Artificer wrote:
Rob: I've always wondered: in a heating area like Wisconsin, why do contractors use foam and seal/tape all joints on the OUTSIDE of the house? I know about the vapor barrier on the inside, but the outside taping seams to be a moisture trap.
You are not the first to have that concern.
Under ideal circumstances the second layer can act as a vapor barrier, in theory trapping moisture in the cavity.
Studies have shown however, that unless you have a very poor, or nonexistant vapor barrier inside, that condensation rarely forms on the cold side (in the cavity next to the exterior wall).
It amounts to keeping as much warm moist air in (the living space) and as much cold outside air out (of the cavity), as possible.
When those goals are met, the cavity performs well as a dead air space.
There are still so many studies, and theories floating around that the questions of "how, when, and why" are hard to define with a "one size fits all" answer.
Some advocate no barrier, some a breathable barrier, some suggest air infiltration control will negate the effects...
My answer? Seal the exterior well, use a vapor barrier or retarder, and remember ventilation is the key.
Control the humidity level, and there is no issue.
As Fred pointed out, all the interior sealing won't stop water vapor from being transmitted through wood framing, but, if the
warm moist air is properly contained inside the structure, and there are no exterior leaks, there will be no excess moisture inside the walls to do harm.
There will always be some moisture inside a wall.
The air always has some moisture in it.
As long as the air inside the wall, and the air outside the wall are the same general temperature, the moisture won't condense, and be deposited inside the cavity.
As one might imagine, those who are successful in getting a perfect seal, are those who will soon have very stale air, venting is a must!
Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...