Vapor open exterior - without sheathing - maximizes outward drying potential

2x4 Structural wall - airtight tape plywood sheathing, vapor open dense packed insulation and SOLITEX MENTO Plus membrane - click for larger view

A short case study, with many pictures......The theory is quite simple - in a heating dominated climate, for the most robust assembly - you want a vapor retarding airtight layer on the interior of your insulation and a windtight/airtight and vapor open exterior.

Taped seams at Ecocor's Montville project

This principle will ensure that the insulation is protected from vapor drive from the interior in winter - while any unforseen (interior) vapor or (rain-)water that gets into the enclosure can rapidly dry out to the exterior and thus will not cause any damage/mold.

A straightforward way to build this way has been used by Chris Corson (Ecocor design & build). The principle he employs for his walls is quite simple.

I-joist bolted through the sheathing form cavity for dense pack cellulose - photo: Chris Corson

Build a conventional 2x4 framed house. The exterior sheathing of this wall will be the interior vapor retarder and air tight layer (but it will be the exterior structural sheathing for your code official).  Now continue with taping all the seams of the sheathing with TESCON VANA- on outside corners use VANA 75, on inside corners TESCON PROFIL with split release paper).

SOLITEX MENTO Plus without sheathing behind "pillows" slightly.  Battens, counter battens and flying lath holding back the cellulose insulation and provide  back-venting for cladding. - photo: NCOB Photo

Now bolt the I-joists, with the desired (insulation) depth, through the sheathing into the studs of the structural wall. The window openings cut into the sheathing will have to be integrated into this airtight layer - which will be the topic of another blogpost.

Thick walls, no sheathing (on the exterior) - photo: NCOB Photo

On the exterior of the joist-flanges, you can now staple the SOLITEX MENTO Plus membrane. This vapor open, reinforced WRB is then supported by a system of battens. This combination will hold the dense pack cellulose insulation in place during and after installation, while allowing maximum outward drying in the winter. Notice the slight bulge on the membrane in photo above, showing that the cavities are fully filled, you can pad the membrane and if it feels like a firm mattress you know that the packing is sufficiently dense.

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