A method of forming a semiconductor structure includes forming a semiconductor layer stack including a substrate and a nanosheet channel stack including alternating layers of a sacrificial material and a semiconducting material providing nanosheet channels for nanosheet field-effect transistors. The method also includes forming vertical fins in the semiconductor layer stack, forming a liner on sidewalls of the vertical fins, and forming a sacrificial epitaxial layer over the substrate surrounding the vertical fins. The method further includes replacing the sacrificial epitaxial layer with a first dielectric layer, removing the liner to form air gaps between the first dielectric layer and sidewalls of the vertical fins, and forming a second dielectric layer in the air gaps between the first dielectric layer and sidewalls of the vertical fins. The first and second dielectric layers provide shallow trench isolation regions surrounding sidewalls of the vertical fins below the nanosheet channel stack.