Window Trim & Aluminum Capping: what to know
What capping is for
Aluminum capping wraps exterior wood trim around windows and doors in bent coil stock. Done well it ends the repainting cycle and keeps water off the wood underneath. Done badly it is one of the most reliable ways to rot a window opening from the outside in, because it seals water inside rather than out.
The detail that decides everything
Water that gets behind cladding has to have a path back out. Head flashing above the opening, laps that shed downward, and end dams that stop water running sideways into the wall are what make that happen. Caulk is not flashing. A capping job that relies on a continuous bead of sealant to stay watertight will fail at the first joint that opens, and because it is hidden the failure is invisible until the interior sill or the sheathing gives it away.
Where this shows up on Kentucky homes
Ohio Valley humidity means a wall that takes on water dries slowly. Add repeated freeze-thaw cycles and a small unflashed gap widens each winter. Interior evidence, a stain at the corner of a sill, peeling paint on the inside casing, a musty smell near a window, often points to the exterior trim detail rather than to the window unit itself.
Quoting it with the siding
Trim, corner posts and capping are normally part of a siding scope, and that is the sensible time to correct flashing at every opening, because the wall is open. Retrofitting flashing later means removing cladding again.