In this paper, we consider the diffusion of water vapor into silica glass at relatively low temperatures, 25 degrees C to approximate to 500 degrees C. Extensive studies of such diffusion by others have shown that water diffusion from inert gases behaves differently than diffusion from liquid water. In liquid water, the concentration of water at the surface quickly achieves saturation value. In gas, the concentration of water at the surface does not achieve a constant value, but builds up with time eventually reaching a steady state value. Here, we show that published data can be explained by assuming the existence of a surface barrier to the diffusion of water into the silica glass. We develop equations that quantify the diffusion rate through the barrier; the diffusion results are consistent with published data. We suggest that barrier formation is a consequence of the structure of the silica glass at the free surface and the way that water reacts at the surface in contrast to the way it reacts in the bulk glass. This description gives a quantitative explanation of the development of a diffusion barrier and provides a contrast between the way liquid water and water vapor behave at a free surface.