Hi again. i have heard of OSB panels, et al, as being a ‘smart’ vapour barrier/retarder in that it becomes more vapour permeable the wetter it gets. assuming ‘smart’ is a good thing, why is it a good thing that a vapour barrier becomes more permeable the wetter it gets?
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A smart vapor retarder isn't always desirable. For example, you don't want to install a smart vapor retarder under a concrete slab or on the bare dirt of a crawl space floor. In these applications, you want a stupid vapor barrier like polyethylene.
For some wall and ceiling assemblies, however, a smart retarder makes sense. In these applications, a buildup of moisture is a warning sign that something bad is happening. The wall is getting wet. When that happens, the smart retarder absorbs moisture, thereby becoming more vapor-open. As the smart retarder becomes more vapor-open, it allows inward drying to occur. This drying helps prevent the wall from becoming so wet that mold or rot develops.
i buy that. thanks again, martin