How does "breathable" siloxane water sealant work?

*(This is an interdisciplinary question, between molecular chemistry, materials science & [construction](https://diy.stackexchange.com/questions/15375/what-type-of-concrete-sealer-to-use-to-prevent-frost-heaves-in-my-driveway). I think Chemistry SE is the right fit, but please move it if needed.)*


Silane/siloxane solutions are applied to porous building materials (e.g. brick or concrete) to repel water. Unlike a solid coating, these sealants are "[breathable](https://www.google.com/search?q=silane+OR+siloxane+breathable)", allowing water vapor to exit instead of being trapped, which would cause slow structural damage. How does that work, molecularly?


I presume these compounds affect surface tension in some way, reducing the ability of liquid water to permeate, without completely sealing the pores, but I don't understand it.


Optional: also, how does this sealant affects the permeability of other liquids (polar vs non?) and other gases?




According to [How silicone water repellents work](http://www.dowcorning.com/content/discover/discoverchem/si-repellents.aspx), 
"Because silicones have a very open molecular structure, water vapor trapped in the substrate can easily slip through. Water molecules, however, are too large to enter."


If I remember correctly, siloxanes' Si-O bonds are not *intrinsically* hydrophobic, but the long molecular chains with outward-facing hydrophobic methyl groups, and thermal molecular motion, prevent liquid water from seeping through, which presents less of a barrier to water vapor and other low-molecular-weight gases.


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