I have found a great deal on some used Roofmate XPS insulation that I would like to use to insulate my basement. I want to acheive R25 so I will be using 4" thick R-20 with a second layer staggered of 1" R5. I have a few questions around placement of this on the wall. Some suggestions are to place the XPS directly against concrete using adhesives/furring strips for drywall attachment. I have been unable to find any sort of fastener that would be 7" long to go through strapping and 5" of foam into the concrete wall.
Another concern that I have is allowing for the assembly to dry to the interior. If I used thick XPS directly against the concrete, this would prevent any drying to the interior - would I have moisture buildup against the concrete, but is this not a problem since the concrete is porous and may be damp regardless?
One option that seems like a good idea would be to install 1" strapping directly to the concrete first, serving as a nailing surface for securing the 5" foam with long nails/screws. This would also provide a channel for air movement and prevent moisture buildup - which would then just seep back through the porous concrete?
The basement does not have any serious water problems but there are a few cracks that I will first fill with hydraulic cement as a precaution.
Look forward to hearing some thoughts!