A semiconductor device includes a stack structure of a conductive line and an insulating capping line extending in a first direction on a substrate, a plurality of contact plugs arranged in a row along the first direction and having sidewall surfaces facing the conductive line with air spaces between the sidewall surfaces and the conductive line, and a support interposed between the insulating capping line and the contact plugs to limit the height of the air spaces. The width of the support varies or the support is present only intermittently in the first direction. In a method of manufacturing the semiconductor devices, a sacrificial spacer is formed on the side of the stack structure, the spacer is recessed, a support layer is formed in the recess, the support layer is etched to form the support, and then the remainder of the spacer is removed to provide the air spaces.