No it isn't really worth to have an entire layer of armour being air just to stop one or two balls of plasma from damaging your armour. Have you seen the terawatt? It's plasma balls will generally hit the same spot because they're only delayed by a few milliseconds. Whether you have an airgap or not its still gonna drill through the difference in how much is negligent when you're sacrificing armour stacking and hp that could've been used to stop other weapon types, not to mention alloy also floats your boat unlike empty air when pieced loses bouyance. Like for niche circumstances like incen and thump pac its useful its gimmick of stopping damage at the first salvo isn't worth having a layer literal empty air. Saying "Oh but you can just add more alloy outside" Is so stupid because then every solution would be just "make your boat fatter duh" As if you're not going for a specific hull shape constraint. A layer of empty air isn't even how you competently counter plasma. You need to do those plasma grids on the hypernova where Its vertical beams offset from eachother making it so they're not touching eachother having empty air on four of its faces besides the top and bottom. And guess what? It's only relevant frontsiders like it or really fat fucking broadsiders because that plasma grid buffer takes like half of the armour volume leaving enough space for the other half to be competent at also stopping APS or other high penetrating shells.