A low-k dielectric sacrificial material is formed within a microelectronic structure covered with a layer defining an exhaust vent. At an appropriate time, the underlying sacrificial material is decomposed and exhausted away through the exhaust vent. Residue from the exhausted sacrificial material accumulates at the vent location during exhaustion until the vent is substantially occluded. As a result, an air gap is created having desirable characteristics as a dielectric.