Drainage plane between exterior insulation layers?
kdd225
| Posted in Green Building Techniques on
Hello! Have a question on drainage planes when using multiple layers of exterior foam insulation. Would you recommend doing a textured drainage plane on the face of the sheathing AND between the layers of foam board insulation? Just thinking it would allow for some drainage if water got behind or in between the layers of foam and shouldn’t(?) be so much of a gap that it’d detract from the thermal performance of the insulation. Any thoughts would be helpful!
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Replies
kdd225,
The drainage plane (a rain-screen) directly behind the cladding brings the overwhelming majority of the benefits a gap yields. That's where I would put my energy. I don't see much use to having a dimpled WRB either on the sheathing, or between the foam layers. If you are worried about moisture making it's way into the foam, consider moving the house-wrap to the outside under the strapping.
I considered the same thing; however, any air movement between the layers causes convection and will draw up cold air between your layers. There is a possibility of condensation on you sheathing without the proper ratio of exterior to interior (some studies show you need almost 60% on the exterior to be safe). You could always use mineral wool as it's more breathable; however, it to0 allows for some level of convection (more so on the exterior surface as airflow will decrease with depth) and diminished r-value; which nobody talks about. There was a paper published regarding this issue, but one of the authors is an employee for Roxul and the research was supported by Roxul; for what it's worth any repeatable research publication is supposed to have a specific clause about conflict of interest.
pietrasms5,
" any air movement between the layers causes convection and will draw up cold air between your layers. "
You are over-thinking this. There are all sorts of theoretically possible problems that simply don't manifest themselves in buildings. Sheathing on walls with the proper (code mandated) ratio of exterior foam doesn't experience moisture problems. I'm not sure how you could get voids between the two layers of foam that would sustain convective currents sufficient to make any difference. The foam is fastened to the strapping used to support your cladding, and is by necessity snugged tight as part of its structural integrity.
"You could always use mineral wool as it's more breathable; however, it to0 allows for some level of convection... and diminished r-value; which nobody talks about. "
On the contrary. The reduction in R-value of exterior permeable insulation has been studied and quantified. It isn't worth worrying about. See tables #8 and #9 in this link:
https://www.rdh.com/wp-content/uploads/2017/10/Van-Straaten-Windwashing.pdf