I have been learning a lot abut building insulation over the last few months and recently decided to insulate the exterior frost walls of my workshop building.  Last winter I had noticed that the snow was melting back from the wall by up to 6 inches.  Clearly this represents heat loss through the wall!  Also my radiant floor tubes make their first pass around the exterior edge of the floor before zig-zagging around the interior space.  I want to conserve as much of this heat as possible!

So I decided to dig down as far as practical - which turns out to be 2 feet from the top edge of the concrete frost wall.  Then I added 2" of blue foam board (R-15) covered by T111 hardy board (textured concrete panel).  I also dug out 2 feet away from the wall and buried a 2ft wide foam board that will trap below grade heat.  I sloped this foam board down away from the building so that ice and water would shed away from the foundation. 

The ground temperature remains at 45F year round below the frost level which can be as low as 36" here in Maine.  Trapping that heat below insulation will help to keep the frost wall warmer below grade.  A building expert explained to me that the horizontal foam adds an equivalent depth to the wall insulation.  So by putting 2 foot against the wall, and 2 feet out from the wall I have achieved the equivalent of insulating 4 feet deep down the wall.

I taped the joints in the foam with aluminum tape to prevent ice from creeping in behind the foam.  Then I glued the foam to the wall with construction adhesive and glued the (pre-painted) hardy board to that.  The top edges of the hardy  boards were screwed to the bottom edge of the vertical barn boards, then I sealed the top edge with dark brown caulk - again to prevent ice infiltration.