* Structure: Cardboard is made of compressed fibers, creating many air pockets. Air is a very poor conductor of heat.

* Low Conductivity: The materials used in cardboard (primarily paper) don't conduct heat very well.

Therefore, cardboard slows down the transfer of heat, making it a good insulator.

However, it's important to note:

* Moisture: If cardboard gets wet, its insulating properties decrease significantly. Water is a better conductor of heat than air.

* Thickness: Thicker cardboard provides better insulation than thin cardboard.

* Specific Uses: While cardboard is generally an insulator, it can be used in some specific applications where a low level of conductivity is needed.

Overall, cardboard is primarily known for its insulating properties when it comes to heat.