ventilation. The natural or mechanical process of supplying conditioned or unconditioned air to, or removing such air from, any space.

ventilation air. That portion of supply air that comes from outside (outdoors) plus any recirculated air that has been treated to maintain the desired quality of air within a designated space.

vertical fenestration. All fenestration other than skylights.

visible transmittance [vt]. The ratio of visible light entering the space through the fenestration product assembly to the incident visible light, visible transmittance, includes the effects of glazing material and frame and is expressed as a number between 0 and 1.