Revisions log for Answer #2818

The vapor barrier must prevent moisture from getting from the living space into the insulation so that it does not get moldy

Is an OSB board a substitute for vapor barrier? I've never heard and have a hard time believing that OSB is inherently vapor-proof, but I'm willing to prove that it isn't.

There is a difference between a vapor barrier and a vapor barrier.

There is also summer (warm outside = warm insulation) and winter (cold outside = cold insulation).

If steam goes from the inside (quite warm) to the insulation (if warm), not much happens, the steam passes through the insulation to the outside and the insulation dries out.

If steam from inside (quite warm) reaches the insulation (when cold), the steam condenses and the insulation becomes damp/wet and can't dry out because of the cold. Mold can form.

A "climate membrane" seems like the perfect hero. When it's warm outside, it's "automatically" open to diffusion (more like a vapor barrier). When it's cold outside, the membrane makes you (barrier). Overall, there is less risk of mold (at least theoretically).

A vapor barrier is only necessary where steam is generated (people) and insulation protection is needed.

OSB as an attic floor has no effect because it is installed over the insulation.