Editing Polystyrene (section)

==== Water absorption of polystyrene foams ====
Although it is a closed-cell foam, both expanded and extruded polystyrene are not entirely waterproof or vapor proof.<ref>Gnip, Ivan et al. (2007) [http://leidykla.vgtu.lt/conferences/MBM_2007/1pdf/Gnip_Kersulis.pdf LONG-TERM WATER ABSORPTION OF EXPANDED POLYSTYRENE BOARDS] {{Webarchive|url=https://web.archive.org/web/20180128002027/http://leidykla.vgtu.lt/conferences/MBM_2007/1pdf/Gnip_Kersulis.pdf |date=28 January 2018 }}. Institute of Thermal Insulation of Vilnius Gediminas Technical University</ref> In expanded polystyrene there are interstitial gaps between the expanded closed-cell pellets that form an open network of channels between the bonded pellets, and this network of gaps can become filled with liquid water. If the water freezes into ice, it expands and can cause polystyrene pellets to break off from the foam. Extruded polystyrene is also permeable by water molecules and can not be considered a vapor barrier.<ref>[http://www.foamular.com/assets/0/144/172/174/e45fe07d-5cc9-4e4b-866a-5e35d75090ec.pdf Owens Corning FOAMULAR Extruded Polystrene Insulation: Resisting Water Absorption, the Key for High-Performance Foam Plastic Rigid Insulation], ''Technical Bulletin'', Pub. No. 10011642-A, September 2011,</ref>

Water-logging commonly occurs over a long period in polystyrene foams that are constantly exposed to high humidity or are continuously immersed in water, such as in hot tub covers, in floating docks, as supplemental flotation under boat seats, and for below-grade exterior building insulation constantly exposed to groundwater.<ref>[http://epsindustry.org/sites/default/files/EPS%20Below%20Grade%20105.pdf "XPS Insulation Extracted After Field Exposure Confirms High Water Absorption & Diminished R‐Value"] {{Webarchive|url=https://web.archive.org/web/20150206100709/http://epsindustry.org/sites/default/files/EPS%20Below%20Grade%20105.pdf |date=6 February 2015 }}, EPS Below Grade Series 105, March 2014, Technical Bulletin, EPS Industry Alliance.</ref> Typically an exterior vapor barrier such as impermeable plastic sheeting or a sprayed-on coating is necessary to prevent saturation.