Thermal Conductivity

Thermal conductivity is the measurement of the speed at which heat travels through a material through conduction. In the United States thermal conductivity (also referred to as the “k” value) is commonly expressed in terms of the number of BTUs of heat which will travel through one sq. foot of material which is one inch thick when there is one degree F temperature difference across the material (ie. Delta T). This expression is often stated as btu/in/hr/sq.ft/°F. The lower the “k” value the better the thermal insulation. The term “R” value is frequently used to describe the performance of insulation materials. The “R” value is simply the reciprocal of the “k" value. Therefore, the higher the “R” value, the better the insulation quality.

For example: Polyurethane foam insulation board is commonly rated at a thermal conductivity of 0.17 (point one seven). This means that a 1” piece of foam 12” square would permit 0.17 BTUs of heat to move through it in one hour if there were a temperature difference of 1°F on either side. Were the temperature difference across the material to be increased to 10 degrees, then the 1.7 BTUs would move through it in the same hour.

Listed below is the thermal conductivity of some common materials.

Convection

In some cases the contributions of convection and radiation play only a minor part in comparison to that of conduction. However, under some conditions, the effects of one or both can be very significant. Convection is the term used to describe the motion or, circulation current, which is set up in any gas or liquid as it is heated or cooled. Convection is not, in itself, a singular heat transport vehicle as is conduction and radiation. Instead, it greatly increases conduction by constantly circulating colder material to the warm surfaces, thus increasing the effective delta T.

A closer look at the role of “trapped air” in traditional insulation materials provides a good example of how convection effects heat transfer. As you can see from the table above, air, by itself, is a very good insulator with a “k” value of only 0.16. Further examination of the table shows that nearly all traditional insulation materials have a higher thermal conductivity. Therefore, one might reasonably ask, “Why use insulation at all?”  The answer is found when one also considers the effect of heat transfer through convection.

The “k” value given for air (0.16) describes the amount of heat which will travel directly through perfectly still, and dry, air. However, air used as an insulator never stays completely still. Instead it sets up an active circulation as one side of the containment chamber is heated. The heated air rises and the cold air falls. This circulation constantly exposes the colder air to the warm wall, thus increasing the delta T across that wall and greatly increases the rate of heat transfer through the chamber. This is where traditional insulation helps. In these materials the air is “trapped” on a great many small chambers called “cells”. While each cell still sets up its own convection current, heat transfer is reduced in direct proportion to the size of the cell. The smaller the cell, the greater the reduction in convection.

In standard insulation foam, the size of the air-trapping cells is described in terms of the foam “density”. A high-density foam will have a greater number of smaller cells than will a low-density foam. However, before jumping to the conclusion that the highest density foam is, inevitably, the best insulator, there is one more factor to consider. This is the thermal conductivity of the cell walls themselves. These are typically PVC, polyurethane, polyethylene or polystyrene and often have a greater thermal conductivity than does still air. The greater the number of cell walls, the more material there is present to transmit heat through conduction. This is why the best insulation foams must reach a compromise between small cell size (ie. high-density) and minimal cell wall material (low-density).

Radiation

Like conduction, radiation is a unique and independent form of heat transfer. Ignoring the conflicts of wave and quantum theory, it will suffice to say that radiation, in this case, refers to the transmission of electromagnetic energy through space. While the term radiation applies to the entire electromagnetic spectrum, our concern is with that portion which falls between visible light and radar, the infrared rays. Infrared rays are not themselves “hot”, but are simply a particular frequency of pure electromagnetic energy. Sensible “heat” does not occur until these rays strike an object, thereby increasing the motion of it's surface molecules. The heat then generated is spread to the interior of the object through conduction. Therefore, radiation is fundamentally different from conduction in that it describes the transfer of heat between two substances which are not in contact with each other. All matter above absolute zero (-456.7°F) radiate heat to some degree. How much heat an object radiates is determined by it's temperature, the temperature of the surrounding environment and the object's emissivity factor.

Recently there has been a great deal of emphasis placed on the importance of radiant heat “barriers” by some insulation manufacturers. Such barriers inevitably consist of a piece of aluminum foil glued to a traditional air-trap type insulation foam or similar material. Although these manufacturers claim greatly increased insulation performance as a result, the truth is that the improvement, if any, is highly dependent on the application.

A radiant heat barrier works by reflecting radiant heat back toward the source. It does not reflect conducted heat, nor can it reflect heat within a solid object. In a vacuum, such as outer space, radiant heat barriers are very effective. In this air-free environment there can be no conduction (except through solid objects) so all heat transmission is by means of radiation. A radiant heat barrier on the outside of a space ship or satellite proves a very efficient insulator by reflecting back up to 95% of the radiant heat which strikes it. For this same reason, thermos bottles (ie. Dewars flasks) are also coated internally with aluminum radiant heat barriers.

However, once our space ship returns to the atmosphere of earth, it's radiant heat barrier becomes considerably less efficient. This is because the source of the heat is no longer completely radiant in nature. The warm air is also transferring heat to the ship's skin through conduction. How much good the radiant heat barrier is now doing depend on several factors. If the ship is parked on the runway in the bright sun, the percentage of radiant heat to conductive heat would be very high and the barrier would still be quite helpful. Once the ship is moved into a hot hanger, the conductive heat of the surrounding air become much more dominate and the barrier contributes little.

One example of the misuse of radiant heat barriers can be seen in the common practice of laminating foil sheets between pieces of standard foam insulation. In this case, all heat reaching the foil barrier is conductive and passes straight through making the barrier useless. Radiant barriers reflect infrared most effectively back into a vacuum. As the density of the material in contact with the barrier increases the effectiveness decreases. A barrier which would be highly efficient in space would be totally ineffective if sandwiched between insulating foam.

The chart below gives the infrared radiation reflectivity (emissivity) of some common materials: