My recollection is that glass has drastically higher dialetric constant than air, but more creep (still very little creep, but more than the ~0 of air). If you

My recollection is that glass has drastically higher dialetric constant than air, but more creep (still very little creep, but more than the ~0 of air). If you're arcing kilvolts, you want that higher dialectric constant, and probably don't care about small amounts of creep. If you're building a supercapacitor, that equation might be different because the voltages can be quite small. I've been out of EE for a long time though, so maybe I'm misremembering things here.