The X cut is no longer recommended. And a flashing should be installed under the window and up a few inches on either side. If possible install a backdam on the sill as well. If these are flanged windows, then the flashing tape goes on afterwards Watch the order so that it laps correctly.

Here is a link that might be useful: Fine Homebuilding: Installing and Flashing Windows

Thank you so much! Very helpful info!

The flashing tape will be determined primarily by the kind of housewrap and the window classing. Do not use the house wrap seam tape at the windows.

worthy:

Excellent link. Thanks.

We use both a Vycor-like tape and Tyvek tape where appropriate, and you need both!
Casey

Here is a link that might be useful: Vycor, because why not?

Grace Vycor and Vycor Plus are modified asphalt based flashing tapes. They are not compatible with certain sealants and flexible PVC. If you are concerned with asphalt compatibility a better tape might be Grace Vycor Butyl or Vycor Pro.

Grace was the original fabricator of these types of flexible self adhering flashings and they have consistently been the best source of expert advice for all kinds of waterproofing and flashing conditions for many decades.

I have never liked house wrap event the good stuff like Tyvek and Typar so I now use Huber's Zip System and their tape.

This post was edited by Renovator8 on Mon, Sep 15, 14 at 14:58

Great thread. More important than the tape, and the detail I would highlight from the FHB link, is the site-made sloped, rough sill.

I differ from the author and it technically might void some warranties to not fasten the bottom flange. This is the most vulnerable area of the window installation and the whole point of the flashed sill detail. In theory the tape should seal the fastener penetrations but.. The window needs to be shimmed and supported from the bottom anyway to create a good drainage gap.

The only sealing in the sill area should be done from the inside which includes flashing tape. Dont tape the bottom flange! Backer rods and caulk and/or interior flashing tape are more reliable than spray foam and are a nice belt and suspenders addition. Avoid filling the sloped sill drainage area with spray foam.