Re: foam insulation board over fg insulation
In a sense yes this could potentially trap moisture between the foil backed foam board and the kraft paper faced batts .... considering both are vapor retarders.
In the case warm moist air was able to get in behind the drywall through penetrations from electrical or plumbing for example .... this may be trapped between the 2 layers and not allowed to breath out.
If you decide to use the foil backed foam then use unfaced batts .
In my opinion save yourself a few bucks and use extruded foam sheets without the foil .... for the method you're using it this isn't really benificial.
Regardless .... ensure to seal the butt joints where the sheets come together with house wrap tape . Also where the edges of the foam sheets meet the ceiling , corners and floor leave a gap of around 1/4 inch and fill with canned foam.
Having the foam sheets completly covering the studs directly behind the drywall will definately help the insulation performance.
Hopefully this helps.
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