Multiple ultra-short laser irradiation enabling direct writing of high aspect ratio barriers is used for structuring of nanoporous glass. Shape and morphology of laser-modified regions are examined, and high aspect ratio laser-induced material densification is founded. Experimental results are analyzed by modeling describing laser propagation, non-linear ionization and thermal effects. The role of laser focusing, laser energy and pulse number are examined. Several regimes are distinguished. Particularly, high-aspect ratio densified zones are obtained for the numerical aperture of 0.25, whereas either more symmetric densified regions or spherical cavities are shown to be formed for numerical aperture of 0.4. The resulting laser irradiation conditions required for deep and prolonged densification are explained by a lower ionization rate, leading to the under-critical free electron plasma density, longer filamentation and pulse-to-pulse elongation effects. Furthermore, filling of the porous glass with water is demonstrated to particularly extend the length of the densified region in depth. The presented study provides insights facilitating laser-based fabrication of barriers, membrane and patterns suitable for the environmental gas-phase analysis.