What's The Difference Between Air Barrier and Vapour Barrier?

Casey Grey is back to answer another question from a viewer. This week he answers the difference between an air barrier and a vapour barrier.
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[@twofacepalmer1212]

Consensus on this subject has not been established. I live in SoCal and have spoken with a building contractor (because I'm in the process of renovating my bathroom ) about which type of barrier I should use because when I tore the original gypsum board down there was a .006 Kraft paper type of barrier (not the asphalt based type) all along the southern wall. I initially was going to replace it with 6 mil poly and when I mention that, he laughed and said not to even put up a barrier. After that I started watching numerous CZcams vids on the subject and found out there are two different camps on barriers. One side says they can actually create problems instead of preventing them, especially poly barriers. Others say it depends on where you live and what rooms are being discussed. So basically, for every yes... there's a no on the subject. Even one CZcamsr said to disregard vapor barriers altogether... doesn't matter where you live.
So what is one supposed to do?... lol. These barriers have been around, I guess since the 1940's, but I think the problem is the fact - even though there have been mold problems in houses without barriers... there have supposedly been houses with barriers which still suffered from mold build up. Of course, those could be attributed to small holes allowing humidity from the outside to enter the house or cold air (from air-conditioning?) to penetrate through the walls and the water from the cold air gets trapped.
You bring up the analogy of the body needing to breath... well... live in a body wrapped up in plastic and see how well the body does after awhile. Poly traps moisture and water droplets form up on plastic sheeting and that's where problem begins. Perhaps going back to the asphalt based Kraft paper is the way to go. Do you put the barrier on the exterior (behind the exterior siding) or in the interior walls (behind the drywall)? Do you put it under the subfloors in the construction of a new house? Do you install it into the ceilings? Are you trying to keep moisture in or keep it out, or both?
Way too many variables involved and IMHO, the house DOES need to breath and it can't breath if it's all wrapped up in something that will not allow air/vapor/moisture to pass through it's walls without becoming trapped.
I came across a short video uploaded by 'Hat Home Inspections' called *Vapour barrier on interior walls, bad idea! "On the home inspection today"* who gives a pretty good explanation.
I think barriers have been code for many areas but until it's set in stone, I don't think it's a good idea to generalize on barriers until a scientifically based test and standard has been set and all contractors and inspectors can agree.
One thing that interest me is the permeable air barrier. I might be looking into that.
anyways, just MHO. Cheers... 😁

[@Theconsciousbuilder]

There is a general consensus among building science experts and product developers on how air and vapour barriers should be used. Consensus among contractors is another matter. There are many factors based on location, project details and climate that vary the best course of action. Good luck on your project!

[@andreycham4797]

I would insulate all walls in a bathroom with closed cell foam only with a weather barrier on outside . Where you put your vapor barrier inside or outside depends on heating season , let's say I have to heat my house almost 7 months a year and only 2- 3 months to cool since energy goes from hot to cold vapor pressure will be from inside out for 7 months a year and only 2 months from outside in . So to stop most of the vapor coming to my walls I would put a vapor barrier on inside which I would consider as an air barrier as well