Semiconductor devices and method of forming the same include forming a stack of vertically aligned, alternating layers including sacrificial layers and channel layers. The sacrificial layers are recessed relative to the channel layers to form recesses. A dual-layer dielectric is deposited. The dual-layer dielectric includes a first dielectric material formed conformally on surfaces of the recesses and a second dielectric material filling a remainder of the recesses. The first dielectric material is recessed relative to the second dielectric material. The second dielectric material is etched away to create air gaps. Outer spacers are formed using a third dielectric material that pinches off, preventing the third dielectric material from filling the air gaps.