Vapour-pressure deficit the plant may dry out and die. For this reason the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 and 0.95 kPa. In ecology, it is the difference between the actual water vapour pressure and the saturation water vapour pressure at a particular temperature. Unlike relative humidity, vapour-pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation. To compute the VPD, we need the ambient (greenhouse) air
Air barrier perform the function of a vapour barrier. This is because water fits through narrower pores than the other main constituents of air, oxygen(O2) and nitrogen. Air barrier Air barriers control air leakage into and out of the building envelope. Air barrier products may take several forms: Air barriers are divided into air barrier materials, air barrier accessories, air barrier components, air barrier assemblies and air barrier systems. Air barrier materials – Building materials that are designed and constructed to provide the principal plane of airtightness through an environmental separator, which has an air permeance rate no greater than 0.02 L/(s•m²)