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Tracy
Condensation between fiberglass insulation and sheathing in basement
Tracy

Seeking some answers on why we are getting condensation between fiberglass insulation and sheathing in the basement.  We are located in Chicago and are renovating our basement.  We air sealed and re-insulated the existing exterior 2x6 walls with unfaced fiberglass (basement is not a walkout but a garden style basement with some full size windows).  The new basement walls are built roughly an inch off the foundation and we plan to insulate those with R-13 and then cover with 1/2" drywall.  We were about to move on to insulating the new walls when we noticed that the sheathing behind the 2x6 exterior wall is wet.  It's been very cold here but what is strange is that this is happening on only the east side of the house.  I pulled back the insulation on the south wall and the sheathing is dry. 

As far as I know, the exterior consists of cedar siding, house wrap, sheathing (not sure how thick), & R-20 unfaced batts.  The basement is cool at around 62-64F and the humidity is around 20-30%.  Originally there was no heat down there so we added some new forced air supplies but don't have a return down there yet.  No evidence of mold from when we replaced the original insulation on the exterior walls so I have no idea if this has been happening over the life of the home (built in 1999).  

Exterior temps have been in the teens with wind chills in the negative.  We are afraid to go any further with the basement construction before we resolve this issue.  Does anyone have any thoughts on how to avoid/resolve this issue and avoid any major mold issues in the long term.  Could this be happening on the east wall of the floors above?!  Why is this only happening on the east wall?    

On another strange note, in the warmer months, when it rains for long periods, we can see water on the sheathing in certain areas (likely coming from the window on the first floor) on that same east wall in the basement.  We don't notice anything wrong with the cedar on the exterior so this issue could be related to a window issue, and not a temperature issue like we are seeing in the winter.  

 

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