Housing and Community Development

Whatever happened to the KC that was about technics?  But that's not my real question here.  I am trying to effectively retrofit additional insulation on a house in Alaska.  I would like to put a ventilation channel between the added external insulation and the exterior sheathing of the now uninsulated exterior wood framed wall.  But I wonder if I am thereby isolating my insulation from the interior of the building.  Put another way, will this ventilation channel come to be the exterior ambient temperature, thus defeating the value of the insulation outside of it.  I note that dryvit has a system where they provide ventilation behind their insulation.  Do we know this works?  Do we know it does not defeat the purpose of the insulation?  Is there a standard for the size of this ventilation space beyond which the insulation becomes irrelevant?
Hope the CRAN guys are still reading this discussion forum.  I would send this to you but I've not yet been certified as a member of your community.
Mike Mense FAIA
Owner
mmenseArchitects
Anchorage AK
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Rand Soellner AIA
Architect/Owner/Principal
Rand Soellner Architect
Cashiers NC
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Hi Mike,
You might want to see what Building Sciences Corp. has to say about such matters.  I believe they will likely agree with your concerns.  And the larger the ventilation gap, the more ambient air temperature will render your outboard insulation irrelevant.  It might be better to directly adhere rigid insulation to the exterior wall sheathing, assuming that the type of rigid insulation will also perform well as a vapor barrier, to prevent water from condensing on the face of the sheathing.  The University of Mass. also has a study that indicates that good old asphaltic felt (15#) perform almost perfectly as a proper vapor barrier, because it will also allow water vapor coming from within the wall itself (and from the interior of the house or building) to move slowly from the interior through to the exterior.  Let me know if you would like to see that study and I'll send you a link.  Then install a vent gap, using p.t.t 1x2s or something similar, if you want to vent your finish material, then install your finish materials over that.  In that manner, your finish materials are vented front and rear and they will probably never rot, if they are of a hardy species know to perform well in your climate zone. 
I agree with you completely that install a vent area behind exterior insulation doesn't seem to make a lot of sense.  I know that Dryvit used to have a grooved rear surface behind its rigid insulation that allowed any moisture there to drain out through weeps.  Having microscopic weeps is far different from installing much larger vents.  At least, that's the way I see it.  I wish you the best up there in the cold North. 

Okay Mike, here is the link to one of my webpages:

http://www.homearchitects.com/vapor-barriers-in-residential-construction

this page talks about vapor barriers in frame construction for houses and also, if you scroll down through it, you will see a link to the study about which I mentioned at the Univ. of Mass.  All my best. 

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Rand Soellner AIA
Architect/Owner/Principal
Rand Soellner Architect
Cashiers NC
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