Raise the Hammer

To understand the position of the vapour barrier, one has to consider where in the wall depth the dew point is reached. That is, based on both the level of humidity and the temperature outdoors vs. indoors, the point at which dew will form. If you were to take temperature readings at different depths in the wall, it is clear to understand that the temp will be fairly warm in the inner positions, and likewise quite cool towards the outside (winter). By placing the VB closer to the inner side, it will remain on the inside of the measured dew point. The moisture will never be able to get further out to the dew point, precisely because the VB will prevent that from happening.

The general consensus in the low energy construction technology business is that the VB should have about two thirds of the insulation to the outside (thus keeping it nice and warm, and well above the dew point) and the remaining third on the inside of that layer. This will ensure integrity 99% of the time. The one percent would entail more detailed analysis for buildings in extremely hostile and unusual environments.

The second key to the VB working, is that it cannot leak air. If it does, then the humid indoor air will get past the VB and meet up at the dew point in the middle of the insulation somewhere and condense. Detailing the VB is 100% critical. And yes, as a tradesman myself, I agree that thus far almost no trades appreciate this, let alone understand it. Whenever I have discussed this to others I have been viewed as if I had three heads. It is a shame because I would wager a guess that if the current (building code) VB were detailed properly, Canadian homes would likely use 10% to 20% less energy to heat. I am not personally a big proponent of Kyoto, but for those that are, that gain in efficiency would likely get us to where we need to be all on it's own, or close.