Thermal insulation usually refers to the use of appropriate insulation materials and design adjustments to buildings to slow down the transfer of heat through the envelope, thereby reducing heat loss. Heat transfer is caused by the temperature difference between indoor and outdoor. Heat can be transferred through conduction, convection or radiation. The transmission rate is closely related to the propagation medium. For example, heat is lost or gained through transmission through ceilings, walls, floors, windows and doors. This heat reduction and gain is usually undesirable. It not only increases the load on the HVAC system, leading to more energy waste, but also reduces the thermal comfort of people in the building. Building insulation is an important factor in achieving thermal comfort for occupants. Insulation can reduce unnecessary heat loss or gain, and can reduce the energy demand of heating and cooling systems. It does not necessarily address the issue of adequate ventilation, and may or may not affect the level of sound insulation. In a narrow sense, insulation can only refer to insulation materials used to slow down heat loss, such as: cellulose, glass wool, rock wool, polystyrene, polyurethane foam, vermiculite, perlite, wood fiber, extruded board, plant fiber (hemp, flax, cotton, cork, etc.), recycled cotton denim, plant straw, animal fiber (wool), cement and mud or soil, reflective insulation (also known as radiant barrier), but it may also involve a series of designs and technologies to address the main heat transfer modes of conduction, radiation and convection materials.
Most of the materials in the above list only retain a large amount of air or other gases between the molecules of the material. Gases transfer much less heat than solids. These materials can form air cavities that can be used for insulation, with low heat transfer efficiency. This situation also occurs in animal fur and bird feathers. Animal hair can utilize the low thermal conductivity of small air pockets, thereby achieving the purpose of reducing heat loss.
The effectiveness of reflective insulation (radiant barrier) is usually evaluated by the reflectivity (emissivity) of the surface of the air space facing the heat source.
The effectiveness of bulk insulation is usually evaluated by its R-value, of which there are two metric (SI) (unit K⋅W⁻¹⋅m²) and customary (unit °F·ft²·h/BTU), the former is 0.176 times the latter, or the reciprocal of thermal conductivity or U-value WK⁻¹⋅m⁻². For example, the insulation standard for attics recommends at least R-38 US units (equivalent to R-6.7 or U-value 0.15 in SI units). Equivalent standards are technically comparable, and approval documents usually require the average U-value of the roof area to be between 0.11 and 0.18, depending on the age of the property and the type of roof structure. Newer buildings must meet higher standards than those built according to previous versions of the regulations. It is important to achieve a single R-value or U-value, regardless of the construction quality or local environmental factors of each building. Construction quality issues may include insufficient moisture barriers and windproof issues. In addition, the performance and density of the insulation material itself are also critical. Most countries/regions have some system of inspecting or certifying installers to ensure that good standards are maintained.
History of insulation
The history of insulation materials is not that long compared to other materials, but humans have long realized the importance of insulation. In prehistoric times, humans began the activity of building shelters to resist wild animals and harsh weather, and humans began the exploration of thermal insulation. Prehistoric humans used animal skins, furs, and plant materials such as reeds, flax and straw to build dwellings. These materials were originally used as clothing materials because their dwellings were temporary and they preferred to use the materials they used for clothing, which were easy to obtain and process. The materials of animal fur and plant products can accommodate a large amount of air between molecules, forming air cavities and reducing heat exchange.
Later, the longevity of humans and the development of agriculture determined that they needed a fixed place of residence, and earth houses, stone houses, and cave dwellings began to appear. The high density of these materials leads to a time-lag effect in heat transfer, thereby making the internal temperature change slowly. This effect of warm in winter and cool in summer, also because materials such as earth and stone are easy to obtain, this design is very popular in many places.
Organic materials were the first materials that could be used to build shelters for people to protect themselves from harsh weather conditions and help them stay warm. But organic materials such as animal and plant fibers cannot exist for a long time, so these natural materials cannot meet people's long-term insulation needs. So, people began to look for more durable alternatives. In the 19th century, people were no longer satisfied with using natural materials for insulation. They processed organic materials and produced the first insulation board. At the same time, more and more man-made materials began to appear, and a large number of man-made insulation materials were developed, such as rock wool board, glass fiber, foam glass, hollow bricks, etc.
English