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.