The Concept of Radiant insulation

Standard types of insulation, such as fiberglass, foam, and cellulose primarily reduce heat transfer by trapping air or some type of a gas. Thus, these products or technologies reduce convection as a primary method of reducing heat transfer. They are not as effective in reducing radiant heat transfer, which is often a primary mode of heat transfer in a building envelope, especially in warm Australian climates. Rather, these products, like most building materials, have very high radiant transfer rates. In other words, the surfaces of standard types of insulation are good radiators of heat.

Reflective insulation uses layers of aluminum and plastic (as in the case of RadiantShield) to trap air and thus reduce convective heat transfer.

However, the effect of this is negligible compared to the benefit of a materials reflective foil surfaces, as they are very effective in reducing radiant heat transfer. In fact, the foil used in our reflective insulation materials will reduce heat transfer by as much as 97%

The reflective surface of foil insulation reduces the radiative heat transfer, so the cavity adjacent to the reflective surface becomes radiant insulation. For example, a reflective foil product may be only a 0.5mm thick, and have a material R-value of just R0.05, but its presence will transform the adjacent air space into a radiant insulation system. The point is that the reflective material needs to be installed in combination with air spaces to create these radiant insulation systems.

Typically we find that the total R-value of a building system is improved drastically with the addition of a correctly installed reflective material. In the above example, a material might have an R-value of just R0.05, but if we include the reflective air gap adjacent to it, the total R-value might be around R2.0. Keep in mind that the air gap without the reflective surface might be around R0.15 – so a very significant improvement.

Therefore, reflective insulation is superior to other types of insulating materials in reducing radiant heat. The term reflective, in reflective insulation, is in some ways a misnomer, because aluminum either works by reflecting heat (reflectance of 0.97) or by not radiating heat (emittance of 0.03). Whether stated as reflectivity or emissivity, the performance (heat transfer) is the same.

When reflective insulation is installed in building cavities, it traps air (like other insulation materials) and therefore reduces heat flow by convection, thus addressing all three modes of heat transfer (the benefit of this effect is accounted for in reflective insulation Total R-values).

Remember that in all cases, the reflective material must be adjacent to an air space. Aluminum, when sandwiched between two pieces of plywood for example, will conduct heat at a high rate.

• What is radiant heat?

Radiant heat is energy. Radiant heat is heat that is transmitted from a heat source through space. It is the heat we feel from distant objects like the sun or a fire. Radiant heat is also called infra-red heat or infra-red energy. Around 80% of the heat transfer in your building envelope is via radiant heat energy, which heats up your house in summer.

• Bulk Insulation and R-values?

Material R values are determined in a laboratory by placing a product between two fixed temperature plates set at 33 and 13 degrees Celsius (a temperature difference of 23 deg) and measuring the heat transfer that takes place over a four hour period. The heat transfer measured is then presented as the material R-value.

The problem is that the quoted R values are only valid for the standardised conditions in which the test took place. If the temperature difference is increased, the R-value decreases. This is the case in real life conditions, where temperatures of up to 50-70 degrees are common in roof spaces during summer. In these sorts of conditions the actual R-value your bulk insulation would be much lower than what is quoted.

Also, keep in mind that no matter how high the R-value might be, bulk insulation does not stop any heat flow at all. It slows it down by absorbing it, but later, when the material has reached its thermal capacity, it will give this energy off as heat.

• How Radiant Insulation Works?

Radiant insulation like our RadiantShield product works differently to traditional bulk insulation and doesn’t merely slow down or ‘resist’ heat transfer. Radiant insulation blocks radiant heat (up to 97% is reflected) by reflecting it back towards the direction it came from. This ensures that the heat never reaches the insides of your house.

A simple test to prove the effectiveness of reflective insulation is to place a layer of bulk insulation (such as a fiberglass batt) between yourself and a source of radiant heat (such as an oven with the door pulled down or near an open fire). After a short time you will find that your side of the batt is warm to touch and it will be radiating heat at you.

Then place a sheet of foil between you and the source of radiant heat. You should find that nearly all of the radiant heat has been blocked and very little reaches you – even though the foil will be warm to touch. It will stay this way.

It is the foil that does the summer insulating, not the bulk insulation.

• What is the R-value of your material?

As already explained, R-values are a measure of a materials resistance to conductive heat transfer. Radiant insulation works on a totally different principle and thus simply knowing the R-value of a reflective material is useless.

