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August, 2008: Insulating the frost wall

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.

exposed frost wall before installed foam and board
2X8 sheet of foam ready for installation detail of foam
  finished exterior
October 2008 update
I waited until it got down to 23F outside and took some temperature measurements along the inside of the frost wall.  I keep the inside temperature at 65F downstairs.  I found that the insulated section measured 59-62F, while the un-insulated section was 48-51F.  So the difference is about 11F, not bad!  Since my radiant floor plumbing makes its first pass around the exterior wall I expect to be saving a little propane this year.

December 2008 update
I borrowed a thermal imaging camera to document the improved insulation from the outside.  There's nothing like color to tell the story.  The thermal images below were taken at night when the ambient temperature was around 20F.  Note that the cross hair that is the point where the temperature reading is taken.  Clearly the insulation is retaining about 11F in the building.

I will definitely be completing this insulation project next year.  So far I have only done 2 sides of the exposed frost wall.  Lot's more digging to go!

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