Windowsill/jamb wall design feedback/possible issues
GBA advisors,
Hello all, looking for feedback on the attached design Climate zone “5A” , Connecticut Building Codes.
Attempting to follow “Joseph Lstiburek’ perfect wall design as well as info gathered from the GBA site.
https://buildingscience.com/documents/insights/bsi-001-the-perfect-wall
(Open Joint Vertical Cladding with rain screen, 4″ External Rigid foam insulation, no cavity insulation, 2×4 construction, European aluminum windows).
Main concern is moisture control within the foam and internal OSB sheathing. Thinking of having a gap (silicon dimples between internal OSB and first layer of ridged foam to allow wood to breath).
Thanks in Advance
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Replies
whereditgo,
First off: well done with the drawings. It’s rare to see details that show the attachment of the various pieces.
I don’t know how necessary it is, but a dimpled house-wrap does seem to be recommended by Joe L. and others between sheathing and exterior foam - primarily as a drainage plane, not to help the wall “breath”. Probably worth doing as there is very little energy penalty from including the gap.
One area of vulnerability I see on your details is the intersection of the cladding and the window trim. To keep a consistent gap (presumably for aesthetic reasons) between the cladding pieces, you end up with an opening between the siding and trim - which is one of the most important points to protect against bulk water intrusions. I’m not sure how you handle this effectively, but it needs some thought.
whereditgo,
I agree with Malcolm that dimpled housewrap between the structural sheathing and the innermost layer of foam seems like inexpensive insurance, especially since you're using foil-faced polyiso which should effectively be vapor-impermeable.
I was about to ask if you were going to black out the open joint somehow, but it looks like Delta Fassade S is black. Along with the black Cor-A-Vent, anything between joints ought to vanish.
Nice drawings.
Appreciate the feedback and fully agree.
Would like to place a ventilation gap between the outside of the "STRUCTURAL OSB" and the inside layer of "POLYISO" to allow the "STRUCTURAL OSB" to breath to the inside.
The double layer of foil POLYISO and Sealed/coated "WRB ZIP OSB" sandwich is not going to let much/any moister/vapor to pass.
Structure is basically a SIP panel and from research failure usually occurs moisture/condensation entering/trapped within the panel.
That said would you recommend installing the dimple mat with dimples facing inward, towards the "STRUCTURAL OSB"?
Would it be better to use a fully permeable product like "Slicker® Classic Rainscreen"?
https://benjaminobdyke.com/product/slicker-classic-rainscreen/
Assume POLYISO seams/gaps so should be foil taped to reduce vapor transmission through the panel?
Thanks Again!
whereditgo,
The sheathing is what Joe L calls The First Condensing Surface of Interest. Because it is kept warm by the exterior foam It will be well above the dew point at all times in your wall, so it is not in danger of accumulating moisture, and is essentially in the same conditions as the wood and drywall in front of it. What you call “breathing” is being vapour-open, or permeable. That matters when material can take up moisture and need to be able to dry. That’s not the case with the sheathing in your wall assembly - and is why it's termed The Perfect Wall.
The dimpled WRB is there to provide path for incidental moisture that gets through the foam to drain. It won’t help your sheathing “breath to the inside”. It needs to face outwards, and yes being permeable is probably a good idea, but not essential.
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-111-double-play