In the case of reflective insulation, Total R-values must be used. The Total R-value simply indicates the thermal performance of an insulation system, determined by a calculation process of airspace values and individual R-values of building materials.

• Doubling the product doubles the benefit?

This entirely depends upon the method of installation. If by doubling up one means just adding a second layer of reflective insulation without increasing the number of air spaces, that is if both layers of insulation are in contact with each other, essentially the only increase in thermal value would be the intrinsic R-value of the added layer. In the case of a RadiantShield, that would represent about an added R0.15. This would not represent an economical insulation option. If, however, doubling up were meant to mean dividing the wall cavity into 3 separate and equal air spaces in place of 2 with only one insulation layer, then the result would be an increase in the total R-value.

• There is no way a product that thin provides much benefit.

The thickness of reflective insulation has much less to do with the overall thermal performance than does the reflective, (low emittance), surface facing the adjoining air space. With respect to reflective insulation materials, the application, including the size of the adjoining air space(s) and direction of the heat flow are the major contributors to the R-value and thermal performance of the insulation system.

• Radiant Barriers and Reflective Insulation only work in hot regions. They are not suitable for use in cold climates.

Reflective insulation products excel in hot climates and often are the first choice for insulation in those regions. However, they also provide significant benefits in cold climates where they are used alone and/or when used in combination with other insulation materials in specific applications.

• Does dust accumulation on the foil surface render foil insulation useless?

Remember that an aluminium surface has both reflective and emissive properties. Reflectivity is the ability to reflect radiant energy and emissivity (the reciprocal of reflectivity) is the ability to not release radiant heat.

Over time, there will be some dust accumulation on the top surface of the foil. In practice and through various studies it has been shown that a significant amount of dust is required to reduce the reflective performance of a foil surface.

However, with double sided foil products, there will always be a foil surface facing downwards which won’t collect any dust. This is there the emissive property of aluminium foil is important to us. Theoretically, even if an upwards facing foil surface is covered with dust and starts to absorb all radiant heat rather than reflect it, only 3-4% of this energy will be further transmitted downwards through the downwards facing foil surface. This is because the downwards facing foil surface, as a function of its physical property, can only ever release 3-4% of it’s absorbed energy via radiant heat.

SuperShield can be cut to shape and fused directly to painted panels or onto bare sheet metal after cleaning. It is used to treat vibration produced noise in many different applications including; metal panels, doors, ducts etc in automobiles, buses, rail cars, planes, ships and so on. It can also be used on ventilation ducts and other HVAC equipment, computers, home appliances and many other objects.

If you have any further questions on this product, don’t hesitate to check out the product page or contact us directly.

So watch this space for articles and additional products as we continue to add to our range.

Aluminum is the material of choice to produce low-emittance facings. Aluminum has an emittance as low as 0.03, but as with many metals, oxidation can occur. Here we will attempt to clarify the process of aluminum oxidation, its effect on surface emittance, and its effect on a facing’s overall performance.

The Process of Aluminum Oxidation

Aluminum oxidation is a chemical reaction between oxygen and aluminum. If bare aluminum is exposed to an oxygen-rich environment, then a process called passivation will occur. Passivation is the spontaneous formation of a thin, protective oxide film which limits the potential for further corrosion. This process – for aluminum – can be expressed by the following reaction:

4Al + 3O₂ = 2Al₂O₃

The process can be described as aluminum reacting with atmospheric oxygen, to produce an aluminum-oxide barrier. Over time – as this barrier grows – the ability of oxygen molecules to diffuse down to the metal surface is diminished. The process of aluminum oxidation will actually protect the metal’s surface, slowing the rate of oxidation to near zero.

The rate at which aluminum-oxide forms, depends upon a number of factors including: metal purity, atmospheric conditions, and the presence of an existing oxide film.

Aluminum Oxidation and Surface Emittance

A low-emittance surface is the key component in any reflective insulation system; as such, the preservation of a facing’s emittance value is essential in maintaining optimal thermal performance. Concerning the time line for aluminum-oxide formation, it should be noted that industry testing of surface emittance is carried out on finished product – or at the very least, aluminum material which has already accumulated its protective layer of aluminum-oxide.

