The insulation puzzle: looking past thermal resistance

The three ways heat travels

Conduction, convection and radiation. These three modes of heat transfer are where any honest answer has to start. Conduction is the warmth you feel through direct contact. Convection is the movement of warm air. Radiation is the infrared exchange between surfaces, silent and invisible, and the one most often left out of the conversation.

We start by separating these three, and then show that thermal resistance is one indicator among several rather than the whole story.

Thermal resistance: a wall against the cold

Thermal resistance, written as R, behaves like a wall against the cold. The higher it is, the more solid that wall appears. But what does it actually do against conduction, convection and radiation, taken one at a time?

The igloo, and what it explains

The igloo makes the point better than a diagram. Snow, cold as it is, becomes an insulating structure, because of the air trapped between its flakes. Air is a poor conductor of heat, and that is what does the work.

Even so, the sturdiest igloo is not immune to convection. Cold finds its way in through the smallest gap. The thickness of the walls slows conduction down, but it does not stop it.

The missing piece: airtightness

The answer lies in airtightness. Once you are inside an igloo, simply opening your coat is enough to melt a thin layer of snow, which refreezes as ice and turns the wall into a barrier against convection.

Radiation, the third factor at work in an igloo

Radiation is the third factor, and it matters. In an igloo the white walls behave like mirrors for heat, sending it back inside and limiting what is lost.

In conclusion: three principles working together

A well built igloo shows the three principles working together, which is what effective insulation actually looks like.

In short, thickness on its own is not enough. Good insulation combines airtightness with the reflection of invisible heat. Insulation is not a single calculation of thermal resistance, and treating it as one is where most disappointments begin.