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Vapor open exterior – without sheathing – maximizes outward drying potential

September 30, 2012

2×4 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 2×4 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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4 Responses to Vapor open exterior – without sheathing – maximizes outward drying potential

  1. Robert Riversong October 30, 2012 at 11:20 am #

    This is the principle I’ve been using for 30 years in cold-climate construction. I aim for an outside “skin” that is five times as vapor permeable as the inside “skin”. Since code now requires a 1 perm interior vapor retarder, that requires a minimum five perm exterior, which coincidentally is the perm rating for #15 felt which is the IRC standard for an exterior weather barrier.

    However, I’ve accomplished this with no sheathing at all (let-in metal T-bracing instead) and no need for a rainscreen cladding space, allowing for a 12″ deep single insulation cavity for dense-pack cellulose, housewrap and horizontal shiplap wood siding, preprimed on all sides.

    see Pictures and Details for more.

  2. ergodesk November 5, 2013 at 12:49 am #

    Why are you using all that OSB, that rots and creates mold when wet?

  3. foursevenfive November 5, 2013 at 12:03 pm #

    Ergodesk. The OSB is on the warm side of the insulation – it won’t get wet or humid. Please take a look at the detail. We use it as an airbarrier and vapor retarder, taped w. TESCON VANA. The SOLITEX MENTO WRB over the exterior fibrous insulation will keep it warm and dry.

  4. Miles Scott February 9, 2014 at 6:37 pm #

    I am very interested I this concept but am somewhat worried about insects entering through the somewhat “loose” exterior envelope. Has anyone had problems with insect infestations in the walls?

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