How is that a material can be both airtight and vapor permeable?
Can anyone point me to an answer that explains the physics? Obviously to be vapor permeable a material must have penetrations that are bigger than vapor molecules; why doesn’t air, driven by an air pressure differential, flow through the same holes? I’ve heard it said that the pores are big enough for H2O molecules but too small for O2; is that it? Or is it simply that the rate of air leakage is too low to matter given the grosser methods used to measure air leakage compared to vapor diffusion? Or is there some force binding the molecules in a volume of air that prevents them from convecting through very small openings. Or perhaps a force greater than air pressure that retards them within the pores? I discovered in Straube’s Building Science for Building Enclosures that liquid water tends to exist in clumps of about 80 molecules and that this phenomena allows for materials to be at once watertight and vapor permeable. Is there some similar clumping phenomena occurring in air?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part