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Integrating metal drip cap with Vapro Shield IT and 3M flashing tape?

benjamin_wooten | Posted in General Questions on

Hello! First post, however I am a long-time appreciative reader of this site’s articles and forums.

I live in Halifax, NS, Canada. Climate zone 6. A carpenter friend, myself, and my spouse are doing a renovation/addition on our home, and we have a question regarding our exterior building envelope.

From inside-out, we currently have: plastic vapor barrier, 2×6 wall studs, interior cavities sprayed with 1/2lb. open-cell foam, 3/4″ plywood sheathing, Vapro Shield IT wrap. The final buildout will add to that: 1 1/2″ Roxul Comfortboard rock wool insulation, 1″ strapping, and finally 5/8″ wood siding. The roof deck is sprayed out with 2lb. closed-cell foam, 3/4″ plywood sheathing, full Ice and Water, and 24ga. galvalume on the roof (standing seam, unexposed fasteners).

Our windows are “innies,” with flanges on the sheathing plane. We used Vapro Flashing and the sonic welded corners in the window install, caulked the window flanges top and sides, nailed all-around. The house wrap runs right up over the window flanges on top and sides, to the base of the brickmold, with DOW Corning 758 holding it in-place on the sides and left loose on top (for the eventual insertion of a metal drip-cap as part of the metal window flashing system).

It was suggested by another builder that we pick up some 3M all-weather flashing tape to really seal up the joint between the Vapro Shield IT and the window flanges. We have the 4″ tape, but now are scratching our heads about how it should integrate with the metal drip cap and the Vapro Shield IT along the header.

The following order of events makes sense to us…

1. The bottom flange of the window should receive NO 3M tape;
2. the side flanges would get taped vertically, all the way up the window, sealing the Vapro Shield IT firmly to the flange, with the tape extending up past the header;
3. the metal drip cap should then slide in underneath the loose flap of Vapro Shield IT, from edge-to-edge of the top of the window, folding the metal over the corners of the brick mold…

…and here is where we get lost. Should the drip cap be caulked to the sheathing plane? Or just nailed? Both? Should it’s top edge be 3M’ed to the sheathing? Does the drip cap get caulked to the brickmold anywhere? If so, where? When the Vapro Shield IT flap goes back down over the drip cap, should that be DOW Corning’ed, 3M’ed, anything?

We are leaving the exterior insulation/strapping/siding off until Spring, so the idea of just putting on the drip cap and taping the edges sounded appealing for the Winter, while we work on the interior.

However, come Spring, some pretty wide metal flashings are going to have to go around all sides of the windows to cover up that exterior buildout, and we don’t want to find out we somehow painted ourselves into a corner.

A window detail is enclosed to illustrate what we are looking at.

Any advice would be much appreciated!

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not a fan of the roof assembly you describe -- with Ice & Water Shield on the exterior, and closed-cell spray foam on the interior, your roof sheathing won't dry out in any direction. I hope that the roof sheathing was bone dry, and the RH was very low, on the day you installed the spray foam.

    Your question is a little confusing, but I think that you plan to install wood brick-mold casing on the exterior side of your installed flanged window. Is that correct?

    If so, it is a good idea to install a metal drip cap to protect the wood head casing (the brick-mold). The vertical leg of the metal drip cap can either be tucked under your WRB, or you can use the 3M tape to seal the metal drip cap to your WRB. Either method will work.

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