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The vapour barrier must be installed on the warm side of the wall, that is on this interior side of the house. It is permissible to have it sandwiched in the middle between two layers of insulation, but only if the cold side insulating layer is at least two thirds thicker than the warm (source).

My question is what happens, exactly, if the barrier is actually on the colder side? e.g. two-thirds warm insulation, barrier, one third cold insulation? This is in a very cold climate (-40 in winter not unusual). And what can be done to mitigate this situation?

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You put the vapor barrier on the inside of insulation to prevent cold air from getting near the vapor barrier. If warm, humid air is touching a cold surface, the water will condense out, just like you see on a glass of ice water on a hot, humid day. This condensation is a bad thing, since the presence of water over a long period of time will cause mold to grow. The moisture may reduce the effectiveness of your insulation. It may cause dry rot in your home. It may cause other problems, attracting bugs who like moist places.

How can you fix this? Turning the insulation to lie in the proper orientation is clearly the best solution.

I suppose one could put an additional layer of vapor barrier on the inside. That will stop the inside, moisture laden air from getting to that middle layer. The problem is, this will create a layer of insulation that does not breath. Any moisture in there will never escape, and you will have the same problems as I mentioned above. So this could potentially be a bad solution.

Better seems to add more insulation to the outside, with NO vapor barrier on it. That will prevent the cold air from getting near the vapor barrier. It may not be possible, or even terribly easy. But this is why you try to do things the right way the first time.

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  • unfortunately adding more insulation to the outside is not feasible, exterior masonry etc. is already in place and there's no room in the studs. Looks like we will have to to do the almost as difficult thing of removing insulation from the inside to compensate. Or perhaps remove inside, punch a bunch of holes in the existing vapour barrier, put the insulation back with a proper barrier on the inside.
    – matt wilkie
    Sep 28, 2010 at 0:06
  • Soft insulation such as fiberglass doesn't achieve its full insulation value if it isn't air-sealed on all sides. (Look up the term "air-washing" and "convective heat loss".) When adding soft insulation to the outside, use an air barrier that is not a vapour barrier.
    – Ian D. Allen
    Jul 30, 2015 at 5:28
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Water could condense on the inside of the vapor barrier, which is what you don't want to happen.

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