GPS vs EPS vs XPS
I would appreciate any comments about Graphite Polystyrene (GPS) as an insulation product. It appears to have an R-value of 5 (per inch) and a permeance of 4 for a one-inch product depth. In addition, the graphite in the polystyrene product causes the R factor to trend higher (!) with colder temperatures (for example, 5.0 at 75 degrees F; 5.2 at 40F; 5.4 at 25F for Type I).
Is there anything not to like about this product? I am considering it for continuous insulation under vinyl siding. I do not have room to have a thick layer of insulation (to overcome the dew point concerns), and need to have a product that allows for drying to the outside.
For background information, I am in climate zone 5 (New England), using 2×6 studs and planning to use R-23 rock wool insulation in the cavities. The sheathing is 1/2″ plywood which will be taped for air sealing, then covered with a WRB (HydroGap?) before some insulation and vinyl siding.
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