Using double layer of foil faced rigid insulation together

[cbc58]

Someone is selling rigid insulation panels that are enclosed in foil for a very good price.  They are 1.5" R13.  Wonder if there would be any issues with condensation or moisture if you were to use two panels together in a 2x4 wall.  Tried searching the web for info but finding varied opinions.  Any info appreciated.

[Don_P]

I've used 3 layers of foil faced polyiso, continuous(no breaks) on a roof assembly. I've done two 2" layers of unfaced closed cell XPS between studs cut ~1/2" undersized and shot in with great stuff. Actually I've done that on a couple of old houses, they are 5-7 years out with no reports of trouble. If it is white beadboard, EPS, I would be hesitant as it can hold water.

If vapor passes through an assembly in winter and hits the cold outer sheathing it will condense. If the sun drives vapor inward through the assembly in summer and you air condition the wall cooling the drywall surface below dew point, then again vapor will condense. The question is, how much, how much hygric capacity do the materials have, and how fast can they dry. There is the art and risk as local conditions, modes of HVAC, methods and materials are constantly changing. Think it through as clearly as possible and roll the dice. Totally OT aside, I'm trying to figure out how and where to bury some sensors in the current old building rehab. Things will change as I improve the fire rating and energy efficiency but it might not be for the better. In my case when we insulate the masonry walls I need to know that the wood beams and rafters that are inserted into pockets in the walls are still staying dry and that condensation is not happening on them or in those pockets that were heat leaks but free to dry. 

[cbc58]

Thank you for the info.