The drywall screw perforates the vapor barrier

[b8q]

And all the hundreds of thousands of houses that don't have any plastic film at all, how come they haven't rotted away?

Now, I don't have a research report at hand, but it COULD be for the same reason that old cold attics with only wood shavings as insulation fare better than if you add insulation with mineral wool or glass wool - the shavings can absorb moisture and dry out again without significant damage.

It's probably also about the temperature gradient in the wall, _where_ moist air condenses. In my old house where the outer wall lacks plastic and basically consists of two standing plank walls with a 2-3 cm wood shavings gap between them, the wall becomes so cold that I've sometimes gotten frost on the inside(!) of the wall. So if old walls insulate so poorly that the wall is cold all the way through, maybe no condensation occurs _in_ the wall?.. An old log cabin clearly works well in terms of moisture, but is it energy efficient? No.

 

[Alfred Jonsson]

I don't have a research report in my pocket, but it MAY be the same reason that old cold attics with only wood shavings as insulation fare better than if you add insulation with mineral wool or fiberglass - the shavings can absorb moisture and dry out again without significant damage.

It's also about the temperature gradient in the wall, _where_ moist air condenses. In my old house where the outer wall lacks plastic and basically consists of two standing plank walls with a 2-3 cm wood shavings gap in between, the wall becomes so cold that it's happened that I've gotten frost on the inside(!) of the wall. So if old walls insulate so poorly that the wall is cold all the way through, maybe no condensation ends up _in_ the wall?.. An old log cabin evidently functions well in terms of moisture, but is it energy efficient? Nope.

There's certainly a lot of truth in what you're saying, but the fact remains, in the houses where I as a carpenter most often see moisture/mold (when it's not a matter of water damage) is when there's been plastic sheeting.

A significant problem is likely that people don't have the heat on year-round in their homes and that the houses are too airtight.

I was in a "summer cottage" a few months ago and the mold smell was so horrible that I had to apply tiger balm on the dust mask to avoid smelling the disgusting mold odor while we were tearing it down.

I myself have a number of summer cottages/small sheds and don't have a moisture barrier in any of them, and all have fared well over the years without moisture or mold.

 

[injonil]

In summer cottages, it's very inappropriate to have plastic, that's common knowledge, isn't it?