The Need For Insulation
When installed correctly, insulation reduces the heat transfer through the envelope of a building. When ever there is a temperature difference, heat flows naturally from a warmer space to a cooler space. To maintain comfort in winter, the heat lost must be replaced by the heating system: and in summer, the heat gained must be removed by the cooling system. Statistics show that 50% to 70% of the energy used in the average home in the U.K. is for heating and cooling. It makes sense to use thermal insulation to reduce this energy consumption, while increasing comfort and saving money . Naturally, less consumption of fossil fuels and the energy produced from them relieves the burden our eco-system must bear.
To summarise, insulating the envelope of a builing`s conditioned space yields these key benifits:
Provides a much more comfortable, productive and livable structure. In addition, the effects of moisture condensation and air movement are minimized in well-insulated buildings. This results in lower maintance costs and increased logevity of the building structure.
Reduces energy requirements, which lower utility bills.
Supports economic, environmental and energy conservation goals. This is evidenced by the numerous studies sponsored by the energy commission.
Heat moves through wall cavities or between roofs and attic floors by radiation, conduction, and convection with radiation the dominant method of heat transfer. A reflective insulation is an effective v barrier against radiant heat transfer because it reflects almost all of the infrared radiation stricking its surface and emits very little heat conducted through it. By virtue of its impermeable surface, reflective insulation also reduces convective heat transfer. Mass insulation like fibre Glass, cellulose or rock wool, primarily slow heat flow by eliminating convection and reducing some radiation. Reflective isulation provides a dramatic reduction in radiation heat flow as well as some convection. Foam boards and Spray Foam can provide increased resistance to conductive transfer until the cell gas is lost or diluted by air.
What Is Radiant Barrier Reflective Insulation?
Radiant barrier insulation is a reflective insulation system that offers a permanent way to reduce energy costs. Radiant barrier insulation systems reflect radiant heat energy instead of trying to absorb it. A pure aluminum radiant barrier reflective insulation is unaffected by humidity and will continue to perform at a consistent level no matter how humid it may be. A radiant barrier insulation system is a layer of foil facing an airspace and is installed in the envelope of a building.
Most people are familiar with traditional insulating materials such as fiberglass, cellulose, Styrofoam, and rock wool. These products use their ability to absorb or resist (slow down) convective and conductive heat transfer to insulate (R-value). A third, seldom discussed but dominant form of heat transfer exists: radiant heat transfer. What are the differences among the three forms of heat transfer?
Conductive: Direct contact. If you touch a pot on the stove, this is conductive heat transfer.
Convective: Steam, moisture. If you put your hand above a boiling pot, you will feel heat in the form of steam. This is convective heat transfer.
Radiant: Electromagnetic. Step outside on a sunny day and feel the sun's rays on your face. You are feeling radiant heat transfer. All objects above absolute zero (-459.7 degrees F.) emit infrared rays in a straight line in all directions.
A radiant barrier reflects radiant heat energy instead of trying to absorb it. What does this mean in your home or business? During the winter, 50-75% of heat loss through the ceiling/roofing system and 65-80% of heat loss through walls is radiant. In the summer, up to 93% of heat gain is radiant. If you are depending on R-value (resistance) alone to insulate against heat gain and loss, remember that traditional forms of insulation are virtually transparent to radiant energy and are affected by changes in humidity (moisture levels). A 1-1/2% change in the moisture content of fiberglass insulation will result in a 36% decrease in performance (referenced from HVAC Manual 10.6; McGraw-Hill). A pure aluminum radiant barrier is unaffected by humidity and will continue to perform at a consistent level no matter how humid it may be.
Concept of Reflective Insulation
Different types of insulation products reduce the heat transferred by conduction, convection and radiation to varying degrees. As a result, each provides different thermal performance and corresponding "R" values. The primary function of reflective insulation is to reduce the radiant heat transfer across open spaces, which significant contributor to heat gain in summer and heat loss in winter. The low emittance metal foil (Durafoil) surface of the productblocks up to 97% of the radiation and therefore a significant part of heat transfer.
There are many types of material that reduce heat gain and heat loss. Some materials provide greater resistance than other, depending on the mode of heat transfer: convection, or radiation. Most insulating materials work on the principle of trapped air, gas being a good insulator. Mass insulation like fibreglass, foam, and cellulose use layers of Glass fibre, plastic and wood fibre respectively to reduce convection thereby decreasing the transfer of heat. These materials also reduce heat transfer by conduction due to the presence of trapped air. ( However, these products, like most building materials, have very high radiant transfer rates.) Heat flow by radiation has been brought to the public`s attention with high efficiency windows which commonly use the term "low E " to advertise the higher performance ratings. This value is measured in emittance or "e" values ranging from 0 to 1 (lower "E" value indicates better performance). Most building materials, including fibreglass, foam and cellulose have "E" valuesin excess of 0.70. Reflective insulation typically have "E" values of 0.03 ( again, the lower the better ). Therefore, reflective insulation is superior to other types of insulating materials in reducing heat flow by radiation. The term reflective insulation, in some ways a misnomer because aluminium or polyester either works by reflecting heat (reflectance of 0.97) or not by radiating heat (emittance of 0.03) whether stated as reflectivity or emittance, the performance (heat transfer) is the same. When reflective insulation is installed in wall cavity, it traps air ( like other insulation materials) and therefore reduces heat flow by convection thus addressing both modes of heat transfer. In all cases, the reflective material must be adjacent to an air space. Foil, when sandwiched between two pieces of plywood for example, will conduct heat at a high rate.
