Abstract: A vertical-flow air diffuser plenum assembly wherein a generally rectangular hood is formed of a single piece of sheet material in a generally pyramidal rectangular shape and a HEPA filter is mounted beneath the hood in a filter housing in airtight relationship with the hood. The hood has a plurality of smooth continuous horizontal ridges extending 360.degree. around the hood between the apex of the hood and a bottom opening thereof. The ridges are spaced between the apex and the bottom of the hood so as to deflect air flowing along the hood inside surface downwardly through the filter medium in a substantially uniform flow pattern thereacross. The ridges are joined by sloping wall portions and have smoothly rounded corners free from creases or breaks in the surface.