Microfluidic systems for polymerase chain reaction (PCR) should be fully closed to avoid vapor loss and to exclude the risk of contaminating the test environment. In closed systems however, the high temperatures of up to 95°C associated with PCR cause high overpressures up to 100 kPa, dominated by the increase of vapor partial pressure upon evaporation. Such high overpressures pose challenges to the mechanical stability of microfluidic chips as well as to the liquid handling in integrated sample-to-answer systems. In this work, we drastically reduce the pressure increase in fully closed PCR systems by integrating a microchannel that serves as a vapor-diffusion barrier (VDB), separating the liquid-filled PCR chamber from an auxiliary air chamber. In such configurations, propagation of vapor from the PCR chamber into the auxiliary air chamber and as a consequence the increase of pressure is limited by the slow diffusion process of vapor through the VDB. At temperature increase from 23°C to 95°C, we demonstrate the reduction of overpressure from more than 80 kPa without the VDB to only 35 kPa with the VDB. We further demonstrate proper function of VDB and its easy integration with downstream processes for PCR based nucleic acid amplification within centrifugal microfluidics. Without integration of the VDB, malfunction due to pressure-induced delamination of the microfluidic chip occurred.