Heat transmission through clothing largely depends upon its thickness, types of fibres used and fabric construction. Air gaps between layers (in the case of multilayer garments) can also affect heat transmission.

A horizontally oriented specimen is subjected to a gas flame from gas a burner beneath it. The heat passing through the fabric is measured by means of calorimeter. The rise in temperature to 24°C/75°F (HTI 24) is recorded in seconds for classification.