Heat Loss/ Gain

Heat is transferred through three means: convection, conduction and radiation.
Traditional bulk insulations only worked by slowing down conduction and convection, they never addressed radiant heat loss.

Convection is the movement of air upwards or sideways carrying heat toward a cooler point.  Convection is visible as wavy lines above a hot road.  In buildings, convection currents are more of a concern in the winter because we heat the interiors.  During winter months convection can account for up to 45% of heat transfer, but almost nothing in the summer.

Conduction occurs when hot travels to cold. It carries energy when two building materials come in contact.  The roof is connected to the trusses, the trusses are connected to the drywall, etc. Conduction only accounts for between 5% and 7% of the heat transfer into a building. 

Radiation is electromagnetic waves that can only be seen as part of a rainbow.  It travels at 186,000 miles per second.  Its infrared frequencies travel in all directions seeking a cooler object to strike and then that object generates heat.  Microwave ovens work on the same principle in that they heat the object not the air.  Radiant energy passes through most standard building materials including metal, wood, asphalt, and fibreglass. 

Radiant energy is responsible for between 45% and 93% of the heat transfer into, or out of a building.

“Radiation is the primary mode of all heat transfer.  The other two modes, conduction and convection, come into play only as they interfere with the primary mode.”
Energy Design Update, Feb. 93.  Stated by C.M. Pelanne, Senior Research Specialist for Johns Manville

What should I know?

Before we talk about Low-E Insulation we need to define three thermal values. The first being ‘R’, the second being ‘U’ and the third being ‘E’.

U-values are a measure of a materials ability to conduct heat. The thermal performance of windows and doors is commonly stated in U-values. U-value is equal to 1/R (R-value).

R-values are the reverse of U-values: the resistance to heat flow.

Low-E insulation products do have an R-value but the reason they work so well is in the name.

E-values or the ‘E’ in Low-E stands for Emissivity. Emissivity is the ability of a surface to emit or transfer radiant energy through itself - everything has an E-value.  If you heat the inside of a cast iron radiator you can feel the heat come off the outside. Cast Iron has an Emissivity of 85%. So 85% of the inside temperature is transferred to the outside.

Here are the Emissivity values of some typical building materials:

Asphalt:  .90-.98

Silver:  .02
                
Brick:  .93

Plaster:  .91

Concrete:  .85-.95

Wood:  .90

Polished Iron:  06

Glass:  .95

Fiberglass/cellulose:  .80- .90+

Pure Aluminium Foil:  .03- .05

Remember that conduction only accounts for up to 7% of heat transfer. ’U’ & ’R’ values play such a small part in stopping the heat that is transferred in your buildings maybe we should ask the question.

“ Why do we put so much importance on ‘U’ & ‘R’ values”??

Regardless of how high the R value or low the U value on an insulation product is, unless it has a low E value, it has no ability to reflect radiant energy.  Very high R values don’t stop heat transfer, they simply slow it down.  You cannot ignore the need for good U values, but by addressing the U values alone a large percentage of energy transfer is being ignored. 

The Answer

For optimum thermal performance, you need an insulation product with high R values and low E values.

The primary function of Low-E Insulation is to reduce the transfer of radiant energy.

Low-E, because it is made from pure aluminium, has an e-value of  0.03 which means 3% of radiant energy that comes in contact with its surface will pass through it.  Or another way to look at it is that Low-E will reflect 97% of radiant energy that comes in contact with it.

“If a surface emittance is changed from .90 to .03 the part of the heat flow that is radiation is reduced to 3.3% of its initial value.”

A 1” airspace facing the aluminium is ideal because convection cannot begin to move air in this small a space.  Keeping this in mind, you can see the importance of making certain that an airspace is maintained facing the aluminium on both sides if at all possible. 

The airspace also works as an insulating thermal break.

It would be a serious mistake to ignore U value requirements, just as it would be to insulate solely against radiant energy when high R-values are required.  
That is why you will find many applications in these guidelines that use combination systems, which include Low-E and a secondary insulation.