Most non-metallic substances have a relatively high emittance value. In normal applications, however, the thickness of a naturally occurring oxide-film is too small to have a significant impact on a facing’s emittance. Reflective insulation is aimed at preventing the transfer of radiant heat – or energy carried by electromagnetic waves in the infrared band. It has been shown that the presence of an oxide-film increases emittance only within the [0.5μm - 1.0μm] range of the EM spectrum. Since the infrared band comprises wavelengths within the [1.0μm - 1000μm] range, we can conclude that the presence of an aluminum-oxide film will not have a negative impact on the emittance of aluminum-faced reflective insulation.

Aluminum Oxidation and Corrosion

In addition to its use in reflective insulation, aluminum has become the most widely used non-ferrous metal in the world, across a broad range of industries. A primary reason for aluminum’s popularity is its ability to resist damaging corrosion. This resistance can be attributed to the protective aluminum-oxide film that naturally forms when the metal is exposed to an oxygen-rich atmosphere like air.

In aqueous media, oxide film has been shown to be stable in pH values anywhere between 4.0 and 8.5. Across most of North America, the normal pH value of clean rain is found to be about 5.6 to 5.8, and acid rain has values of 4.2 to 5.0 – well within the tolerance of aluminum-oxide film.

Note: This technical bulletin does not purport to address all potential chemical reactions that can occur with aluminum. The intent of this bulletin is to detail the reaction of atmospheric oxygen with bulk aluminum foil only.

Reference: Reflective Insulation Manufacturers Association International (RIMA-I); http://www.rimainternational.org/technical/tb-index.htm, 2010

1. By radiation from a warm surface to a cooler surface through an air space
2. By conduction through solid or fluid materials
3. By convection, which involves the physical movement of air

1. Convection

Conduction is the direct flow of heat through a material resulting from physical contact. The transfer of heat by conduction is caused by molecular motion in which molecules transfer their energy to adjoining molecules and increase their temperature.

A typical example of conduction would be the heat transferred from hot coffee, through the cup, to the hand holding the cup. Another example, as shown above, the contents of the kettle boils from heat transferred from the burner to the kettle. Also, a poker becomes hot from contact with hot coals.

Heat transfer by conduction is governed by a fundamental equation known as Fourier’s Law. (Rate of Heat Flow) = – k x (Area) x (Temperature Gradient)

The factor k is called thermal conductivity or in the case of many insulation materials “apparent thermal conductivity”. This property is characteristic of the material and it varies with temperature, density (degree of compaction), and composition. Some typical thermal conductivity and thermal resistivity data are given in the following table for the purpose of comparison.

2. Convection

Convection in buildings is the transfer of heat caused by the movement of heated air. In a building space, warm air rises and cold air settles to create a convection loop and is termed free convection. Convection can also be caused mechanically, (termed forced convection), by a fan or by wind.

Typical examples of heat transfer through convection:

• Warm air rising from register. (Forced convection)
• Warm air rising from all surfaces of radiator, (after air in contact with radiator has been heated by conduction).
• Warm air rising from chimney. (Free convection)

3. Radiation

Radiation is the transfer of heat (infra-red radiant energy) from a hot surface to a cold surface through air or vacuum. All surfaces including a radiator, stove, a ceiling or roof and ordinary insulation, radiate to different degrees. The radiant heat is invisible and has no temperature, just energy. When this energy strikes another surface, it is absorbed and increases the temperature of that surface. This concept can be understood with the following example: On a bright sunny day, radiant heat from the sun travels through a car’s window, strikes the steering wheel and is absorbed, causing it to rise in temperature.

Radiation from the sun strikes the outer surfaces of walls and roofs and is absorbed causing the surface to heat up. This heat flows from the outer wall to the inner wall through conduction which is then radiated again, through the air spaces in the building, to other surfaces within the building.

There are two terms commonly encountered while discussing radiant heat transfer:

1. Emittance (or emissivity), refers to the ability of a material’s surface to emit radiant energy. All materials have emissivities ranging from zero to one. The lower the emittance of a material, the lower the heat (infra-red radiant energy) radiated from its surface. Aluminum foil has a very low emittance, which explains its use in reflective insulation and radiant barriers.
2. Reflectance (or reflectivity) refers to the fraction of incoming radiant energy that is reflected from the surface. Reflectivity and emissivity are related and a low emittance is indicative of a highly reflective surface. For example, aluminum with an emissivity of 0.03 has a reflectance of 0.97.

The emittance of various surfaces is listed in the following table:

Reference: Reflective Insulation Manufacturers Association International (RIMA-I); http://www.rimainternational.org/technical/handbook.html