Construction - Insulating the Earth Contact North Wall

Intentional Compromise
In a perfect world, the earth contact north wall would have been a full two stories high, i.e., at least 16 feet.  But to have done so would have increased its cost so significantly that I decided to stop at 12' with the concrete and carry the wall to second story height with a short truss wall.  The latter could be done with salvaged lumber and my free labor as opposed to paying for professional labor and buying several more yards of concrete to increase the height, not just of the 12" thick wall itself, but the three substantial deadmen behind the wall as well.

Moreover, it will be easier with the truss wall to achieve an R-48+ to match the other external truss walls than to insulate a concrete wall to this R-value.  But the advantages of a higher wall of concrete would have been an additional 200 sq ft of earth contact for the AGS system and more of the wall totally impenetrable by air infiltration/exfiltration. However, I am betting that (a) the large floor area of the house, (b) the +/-10' of uninsulated north earth contact wall, (c) a partial earth contact west wall and (d) the insulation/watershed umbrella will provide all of the insulated and dry thermal mass the house will ever need without the additional 4' of concrete for the north wall.  Therefore, I think the additional 200 sq ft of earth contact would have been expensive insurance for a problem that probably does not exist.

Tweaking the Platon Damp-Proofing 
As described in a prior post, the Platon damp-proofing system was used on the lower 8' of

Original Platon installation

the tall section of the wall before backfilling began.  The top 4' still needed damp-proofing before it could be insulated.  I decided to keep the Platon system below the bottom of the metal channel that holds the insulation. Otherwise, the extra bulk would cause unevenness of the insulation because the top of the channel would be directly against the concrete and the bottom would hang over the bulky Platon material. Consequently, I installed only enough additional Platon to bridge the gap between the original material and the bottom of the channel.  As will become apparent presently, the two layers of plastic sheeting that waterproofs the metal channels and insulation will also waterproof the concrete even better than would the Platon material.  The exception to this arrangement was the first four feet at the ends of the wall where the insulation extended down nearly 6' behind what will eventually be retaining walls running more or less perpendicular to the wall.  Here the channels did overlay the Platon material and did flare out somewhat at the bottom.

Materials Preparation
First is was necessary to calculate how far the insulation should extend below the mudsill of the truss wall.  Code specifies a distance of 8" between the grade and the mudsill. Figuring downward from the mudsill, the first 8" will be exposed stucco, the topsoil over the

insulation/watershed umbrella will be 8" deep and the umbrella will be 6-7" thick, so the wall must be insulated to a depth of at least 23" below the mudsill to get below the umbrella. (Below that, of course, the wall should remain uninsulated so that heat can pass back and forth through the wall between the earth and the living space as a function of the AGS system.  For more on AGS, click on the "Featured Post" in the column at the left.)  I settled on 32" of insulation 3 1/2" thick for an R-14 to a depth of 8-9" below the bottom of the umbrella. The nearby photo shows these dimensions drawn on the cement board before stuccoing (click on the photo to enlarge it for better detail).

Parenthetically, in order to achieve an overall R-48, the inside of the wall will have to be insulated to another R-34, which is the subject of a future post.

I used a plywood blade in a circular saw to cut two thicknesses of expanded polystyrene foam board -- 2" and 1 1/2" -- into 24" x 32" pieces .  I also cut 
3 5/8" wide 20 gauge galvanized steel channel (the kind that is used as the top and bottom plates for steel studs for wall framing) into 24" lengths using a metal blade in a radial arm saw.  I cut 1/2" cement board into 48" x 28" pieces using a corded fibercement nibbler.  Next, I cut lengths of 6 mil plastic sheeting into 8' wide pieces.  At this point, all I needed in order to begin insulating the wall were 1 1/4" x 3/8" Tapcon screws for fastening the channels to the wall and 1 1/4" cement board screws with high-low threads for fastening the cement board to the outside of the channels.  I then followed the same installation procedures that were covered in a previous post with a few deviations that were so minor as not to warrant special discussion here.

Installing the Insulation
As described in the previous post, the channels were readied for installation in the shop by cutting to length, drilling holes for the Tapcon screws in half of the channels and then screwing channels with holes back-to-back with channels without holes.  I started the installation at the east (left) end of the 12' wall so that the channel with the holes would be facing to the right each time so as to make it easier to use the hammer drill in my right hand; if I were left-handed, I would have started at the other end of the wall.

