Seal around any penetrations into the space
This step includes checking to see if any pipes, electrical conduit, conveyors, ductwork, or other items go through the wall/floor/ceiling. Sometimes these penetrations are sealed, using caulk, gaskets or other means. When they are not sealed and there is a gap for air to travel into/out of the space, additional sealing is necessary in order to contain the decontaminating agent. If permanent sealing is appropriate, the gaps can be caulked in place. For circumstances where permanent sealing is not allowed, duct tape is the primary sealing tool.
Sealing off the HVAC system
In order to provide the decontaminating agent the contact time necessary to achieve the level of kill desired, the space must be isolated from an active HVAC. Depending upon how the HVAC system interacts with the space and surrounding areas, the supply and exhaust can either be sealed off at the room level or at the roof level.
Finally, once the penetrations are sealed and the space is isolated from the HVAC, the doors are all that is left to be sealed. Once again, simply using duct tape to seal around the door is sufficient to properly seal off the area being decontaminated.