Insulating better: diffusivity, effusivity and thermal inertia

This video deals with thermal insulation and brings out several ideas that decide how an insulation material actually performs. Here is the substance of it:

Who and what

• Speaker: Olivier Dauga
• Subject: explaining the ideas that matter in thermal insulation, and presenting technologies for insulation, heating and sound.

1. Thermal diffusivity

• What it is: thermal diffusivity measures how fast heat travels through an insulation material.
• Why it matters: low diffusivity means heat moves slowly through the material, which helps hold a stable temperature inside a building.
• Thermal inertia: inertia, which is related to diffusivity, is a material's resistance to a change in temperature. A material with high inertia can store heat and release it gradually.
• Thermal lag: a material with low diffusivity creates a lag, delaying the moment a peak of heat or cold reaches the inside of a building.

2. Thermal effusivity

• What it is: effusivity measures how quickly a material absorbs or releases heat.
• A practical example: metal and glass have high effusivity, absorbing heat quickly without their surface warming much, while timber has low effusivity, warming quickly at the surface while absorbing less heat.
• What you feel: effusivity is why the same room temperature feels warm or cold depending on what you touch.

3. Putting it to work: R'BULL

• What it is: R'BULL is an insulation product that uses aluminium foil to catch radiation and turn that energy into heat.
• How it behaves: because its effusivity is low, the energy is not carried into the depth of the material. The exposed surface therefore rises in temperature and radiates strongly, which is what you feel as comfort.
• Fitting advice: R'BULL should be fitted with two air gaps, one above and one below, and alongside a thicker insulation product where the energy balance and the comfort both need to be pushed further.

In conclusion

• The point: thermal resistance, the R value, gets most of the attention, but diffusivity, effusivity and thermal inertia matter just as much when judging how an insulation product performs.
• The takeaway: understanding these indicators is what allows you to choose an insulation product that works, and to save energy with it.

A note on this text

This page summarises what is said in the video. It is meant as an overview of the main points, not as a substitute for watching it.