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Exterior envelope options

user-7233280 | Posted in Energy Efficiency and Durability on

I’m planning a second story addition over my existing home in climate zone 5. 

The existing home has a stone foundation, 2×4 walls @ 16″ o.c., and board sheathing. The addition will be 2×6 walls @ 24″ o.c. and roof framing @ 24″ o.c..  

I’m planning to wrap the existing house and new house with Delta Vent SA to air seal (because of the board sheathing). Then install 2″ of rockwool or recycled XPS (leaning toward recycled because of cost). I will install furring strips over the housewrap and then hardi-plank siding or something similar. Is there any concern with installing XPS over a non-drainable permeable housewrap? or should I install a self-adhered non-permeable housewrap if I’m going with XPS directly over it?

The windows will be installed as outies as I want the large interior sills and it seems simplest to just add 2x material flat around the window opening and liquid flash the bucks and R.O.. However, how do you prevent water that is within the rainscreen or behind the exterior foam from draining behind the exterior window trim and directly onto the window? In the following link from building science he just says to leave a gap behind the trim for drainage and the water will drain over the window.

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  1. Expert Member
    Akos | | #1

    The XPS is fine over the peel and stick. You have enough foam for condensation control so permeability here doesn't matter much. Also since your wrb is your air control layer, you dont need to tape your insulation, the gaps in the seams usually are enough for a bit of diffusion drying of any water that gets behind the foam. A crinkly wrb helps a bit on this front as well.

    For your windows, you need a head flashing above your bucks. This flashing can be either abover or bellow your trim ( I usually go for above). The head flashing needs to go over the window buck peel and stick but bellow the wrb of the wall above.

    This will direct any water out passed the window.

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