Many rooms in Sime Centers, and other buildings in-Territory have insulation built into the walls. This insulation is not only for soundproofing but for nageric insulation as well. Transfer suites are insulated so the pair have privacy from the prevailing ambient nager outside of the room, as well as not allowing the nageric disturbance caused by transfer to spread outward and disturb others.
Rock, thick metal, and other large-mass insulates selyn fields.
Very thick glass can be cross-hatched with thin metal wire (like chicken-wire pattern) to disrupt selyn field propagation, and that is called insulated glass. It works by diffraction, creating a muddy mess of an ambient that is basically neutral. Most Farrises find this solution sickening to behold. And for the Farris, it doesn't work well.
Later technology is developed allowing woven hangings to insulate selyn fields and this is used in Sime Centers. Again, for the Farris channel such hangings may as well be tissue paper, though some are thick enough that most First Order channels would find them opaque.
Still later, as technology developes, insulation is a primary objective for research investment. So progress on insulation marches in step with progress on nageric enhancing projectors such as become introduced first for entertainment. Various devices of dubious satisfaction are deployed commercially to project interference patterns to disrupt selyn field propagation in crowded venues.
For managing crowds, nothing works as well as deploying channel/Donor teams throughout the crowd.
Gradually, for use in medical situations and for home privacy screening, selyn consuming energy projectors come into common usage. By 348 Unity, we find such devices everywhere on Earth -- but again, most Farrises regard them with anything from polite silence to open scorn. Farris-rated insulation is available, but it's ridiculously expensive, so not commonly deployed. Nothing works as well as a deep, underground cave, or the interior of an asteroid.
(Source: Index card file. House of Zeor) It is possible to use selyn-field insulation. Shrines of the Starred Cross are insulated. Imil has something called "Insulated Laboratory."