How does "breathable" siloxane water sealant work? (This is an interdisciplinary question, between molecular chemistry, materials science &amp; construction. 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", 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, 
"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.