I write a lot about the four control layers every building envelope has. There is an order of importance:

1. Water control layer
2. Air control layer
3. Vapor control layer
4. Thermal control layer

A window needs to be able to perform the functions of all four control layers in order to be successful. Window manufacturers design their units to be effective at controlling the movement of water, air, vapor, and heat through good product design and manufacturing techniques, but where they lose control is in the installation of the window. Often the success or failure of a window will come down to the installation, but there are ways to avoid some of the window installation failures I’ve come across.

A missing window head flashing is one of the most overlooked details I see. The idea is to keep water moving so it can’t find a place to sit. If it sits, there’s a chance it can find a hole and end up inside the building. A drip cap with a slight pitch towards the front of the window is effective at keeping water moving.