How to test the Reflective Thermal Barriers from (Airflex Insulation)
The BTR is characterized by: Their reflecting power which opposes to the radiation in infrared thermic (wavelengths included between 3 µm and 50 µm). Their intrinsic thermic resistance which slows down the thermic transfer between 2 external faces of the barrier.
1. Measure of emissivity
The reflecting power of materials is characterized by the emissivity E or the reflectivity (1-E).The emissivity is a number without unit which varies between 0 and 1. When the emissivity is close to zero, the material is very reflective. It is the case of the polished aluminium for example. When the emissivity is close to 1, the material is said absorbing. It is the case of the opaque bodies and most of the common materials (wood, stone).
The emissivity is measured in the range of wavelength for which the material is used. The most used method is the integral sphere's method, the measure is made inside a sphere which constitutes a black body, real trap of infrared. Another possibility consists in using an emissiometer which will detect the variations of temperature between a transmitting and a receiving temperature sensors.
The integral sphere is more used for the standard tests, that is when we make measure the emissivity of a material by an accredited laboratory in order to use the value found as reference. For the measures said about routine in the process of manufacture, the emissiometer is preferred because faster and more practical to use.
2. The intrinsic Resistance
The thermic resistance of a material, expressed in m ². ° K/W is measured in several ways:
2.1. The kept warm plate
The material's sample of a surface about 200 mm x 200 mm is put in the contact of 2 plates, one warm, the other cold of a dimension about 500 mm x 500 mm. At the balance, we measure the difference of temperatures between 2 plates. The sample to be tested is surrounded by a said zone guard's ring which limits the losses of thermic stream to its periphery.
2.2.The fluxmeter
This method implements one or several measure's sensors positioned on both sides of the material to be tested and calculate the energy which is in transit through the material. KdB uses and prefers the measure with the kept warm plate which is recognized as more precise and more accurate than the fluxmeter's method.
3. The global thermic Resistance with air gapes
3.1. Kept or calibrated warm box
This test's equipment allows to measure the global Electric current which crosses a sample to be tested and 2 air gapes whose thickness is included between 3 and 5 cms which are on both sides.The sample to be tested has a relative important dimension (1 m x 2 m) and can be positioned horizontally, vertically or in an oblique way. One side of the warm box is warmed, while the other side remains colder and collects calories emitted by the warm side. Like for the kept warm plate, a guard's ring is arranged in periphery of the sample to be tested and limits the losses of thermic current which is essentially transverse in the product to be experimented. This method is delicate in its application and its precision cannot be lower than 10 %. It integrates the thermic resistance of 2 air gapes which is next to the product to be tested and consider the reflecting's power of product's faces. Nevertheless, it is recognized all over the world by the organizations of certification or of evaluation of the insulation's products performance.
3.2. The measure in 3 dimensions
KdB, in partnership with the CNRS (National center for scientific research), created an equipment of test in which the warm emissive source is placed inside a cube of 1m3 circumscribe, at a regular distance of 3 cms, by another cube constituted by 6 panels of the insulating material to be tested. This second cube is circumscribe by a 3rd cube placed at a distance of 3 cms and constituted by 6 little not very reflective panels and thermically conducting.Those 3 imbricated cubes one in the others are placed in a room regulated in temperature in which the air is renewed by a slow sweeping. The insulating material (intermediate cube) is isolated by the interior cube (transmitter) and by the outside cube (receiver) by 2 aire gapes of known thickness and is held in balance by insulated studs of very weak sections to limit at most the thermic bridges.The power emitted in the center of the equipment is totally evacuated through the three-dimensional structure to be tested. By the measure of the temperatures of wall and of the emitted powers, it is easy to get the thermic current which crosses the 6 sides of the insulated cube.
This method was chosen by KdB to characterize the reflective Thermic Barriers within the framework of a program Prebat on 2005 (Research program on the Energy in the construction) financed by the ADEME (Agency for the Development and the Mastery of the Energy) This method is precise and much easier of application than that of the kept warm box. The thermic bridges between the transmitter and the receiving surfaces are perfectly identified and negligible. The measures are accurate. The results obtained by this method are in the same order of magnitude as those obtained in the warm box kept or calculated from the kept warm plate.
4. The tests in situ
Equipments and methodsØ previously described are interesting because usable in laboratory or in industrial environment. The environment of those equipments is known and under control. The carry out measures are absolute measures, considering a preliminary calibration of equipments. The measures in situ, on the other hand, are measures realized in real situation, that is in a known environment but not under controle. So, it is very difficult, even impossible to take absolute measures in this uncertain environment. Most of the tests in situ are differential or relative tests which compare the characteristics of 2 differents components placed in the same environment subjected to exogenous variations. Their application is extremely delicate and very doubtful, it is such difficult to make sure that the measures made on one of the components would give the same absolute precision if they were applied to the other sample placed in the same conditions. If it is very interesting for a developer to estimate the behavior in real conditions (sun, wind, rain, day and night) of materials used in the construction, the tests in situ can in no case constitute a method of qualification or certification of material because they do not allow the absolute measures and are too much subjected to the hazards of the environment. They are also very long and very expensive to implement.
Roofing Solutions: Thermal Reflective Foil Insulation and Loft Insulation Suppliers
1 North Quay, The Shipyard, Upper Brents, Faversham, Kent, UK tel 01795 597912
1 North Quay, The Shipyard, Upper Brents, Faversham, Kent, UK tel 01795 597912
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