There is a fear that the new exterior insulating sheathing will act as a vapor barrier, trapping moisture in the wall. Such walls may originally have little or no insulation and a separate interior vapor barrier may be absent.

In retrofit applications, STYROFOAM brand extruded polystyrene foam insulation can be used under new siding finishes. Also, some allege that STYROFOAM™ Brand Extruded Polystyrene Foam Insulation does not 'breathe' as some competitive products claim. STYROFOAM™ Brand Extruded Polystyrene Foam Insulation products are usually targeted because of their low water vapor permeance. This moisture is thought to cause the studs to rot and the building to decay. Much has been written on the so-called 'double vapor barrier myth.' This is a misconception that water vapor will become trapped between the vapor barrier on the warm side of the wall and a low permeance sheathing, like STYROFOAM™ Brand Extruded Polystyrene Foam Insulation, on the cold side. Will the use of STYROFOAM™ Brand Extruded Polystyrene Foam Insulation on the outside of a residential wall create a double vapor barrier situation? Now that I see that is has the plastic sheet on both sides of the foam board. I'll read through that document tomorrow and see what I can decipher out of it, but it does explain the foam sheathing I'm seeing. The corners of the building do not have the foam board and instead have OSB, you can see it in the 'before' picture.There is no markings on the foam boards (at least on the inside that I can see) so I am not sure what type of foam board insulation it is specifically. I just checked a piece I had removed during a floodlight installation it is in fact lined with plastic sheet on both sides of the foam insulation. Then it is the foam board insulation, about 9/16' thick.a type of compressed fiber board? Looks like wood siding but is not wood.

Exterior is I think hardi-board or hardi-plank something like that.

Hey, that looks right up my alley! I read through some of the vapor section but some of it was a little over my head so I'm going to read through the whole thing tomorrow when I can get a chance.īut it seems my answer may depend on a few things so here are some details:ĭetached garage, currently not heated/cooled but I would like to do that in the future (1-2 years), until then I had just planned on insulating the walls/ceiling and having some shop fans in there.

I slit a sheathed sheet