A step beyond the envelope house

Envelope, double-shelled houses have advantages and disadvantages. Among the former are true passivity of design (no external power required), low auxiliary-heat requirements, higher minimum temperature in the sunspace, with increased glazing without high heat-loss. Among the latter are higher cost, potential fire-hazard in air plenum or loop, floor-to-floor heat stratification of 5-7 degrees, and much window condensation and frosting in winter in northern states.