[The Exiled]

NZXT blog

The coolant inside of a liquid cooler is a mixture of propylene glycol and distilled water, with the split of these depending on what environment the manufacturer expects the cooler to occupy. This mixture doesn’t “evaporate,” per se, but does slowly permeate the tubes containing the cooler. To reduce the risk of permeation, we recommend to install CLCs with the tubes at the bottom of the radiator (if vertically oriented, e.g. front-mount). Avoid installing CLCs flat in the case, with tubes facing upward.

Izvor: Gamers Nexus