Why did the Lavochkin La-5 pilots frequently flew with their canopies open?

The Lavochkin La-5 was a Soviet fighter aircraft of World War II. It was one of the Soviet Air Force's most capable types of warplane, able to fight German designs on an equal footing. The La-5 had its defects. Perhaps the most serious was the thermal isolation of the engine, lack of ventilation in the cockpit, and a canopy that was impossible to open at speeds over 350 km/h. To make things worse, exhaust gas often entered the cockpit due to poor insulation of the engine compartment. Consequently, pilots ignored orders and frequently flew with their canopies open.