The Hobby-Eberly Telescope (HET) enclosure is receiving a series of modifications to improve dome seeing, including removal of residual heat loads from the optical path, increased insulation of the enclosure, and ventilation of the enclosure ring wall and dome. Analysis indicates that the contribution of dome seeing with the new system will be less than 0.05 arc seconds. The design of the HET enclosure lends itself to very large ventilation openings in the stationary portion of the enclosure also referred to as the "ring wall", with supplementary openings in the rotating dome. The ventilation design implemented has opened approximately 58% of the ring wall, and will open 8% of the dome, in the coming year, in order to achieve natural ventilation of 22 air changes per hour at the minimum design wind velocity of 3.5 mph. A system of ventilation louvers similar in design to the Kitt Peak Observatories 4-meter telescope was determined to be the most practical and cost effective design for use in the ring wall. Conventional off-the-shelf louvers are proposed for the dome, due to constraints in its design. Special considerations for retrofitting an operating facility included a custom hoist mounted on the dome for installation of the louvers (5000lb./ea.), and an inflatable curtain to protect the telescope during foul weather. The ring wall ventilation system has been in full operation since early April 2002 and is part of a program in progress to substantially improve the HET dome seeing.