Building a room within an unconditioned masonry garage

A homeowner in Cleveland with a 100-year-old garage asked whether it makes sense to build an insulated "room within a room" inside it to use as a workshop. Three sides of the garage are double-wythe masonry, the fourth is framed with a door and windows, and the floor is a newer concrete slab. The plan was to leave a gap between the new insulated room and the old masonry walls, use rigid foam and mineral wool (a fire-resistant insulation made from spun rock) on the walls and ceiling, and vent that gap to the outside through louvered openings where old window sashes used to be.

A building science expert who answered the post said the concern about trapping moisture in masonry walls mostly applies when a previously unheated, uninsulated building suddenly gets insulated and heated, cutting off its ability to dry to the inside. Since this garage has already gone a century without interior conditioning, insulating and heating it now shouldn't make things worse, as long as the new workshop doesn't produce a lot of indoor moisture. The simpler fix suggested: foam insulation board directly against the masonry walls with sealed seams, then stud walls filled with insulation, skipping the complicated vented gap and tool-storage cavity. For the ceiling, a vented attic with soffit vents was recommended as a cheaper and more durable option than the sealed foam-and-mineral-wool approach, if there's enough room to do it.

This exchange is mainly useful background for anyone considering converting an old unheated masonry garage or outbuilding into usable, heated space, a project that often raises the same moisture and insulation questions.