The thermal resistance of a particular system is rated by its overall R-value, with m².K/w (under the SI system) or ft².F.h/Btu (under the Imperial system) as its measuring unit. A system with higher overall R-value will have  a better thermal resistance and vice versa. 

   

• Below are recommended overall R-values for roof/ceiling and external wall systems that should be applied for countries in South-East Asian region like Singapore, Malaysia, Thailand, Indonesia, Philippines, Vietnam, Laos, Campuchea, etc. where hot and humid climate prevails all around the year:

   

*   Roof/ceiling system: R = 3.5 m².K/w

*   External wall system, directly exposed to weather: R = 2.0 m².K/w

• Masonry walls (walls made of brick and concrete) generally have very good thermal conductance, thus very poor thermal resistance. A dense clay brick wall of 100mm total thickness and 210kg/m2 mass has an overall R-value » 0.24 m².K/w. Meanwhile, a solid concrete wall of 100mm thick and 240kg/m2 mass has an overall R-value » 0.22 m².K/w. It means if a house is traditionally constructed from 100mm thick masonry external walls of heavy weight, the thermal resistance of these walls will be only 11-12% of the level specified above !!! For 200mm thick heavy masonry walls, their thermal resistance will be better, but merely reach 14.5-16.5% of the required level !!! Per calculation, to attain an overall R-value = 2.0 m².K/w, dense brick wall must have a total thickness of 2.03m whereas a solid concrete wall must be built with 2.66m total thickness !!!

   

• Similarly, houses with metal roof, slight sloped tile roofs or concrete terraces all have very poor thermal resistance. In Vietnam, many people, including architects and civil engineers, misled by a wrong conception, think that tile roofs have good thermal resistance, therefore it is not necessary to insulate tiled roofs with thermal insulation products. Extensive experimental researches in tropical climate countries reveal that about 65-70% of thermal energy radiated by sun rays penetrate into houses via roof/ceiling systems, and about  30-35% via external walls, windows, and doors. Consequently, the overall R-value of a roof/ceiling system must be much higher than that of an external wall system (for instance, 3.5 m².K/w against 2.0 m².K/w).