A basic, affordable system for effective window flashing
We’ve been using a simple but highly effective method to create watertight window for a couple of years. No one wants windows to leak, and its a major hassle to track down the source of a leak once siding or brick or stucco is installed. Over time, we have come up with a standard system for our water resistant barrier - the part of the house responsible for keeping the water out. Siding, brick and stucco don’t do that - water will and does always get in.
I’ve remodeled a lot of houses, and one of the first things to get hit by water damage is often the window sill. When water gets behind that window flange, the water runs down the jamb and camps out on the sill, eventually rotting it out. The standard treatment here is a 6-mil polyethelene vapor barrier draped over the sill, typically with no corner treatment. We just think that the high-impact areas need more durable treatment.
Here is our current list of standard WRB (water resistive barrier) materials:
Sheathing: 1/2” poly-iso rigid insulation
Sealed seams: 3M 8067 All Weather flashing tape
Window Pan: Carlisle CCW-705 40-mil composite adhered membrane
Window Pan Corners: Elastoform or Zip Tape
3M invented acrylic flashing tapes 30 years ago, and they’ve been improving them ever since. This tape has very aggressive adhesion, and as the name says, holds up in all kinds of weather. It is a pressure-sensitive tape, so we roll it on to ensure a good, complete bond to the substrate. Its definitely more expensive than the lumber yard stuff, but this is the water control layer, so we need to be sure. As always, you have to make sure the substrate is clean of dirt, dust and debris before applying - its only as good as its bond.
Shingle-lapped application. Once the window is set, jambs are taped first, then the head.
No tape or sealant along the bottom - if water were to get behind the flange, we want this area to be unsealed so it can escape. On this project we will be installing a 3/4” rain screen for drainage and back venting. Note also the flexible corner flashing - in this case Zip Tape, is stretched out to adhere to the sheathing, and the 3M tape laps over this, creating a tight seal for that corner, which tend to be the weak spots in an installation due to their awkward 3D shape (the reason we use flexible membranes in these areas).
On the inside of the sill you can see how tight and smooth this application is, and the corners are nice and tight. First we apply the primer, then the CCW-705 membrane, then the corner flashing. You want to go up the window about 5-6” in the corners. Carlisle products are not available locally, we drive to Dallas to get them at a commercial roofing supply company. But its worth it to us.
We will come in here in a week or so with backer rod and Dap Dynaflex around the window to create an air and water seal as an added layer of protection. Redundancy can be a good thing…
We lay a coating down of CCW-702 adhesive 20-30 minutes before we install the membrane. They are designed to work together - its basically never coming off.
We only want to install windows once, and we don’t want homeowners to have to deal with water leaking into their home, damaging drywall, wood floors, etc. Its a huge hassle. So we’ve come up with a system that will keep water out for as long as that home and those windows last.