Insulating a shed with no house wrap or sheathing

A homeowner asked a building forum for advice on insulating an old shed that has painted board-and-batten siding nailed straight to the framing, with no house wrap (the water-resistant sheet usually installed under siding) and no sheathing (the solid panel layer under that). They plan to add a mini-split heat pump and had already bought batt insulation sized to fit snugly into the 3.5-inch wall cavities.

The concern is a real one for anyone with an older outbuilding or an old section of a house built without sheathing. Normally, insulation installed against siding like this needs some kind of air gap and a breathable baffle behind the siding, so moisture can dry out rather than getting trapped in the wall. But adding that gap would mean the batts no longer fit the cavity depth, leaving the homeowner unsure whether to stuff the batts in as-is, buy thinner insulation, or switch to rigid foam instead.

An experienced contributor suggested a middle path: staple dimpled house wrap (a plastic sheet with small bumps that creates a built-in air gap) to the back of the siding in each stud bay, then install the batts as planned. This keeps the original insulation, preserves a drying gap behind the siding, and avoids buying new material. If you have an uninsulated shed, garage, or old addition with no sheathing, this kind of detail matters before finishing the interior, since trapped moisture behind siding can lead to rot over time.