Part of my basement has 2" XPS board fastened tight to the block.  It has been like that since I have had the house.  The rest of the basement was just left bare block and no insulation.  I'm in the process of finishing the rest of the basement.  I notice that the tape at the seams in the insulation board has water behind it.  Well, I happened to cut a hole in the board and the walls are completely wet behind the foam.  I'm not sure I understand why this is happening.  This part of the basement is completely underground.  I measured the block wall temp when I took the piece of insulation out and it was within a degree or two of the inside basement temp.  I don't believe its condensation.  I know that the foundation is damp proofed and has a footer drain.  Downspouts are buried and drain far away from the house. 

My question is: Is this something I can ignore and eventually the moisture go away, or do I  have to remove the insulation and try and get the wall dry the replace the insulation?

Thanks

Colby

Thank you for the post. I have experimented with putting up extra insulation board and also foil bubble sheets on interior walls. They do provide significant extra insulation, but build-up of moisture behind them and mold growth is an issue. I have yet to understand the whole vapor barrier strategy and which layer should be the barrier. I gather we don't want two barriers with space between them.

Another issue is the fire resistant characteristics of the foam board, all says the we should add other resistant board in front of it. I've been waiting years for the mushroom foam board insulation to become available in our area. Youtube video's show it resisting a blowtorch.

Any info on this appreciated.

It's condensation alright but not condensation from the room air. It's either fully saturated groundwater moisture wicking up from the footing (drain notwithstanding), or outdoor air that's getting into the empty CMU cores to condense.

As long as you have a good capillary break between the foundation sill and the top of the foundation (EPDM or sheet metal, not 1/6" thick foamy stuff) you can leave it.

If you don't have a sufficient capillary break at the sill, the moisture content of the foundation sill needs to be measured- if it's under 20% it's of little concern- the foundation is drying fast enough to the exterior on the above-grade section. If it's 30% it's a serious rot hazard and you need to do something about it.

Removing the bottom foot to 18" of the foam near the slab should allow the foundation to dry into the basement, without taking a huge hit in thermal performance. If you do that, give it a month or so, and re-measure the moisture content of the foundation sill. (Or just monitor it once a week and keep a logbook of those measurements.)

BTW: The aluminized bubblepack stuff isn't worth the miniscule thermal performance it delivers, and it's a true vapor barrier, which causes more problems than it solves. At 2" the vapor permeance of XPS is about 0.6-0.8 perms- it will allow some drying. Bubble pack is less than 0.06 perms. In a basement application sheet foam trapped against the foundation by a fiber insulated studwall with NO vapor barriers works, as long as the R-value of the foam is sufficient for wintertime dew point control at the foam/fiber boundary. (R4 EPS with R11 batts works even as far north as US climate zone 5 from a dew point control perspectives, but wouldn't quite meet code for thermal performance. R5 XPS and R13s would.) Kraft faced or unfaced batts are fine, foil facered batts are not.

If you're going to use a real vapor barrier, use foil-faced foam of sufficient R for the climate, but stop the foam a foot from the floor if there's any question about moisture wicking up to the foundation sill, unless you have an EPDM sill gasket or metal flashing as a capillary break.

Dana, thanks for the reply.  There is only a foam seal between the sill plate and the block.  I've never seen it done the way my house is,  but the top plate is actually a 2x6 and a 2x4 with about 2inches in between the two.  Not sure if its even allowed by code.  The strangest part is, this is only happening on one wall, the rest of them that are covered are dry as a bone.  The house is located in south central PA.  Just took some measurements, the sill plate is around 9.5% and the floor joist around 12-13%.  If I let it go, will I have a mold issue?