Materials and methods for preparing thick, mesoporous silica monolithic slabs and coatings with high transparency and low thermal conductivity are provided. The transparent silica materials are particularly suited for window or solar applications including insulation barriers for existing or new single, double pane windows or glass panel building components. The template-free, water-based sol-gel methods produce slabs or coatings by gelation of a colloidal suspension of silica or other oxide nanoparticles or by ambigel formation and then ageing and drying the gels under ambient conditions. Solvent exchanges with nonpolar, low-surface-tension solvents help to avoid cracking caused by drying stress. Mesoporous slabs can also be cast in molds on perfluorocarbon liquid substrates to reduce adhesion and enable gels to shrink freely during aging and drying without incurring significant stress that could cause fracture.