Old masonry buildings are worth keeping — and they leak air everywhere the original builders never had to think about. This case study follows a typical retrofit sequence: establishing a continuous interior air barrier with a smart vapor retarder, tying it into new windows, and sealing the penetrations that a century of renovations left behind.

The pencil-line test. Before any product goes on the wall, the airtight layer gets drawn on the section — continuously, around the whole perimeter, floors included. Every place the pencil hesitates is a detail that needs a component: a membrane lap, a tape, a gasket, a sealant joint.

The sequence. Membrane first, laps taped as you go, window connections next with pre-folded corners, then services. Penetrations that come late in the job get dedicated seals rather than improvisation. The blower-door test at the end isn't an exam — it's a flashlight; run it while the layer is still accessible and fixable.

What we'd tell a first-timer. Buy the primer. Respect the install temperatures. And send us the section before you order — matching components to an existing wall is precisely what the Spec Support Desk is for. As always, this article is general education; your design professional of record owns the project decisions.