Choice of rigid exterior roof insulation
The February 2014 issue of the Journal of Light Construction has an article starting on page 41 titled “Insulating Cathedral Ceilings.” On page 46, the second paragraph under “Rigid Foam Above Sheathing,” there first is mention of polyisocyanurate and XPS rigid board as options for this purpose.
Then there is a statement that appears to be contrary to what has been said here and elsewhere about the use of EPS board: “Expanded polystyrene (bead board), which has a high water absorption rate, is not recommended on roofs.” This seems to be at odds with recommendations here on GBA and elsewhere (eg. http://www.nationwidefoam.com/roofing-insulation.cfm). If EPS were inappropriate for a roof for reasons of water retention, why is it OK for use as sub-slab insulation, and why would it be OK to use polyiso on the roof, a material which is NOT recommended for buried applications for the same reason? Am I missing something?
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