I have a 8" wooden ceiling joists. On these there is a plaster ceiling. The joists have 4" batt insulation. However there is a 1.5" to 2" air space between the insulation and the plaster ceiling. The ceiling also lacks a vapor barrier. Is it advisable to fill this air space with an additional 4" batt insulation, thus doubling my R value, or should i leave as is?