The first four feet of the wall needed to be insulated to a depth of +/-6'.  The additional depth was necessitated by the transition from an 8' wall to a 12' wall which will be accomplished by a retaining wall running more or less perpendicular to the wall and butted up against the first four feet of insulation.  (The first book on earth sheltering that I read some 8-9 years ago was Rob Roy's Earth Sheltered Houses in which he cautioned against butting a retaining wall against the concrete house wall without insulation between the two in order to reduce heat loss and to eliminate moisture condensation during warm months.) The same 4 x 6' configuration was necessary at the west end of the wall for another retaining wall.  In between, there was a span of approximately 52' where the insulation was only 32" in height.

Before the channel and the insulation could be started, the 6 mill plastic had to be positioned so that roughly half of it would be trapped under the insulation and half lapped

Insulation installed on about half of the wall with the inner
layer of plastic showing beyond and below the foam;
 bare concrete is in the distance; notice that the metal
 channels are shorter than the insulation

over the insulation, completely sealing off the channel-insulation complex. The rolls of plastic I buy at the farm supply store comes 24' x 100'.  I divided the 24' dimension into thirds.  The 8' wide pieces provided 4' of plastic between the insulation and the concrete wall and 4' between the insulation and the cement board. And the individual pieces of plastic were overlapped by a couple of feet.  In this way, the metal channels were totally sealed against rust-inducing moisture, rather than relying solely on galvanization, and the concrete wall was not just damp-proofed but was actually waterproofed in the process.

Without going into all of the details covered in the previous post, the installation was a

Completed installation of the foam; notice the outer layer
of sheet plastic, thrown back and weighted down on the
floor of the scaffold, that will be brought forward and
 draped over the foam before the cement board goes on 

matter of screwing a pair of channels to the wall over the inner layer of plastic, mating a piece of 2" foam board with a piece 1 1/2" thick, slipping the left side of the foam into the channel, slipping the next pair of channels over the right side of the foam and applying pressure leftward while drilling the holes and inserting the Tapcon screws. The 32" length for the foam and 24"length for the metal meant that the foam protruded below the foam 7-8" in order to keep the metal well up inside the sheet plastic to keep it dry.  I also kept the tops of the channels an inch or so below the tops ot the foam in order to keep any rough edges from perforating the sheet plastic. The inside diameter of the channels is 3 5/8" and the foam is 3 1/2" thick so there is enough tolerance that the insulation can be moved somewhat.  Accordingly, after all of the insulation was in place, I used a mason line to level the tops of the foam to match the bottom of the mudsill that will eventually hang over them.  Any remaining gaps between the sill and the foam board will eventually be sealed with mortar as described in a subsequent post.

Installing the Cement Board
Installation of the cement board required two cordless drills and a driver for phillips head cement board screws.   Before taking a piece of cement board to the wall, I used one drill

Completed cement board installation; notice ample outer
 layer plastic sheeting exposed below the board; the
backfill will press it against the edge of the foam and
the wall below to seal the channel/foam assembly against
moisture

with a 3/16" masonry bit to drill a hole in the upper left corner and the upper right corner.  I As soon as the left side of the board was butted against the right side of the previous piece, aligned flush with the top of it and proved level with a torpedo level, I used the second drill with a bit designed for metal to drill a pilot hole in the metal track through the hole previously drilled in the board in the upper left corner. Then, while holding the board steady, I used the driver to fasten the board to the track with a cement board screw.  The remainder of the screws could then be installed in a similar manner, i.e., masonry bit, metal bit and cement board screw.

Installation of the cement board is best done by two people because it is heavy and difficult to manage working alone.  I overcame the problem by leaning a 2 x 4 against the wall at the level of the bottom of the board as a support.  However, the rest was rarely at the right height necessitating a lot of juggling which probable quadrupled the installation time.  It didn't help either that the ground was uneven and snow-covered part of the time and beginning to thaw the rest of the time.  In either case, the slipperiness underfoot made things interesting.

Stuccoing the Cement Board
This last step in insulating the concrete wall followed exactly the procedures described in detail in the previous post.  The only difference was it had to be done in late December instead of warm weather, which complicated and slowed the process considerably.  Not knowing when the stuccoing might get done, I am publishing this post without pictures of the stucco on the assumption that the curious will be checking out the previous post anyway.

Honest Perspective
The DIY concrete wall insulation described here and in the previous post is a good example of a green building approach that would not be practical in a production setting because it is too customized, cobbled together from disparate materials and time-consuming.  In a way, though, it is a microcosm of our entire house project.  Jo Scheer in the book, Eco Architecture says, "Though extreme eco-architecture may not be a solution to a thoroughly sustainable building industry, it certainly provides ideas. It is a model of ideas and concepts that beg to be assimilated".  Who knows what impact on sustainability some of our "impractical" ideas might have somewhere down the line.