Insulating a Concrete and Framed Basement
I have learned a lot from all the great info here, but I haven’t seen an answer to my particular situation:
We have an addition built about 5 years ago where the basement consists of 6′ tall poured concrete walls below grade and about a 3′ tall section of 2×6 construction above grade. That was originally finished with “blanket” insulation hanging on the concrete and fiberglass bats and drywall on the framing.
We are now working to finish the space, so I am replacing the concrete insulation with polyiso and plan to frame a floor to ceiling wall inside that. That will effectively hide the drywall+framed section (which has a vapor barrier on the exterior, but nothing on the interior of the sheathing.)
So, the questions:
1. Should I just leave the drywall and fiberglass in place? Will that create a moisture issue behind the new wall? Alternately, I could remove the drywall (and the fiberglass?) and run polyiso continuously over the studs (or cut and cobble).
2. If I stop the polyiso at the top of the concrete and also place a horizontal piece on the sill, can I seal to the drywall (can foam) or should I at least cut back the drywall and seal to the sill plate?
Sorry for the long description, but I appreciate any help!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I'll ask a question to give your post a bump.
There is no evidence of bulk water intrusion into the [basement], correct?
Thanks Steve. There is no evidence of any bulk water intrusion into the basement.
>"...which has a vapor barrier on the exterior, but nothing on the interior of the sheathing."
Describe the exterior vapor barrier.
Which IECC climate zone are you located in?
https://beta.greenbuildingadvisor.com/app/uploads/sites/default/files/images/DOE_climate_zone_map.preview.jpg
Forgot to mention the zone -- I am in 5A.
The vapor barrier is Grip-Rite (https://www.grip-rite.com/product/house-wrap/) over OSB. The 2x6 sections also have stone veneer over the top of that. No exterior insulation.
Deleted
One last bump to see if anyone has any insights. I am attaching an image of the existing wall assembly and my proposed finished version.
Your plan is pretty good so far. I would use foil-faced polyiso on the walls, to have a true vapor barrier against indoor moisture condensing on the concrete walls. If you think there is a chance of ever having water on the floor (this is a basement and there's always a chance), then hold the polyiso up 2"-3" off of the floor and do that last bit with EPS instead. It can take some flooding. Skip the air gap you're showing on the plans. Fill the whole cavity with fiberglass or Rockwool. Rockwool can also take some (clean) flooding without collapsing like FG does. I would also recommend applying latex vapor retarder paint on the face of the exisitng drywall on the framed pony wall. Putting the vapor retarder in the middle of the wall is a good thing in your climate. Finally, pay attention to the joints at the pony wall-band joist-subfloor transition. Either very careful caulking or spray foam is generally best to seal and insulate this area.