Almost regardless of local climate and geography, water vapor is constantly evaporating off the water table below ground. Casting concrete over this phase of the water cycle will lead to ~100% RH environment directly below the slab. Enclose a conditioned space above and we create the exact same moisture diffusion scenario in the desert as we would in any other climate, where “wetter” soils might be more expected. Whether we are in Seattle or Phoenix, due to the partial vapor pressure difference inside the building compared to below the slab, water vapor diffuses into and through the slab. Without a low-permeance below-slab vapor barrier to protect the first side of the building, moisture vapor can cause flooring failures, indoor air quality issues, and reduce energy efficiency. Contact your local Stego Representative to learn more on this subject.