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 building`s conditioned space yields these key points:
- 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 maintenance costs and increased longitivity 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 strickeing 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, polyisocyanurate (pir) insulation board or rock wool, primarily slow heat flow by eliminating convection and reducing some radiation. Reflective insulation provides a dramatic reduction in radiation heat flow as well as some convection. Polyisocyanurate boards and Spray Foam can provide increased resistance to conductive transfer until the cell gas is lost or diluted by air normally over a period of 15 years.
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, Polyiscyanurate boards, 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 foi surface of the product blocks 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 emitance or “e” values ranging from 0 to 1 (lower “E” value indicates better performance). Most building materials, including fibreglass, foam and cellulose have “E” values in 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 (emitance of 0.03) whether stated as reflectivity or emitance, 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.
In the UK current regulations require us to provide insulation that will achieve a u value of 0.16, this can be achieved by using several of our foil materials in co- junction with glass wool or polyisocyanurate (PIR) board.To give you more detail it would be advised to consult your local building control on what specification they require as we have seen many councils differ on specifications. We would recommened the following specification and have had many excellent reports on completed jobs around the UK and France. If this is a to gain more insulation and create a warm roof a DIY job, you would use 70mm of polyisocyanurate board or 100mm glass wool between your rafters, then cover the face of the rafters with a quilted foil, we would recommend Actis super 10, Super foil 19, Alumaflex or Aluthermo quatro. all these materials we stock for instant despatch. If using the foil on its own you would achieve a u- value of somewhere between 0.18 – 0.20, which is quite acceptable on a refurbishment project. I f the roof tiles have been removed you would lay the foil insulation on the top side of the rafters and counter pattern would be put in place and a membrane to cover this batons and roof tile. The most important aspect with any foil insulation is to have a 25mm airspace between the foil and any covering material, this will allow for heat transmission to take place, with out this you are eliminating the benefit of reflection from the face of the foil. Bubble foil is also a low cost great way to insulate your home, very popular in the USA and known as house wrap, low cost and very effective, great for keeping your house cool in sumer months as well as retaining heat in winter.