In order to understand this principle we must first take a look at heat transfer in general.
Heat and Temperature
Heat is a measurement of the motion of the molecules that make up a substance, and represents the energy contained in the substance. In hot substances, the molecules are moving relatively quickly. We call something cold when the molecules are moving slower, but their motion is still called heat.
Temperature, on the other hand, is a measure of how something feels to us. An object that feels cold has a low heat content, but the object still contains heat.
Heat always moves from a warm object to a colder one. The important thing to remember is that cold isn’t transferred, because cold is just a qualitative description of low heat. A warmer object may cool off, but that is because it is losing heat, not gaining cold. For example, there is no such thing as letting cold in, rather you are letting the heat out.
Conduction, Convection, and Radiation
Heat can be transferred in one of three ways: conduction, convection, and radiation.
The most efficient method is conduction, which is the transfer of heat between two objects in contact. The transfer occurs as the fast-moving molecules of the hot object bump into the slower-moving molecules of the cold object. The fast molecules give up some of their energy, slowing down, and this energy goes into speeding up (and thus heating up) the slow molecules.
A slower method of heat transfer is convection, which occurs in fluids or gases. A cool fluid in contact with a warm solid will heat up through conduction. The warmer fluid drifts into the cooler fluid, setting up a convective current. Because material must actually be moved, convection is less efficient than conduction.
The least efficient method of heat transfer is radiation. In this case, heat moves through space without the assistance of a physical substance. This is how the Sun’s heat reaches the Earth. The radiant heat is transferred directly into a solid object, but it passes readily through transparent materials such as air and glass.
Insulating a Structure
The first step to insulating a structure is to seal off any openings where air can directly transfer. Sealing all the drafts prevents direct convective currents from forming, and closing the window shade prevents the sun’s radiant energy from heating the inside.
Once this is done, then the only way for the heat to get into or out of a structure is to move through the walls, roof or floor. There is no way to prevent this transfer of heat, but you can slow it down. In a manner similar to electrical wiring, the walls, roof and floor has a resistance to heat transfer. The higher the resistance, the longer it takes heat to move through.
If the walls are solid then the heat is transferred through conductance, which is the most efficient (and thus fastest) method. Resistance can be increased by using a material that is a poor heat conductor.
The ideal insulating approach would be to completely force the heat to be transferred as radiation. This would require a vacuum between the interior and exterior. Vacuum panels exist, unfortunately they are prohibitively too expensive to be considered for structural insulation.
Luckily, just like light, the flow of heat can be reversed by simple reflection. Radiant barrier insulation works on this proven principle. A simple layer of aluminum foil or aluminized mylar can reverse the flow of heat by reflecting it back in the direction from which it came. One of the oldest applications of reflection combined with dead air insulation is the familiar vacuum bottle.
After radiation, convection is the next best option. It’s a less efficient transfer mechanism than conduction, and it turns out that air is a pretty good insulator. If you just leave a gap between the inner and outer walls of a structure, the only way to transfer heat is through convective air currents. This is better than a solid wall, but it could be more efficient.
The problem is that the moving air has a direct path from the warm outside wall to the cool inside wall. It would help a lot to find a way to slow down the air currents. This is where an insulative material comes in handy. These substances slow down the heat transfer by forming many small pockets of air. Most convective insulation materials are held together in a random mat. In order for heat to be transferred through this mat, it must be carried convectively through the tiny air pockets. But the randomness of the fibers means there is no direct path through the mat, forcing the heated air to take a very circuitous route through the insulation material.
The Yomes features a radiant barrier type insulation in the roof and a convective type insulation in the sidewalls. The radiant barrier in the roof is not only effective in the Winter by reflecting the radiant heat from a heat source, in the Summer it keeps the space cooler by reflecting the radiant solar energy from the sun. Unlike most other portable shelters, the Yome does not use a radiant barrier in the sidewall since all the welcome radiant visible light would also be reflected. Instead, the Yome features a series of pillow-type convective panels made out of fiber-fill. These pillows puff out forming a random path with tiny air pockets to help keep the heat in. Unlike the roof insulation which is usually left in year round, the wall insulation is only used seasonally.