Critique idea for basement walls seal, insulate, finish

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Building in a cold climate, snow country, winter design temperature -15 deg F. Sixty percent of a full-depth foundation basement area is to be finished.

The foundation walls are poured concrete and the exterior side has been membrane-waterproofed before backfill.

Inside, what is proposed is 1/2-inch foil-faced isocyanurate closed cell rigid foam board, spot-glued and fastened (as needed) to the concrete wall, extending at top 1-1/2-inches above wall to bottom-of-joists elevation.

All seams of the foam board are to be sealed with squirt-foam.

Against the foam board, a 2x4 framed wall is to be built. The pressure-treated bottom plate, and the top plate are installed first, and squirt-foam is used to seal the joint between foamboard and plate members. Studs are measured, cut, and installed and toenailed to plates on 24" centers.

Fiberglass batt insulation is installed in all stud bays. Sheetrock goes on the framed wall, all joints mudded and taped.

We think of the foam board surface as the vapor barrier, and don't plan to put in an additional barrier in the wall assembly.

Please comment on this system's viability. It exceeds what the local code requires.

[dgbldr]

In order to comment on the system's viability, we need to know your requirements and/or objectives. What is your energy code requirement at your location? Are you in a dry or wet climate? Are you looking for the cheapest method or best performance?

[Dutchman]

Building in a cold climate, snow country, winter design temperature -15 deg F. Sixty percent of a full-depth foundation basement area is to be finished.

The foundation walls are poured concrete and the exterior side has been membrane-waterproofed before backfill.

Before backfill I'd cover the foundation with at least 4" of polyiso. Internal details can change / be reduced for less insulation based on that.

[Ted S.]

Inside, what is proposed is 1/2-inch foil-faced isocyanurate closed cell rigid foam board, spot-glued and fastened (as needed) to the concrete wall

I think that is a very critical detail. The issue is that air could leak between the concrete and the polyiso board. Then you could have condensation, buried within the wall.

The best idea is as Dutch says, insulate on the outside, although that can have its own challenges as to how to finish it on the exterior and what plane it is in relative to the wall above. It takes some planning..

If you could replace the foil faced poliso with closed cell spray polyurethane foam I would do that. You would adhere the foam to the concrete and eliminate the convective pathways.

If you are going to stay with the polyis, the safest detail is using a low expansion foam sealant and apply a continuous bead around the perimeter of the insulation right at the edge in an effort to stop air from traveling behind the board. Make a good effort to build an air barrier at that location, then X's on the back of the board with the sealant to compartmentalize any potential air spaces behind the board.

To answer you question, yes the foil is a vapor barrier or Class I retarder, but technically on the wrong (cold) side of the assembly.