The intrapleural pressure is negative both during inspiration and expiration because
Intrapulmonary pressure is always negative
Thoracic cage and lungs are elastic structure
Transpulmonary pressure determines the negativity
Surfactant prevents the lungs to collapse
The intrapleural pressure becomes more negative at the end of inspiration and less negative at expiration. It ranges between -3 and -6 mmHg. The elasticity of lung and rib cage causes it to recoil and thus creating the negative pressure. The lymphatic drainage of the pleural fluid also creates a negative pressure.