When I first started installing windows in the 1990s we were using Tyvek HomeWrap as our water resistive barrier (WRB) and made the now-infamous X-cut at our windows, folding the housewrap into the rough opening. We bedded the window flanges in caulk, taped them to the Tyvek with housewrap tape, and used canned spray foam to air seal between the window and the rough opening. We thought we were way ahead of the curve.
Maybe we were ahead of the curve; there are still plenty windows being poorly installed in houses all around the country. And maybe that isn’t such a terrible window install. But as my understanding of best practices evolved, so did the building materials that are readily available, and so did the wall assemblies of the houses I build.
Along the way I learned to use lengths of beveled siding to create a slope on the sill of the rough opening, directing water to the exterior. I added a back dam to the sill assembly too. As soon as I learned about them, I upgraded to Vycor flashing to tape the window flanges, and adopted flexible flashing tape to detail the sill pan. The latter was a game changer, and I could see that there was no need to fold the housewrap into the opening at the sill.
From there, I dropped the X-cut from my workflow and started cutting an I-shape in the housewrap to give better protection at the sides of the rough opening. Short diagonal cuts at the upper corners allowed the top window flange to slip under the housewrap, where it belongs. That detail still works when the windows are being installed in plane with the sheathing.
Today, most of the houses I build have a ventilated rainscreen detail, so there is…