Compared to new construction, renovations are often a more complicated game, because you have to play with, and sometimes overcome, the cards that you are dealt. Maybe the original builder was sloppy, or depending on the age of the house, maybe generations of remodelers have left their questionable work for you to clean up. Or maybe the standards that we value today just didn’t exist when the house was built.

In a recent Q&A post, member “Jewell33” describes the process of renovating a slab-on-grade house in Ontario, Canada, Climate Zone 6. Part of the renovation involves living space in the lower level, which at the moment consists of bare stud walls framed inside of concrete walls, some of which are above grade while others are partially below grade. The plumbing and electrical services are in place. Jewell’s contractor wants to insulate the walls by filling the stud bays with fiberglass batts and installing a poly vapor retarder over the assembly. While the construction sounds a bit confusing, the situation also resembles more common basement walls, which we have some pretty solid advice for insulating here on GBA.

But Jewell is on top of the situation: “I started researching and found that putting fiberglass batts up next to concrete is not a good idea, nor is the poly vapor retarder in this situation.”

They’re right to be concerned. Concrete foundations can be a source of moisture and inward vapor drive. They can serve as a cool condensation surface for vapor. A previous generation of builders tried to solve these issues by encapsulating the concrete, insulation, and framed walls with polyethylene sheeting. While the poly may keep moist indoor air from condensing on the concrete, the impermeable poly traps moisture coming from the concrete, and fiberglass batts have not…