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Re: GBlist: vapor permeability
We build with structural insulated panels and get wrapped up in the vapor
permiability issue frequently. Here's mt two cents:
Moisture drive in a built environment's exterior walls is never a good
thing- I don't care where the vapor barrier is (which, by the way,
depends you're in a heating or cooling climate). In heating climates, it
can be especially pervasive and hard to control in the winter, unless (as
you will have the opportunity) you design and build well from the start.
We prefer to insulate extremely well, seal the envelope very well (which
is comparatively easy to do with panels), ventilate the home well
utilizing air-to-air heat exchangers (HRV) pulling air from bathrooms,
kitchens, and other high moisture producing areas, and slightly
negatively pressurize the building (-1 pascal) to help keep moisture from
wanting to drive into the walls. However, this means being keenly aware
of the need to seal combustion on other appliances such as water heaters,
so that combustion gasses are not drawn into the home. It also can be
problematic for wood burning devices, unless they too are sealed
combustion (= have really good outside air intakes).Front costs are a bit
higher with such a system, but payback is relatively fast.
The devil, as they say, is in the details.
That's my opinion. One of many, I'm sure. But it seems to work for us.
Good Luck!
Chris Koehn
Top Ridge Homes
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