We are working on our first ICF build and wanted to get some input on a detail that is concerning us. The house is mostly desert modern with low sloped flat roofs and parapets. It's in Arizona so not a ton of rain however monsoon season brings heavy wind driven rain at times. So no overhangs on the exterior walls. Our wall assembly is ICF. We have engineered trusses that sit on the top sill plate. The sill plate is set to the inside of the ICF walls and the parapet walls are built into the truss ends. The parapet wall design is double sheathing around the parapet exterior walls with WRB and 1" foam to finally match up with the plane of the ICF wall. So, before the foam. There is a 1" ledge at the bottom of our sheathing top of wall. Our stucco guys want to effectively do EIFS and apply directly to the ICF. We are on board with that but fuzzy on how to best tackle where the ICF and wood framed parapet walls meet. Our concern is what WRB direction to go for the parapet wall sheathing. Our concern is water getting behind the WRB either wind driven or from a leak on top of the parapet wall. Then water traveling down the sheathing and just resting on the 1" ledge behind the foam and WRB. The parapet / knee walls inside will be spray foam as part of the roof under deck insulation. Thanks in advance for any input

I've never built in desert conditions. My experience is along the east coast where 100F+ is not uncommon and/or a hurricane passes nearby every year or two. . . . When I have built buildings with similar situations, I installed a waterproof membrane barrier between the sheathing and insulation, front, back, and top of wall. The WB must be self healing, so that the mechanical fasteners holding on the insulation will not let water pass through (similar to WR Grace Perm-A-Barrier). The water barrier terminates at a through wall flashing that directs any water out of the wall.

Posted By A W Warn on 11 Aug 2019 05:09 PM
<> When I have built buildings with similar situations, I installed a waterproof membrane barrier between the sheathing and insulation, front, back, and top of wall. The WB must be self healing, so that the mechanical fasteners holding on the insulation will not let water pass through (similar to WR Grace Perm-A-Barrier). The water barrier terminates at a through wall flashing that directs any water out of the wall.

ps: Asphaltic membranes have a tendency to creep out at the through wall flashing when encountering extremely high temperatures. Another type of membrane may be more appropriate in your area.

Look at a fluid-applied sealant/adhesion promoter (non-asphalt) with a peel-and-stick overlay for this detail.
It'll go up and over the parapet and then down onto the bed of the flat roof.
Just increase the overlap of the layers a bit around the parapet.

The spray foam can go over the top of it.

It's a LITTLE overkill.
But you shouldn't have to worry about water unless you've somehow damaged the roof SEVERELY (to the point where you need to reroof).