Metal is only impermeable if it's a continuous sheet. A building is not a continuous sheet of metal. Go stand in a building with just a metal skin on a cold, windy day and you'll discover that air easily gets through the seams and gaps of the metal exterior, and if air can get through, then moisture can too. A tyvek style moisture barrier will reduce both air infiltration and moisture from the outside. You should also have a vapor barrier on the inside of the wall to keep the warm moist air from penetrating into your wall cavity.
In the form of humidity coming in with air, yes. In the form of water no. A metal sidewall is no more leaky than a metal roof made of the same material. Tyvek won't stop water from leaking in through defects on a sidewall anymore than it would on a roof assembly and we don't use (regular) TYVEK under metal roofs to stop rain from coming in.
What the tyvek does do is allow water that has leaked in through defects to evaporate back out so it doesn't stay trapped in the wall cavity. If there is metal siding, the metal doesn't breathe enough to allow this to take place so the Tyvek is just a waste of $$$$.
Condensation occurs when warm, moist air contacts a cool surface. If you're heating the space inside of your building, then you should have a vapor barrier on the inside of the wall, and insulation within the wall which means the moisture vapor never gets into your wall cavity, and the warm air never contacts the cool metal. Condensation doesn't have the chance to form if the building is done properly.
You use the air-tight condensation blanket to keep interior air from ever contacting the steel. It's air tight as well as insulative in order to keep the surface that interior air will contact above the point where moisture would condense.
You can then fill the cavity with normal bat insulation along with a vapor barrier just as you stated. The difference is the wall cavity is now entirely sealed (air-tight) from both inside and outside air and moisture.
This sounds like the typical method for pole frame construction, where horizontal girts are nailed to the outside of the poles, and the siding is then fastened to those girts. If Tyvek is used, it would go between the girts and exterior siding, directly beneath the metal. There's no need to fir it out anymore because the ribs in the metal panels allow airflow.
In this case you would use the condensation blanket which is an airtight barrier (it's vinyl or something stronger) encasing some type of insulation I.E. foam or fiberglass in lieu of using a breathable building wrap like Tyvek.
The ideal wall cross section should be an assembly like this:
Interior of building>>>vapor barrier>>>insulation>>>moisture barrier>>>exterior siding>>>outside air.
Yes but the siding type matters. Is the siding breathable I.E. wood or maybe hardi or is it non-permeable like metal.
The vapor barrier keeps the interior moisture from getting into the wall in the first place, the insulation keeps the warmth from reaching the cooler surface of the exterior (where it could condense), and the moisture barrier on the outside keeps the bulk of the moisture outside from getting into your wall, while also letting it vent to the outside if necessary.
Yes but interior vapor barriers only stop so much. Humidity still gets through and will pass through the insulation, through the Tyvek and condense on the back of the metal where it stayed trapped and begins to soak any wood that's behind it.
If the metal is firred out, the moisture can pass through the Tyvek to the outside vented cavity.
If there is no way for the tyvek to breathe to the outside then you don't want to use it. You want to use a sold barrier that will block interior vapor and air from being able to contact the siding.
A condensation blanket performs both those functions.
My neighbor, a retired carpenter, was repairing some rot on another neighbors 30 year old pole building caused by condensation pass through insulation.
Point being, he said vapor barriers have always been a topic of endless debate and probably always will be.
