Increasingly complex electronic-photonic integrated circuits, based on strong confinement of photons, has increased the importance of low-refractive-index overcladding materials. In addition to providing high optical transparency and sufficient thickness for adequate optical insulation, overcladding materials must also fill in high-aspect-ratio gaps, withstand high temperatures, and provide acceptable thermal conductivity. Previously, all of these qualities were simultaneously achievable only through tetraethylorthosilicate (TEOS)-based deposition of SiO2. Here, the authors demonstrate how hydrogen silsesquioxane (HSQ) can be used as a lower-cost alternative to TEOS, with superior gap-filling and self-planarization properties. HSQ is a spin-on dielectric designed for low-k applications. The standard curing process for HSQ results in a low-k porous film that is not adequate for photonic applications. It shows very low thermal conductivity, optical absorption due to Si–H bonds, and high intrinsic tensile stress, which limits the achievable layer thickness. By optimizing the HSQ curing process, they eliminate these shortcomings while maintaining HSQ’s excellent gap-filling and self-planarization properties. They demonstrate that HSQ layers can be made almost arbitrary thick, with no detectable Si–H bonds while easily filling sub-100-nm gaps. They demonstrate Si3N4 optical microring resonators with quality factor of 240 000, consistent with the best published values using TEOS as an overcladding.