Tuesday, June 16, 2015

Timeline - Design Evolution -- Earth Contact North Wall

Last Four Years

As a DIYer, it was hard for me to imagine ahead of time the structural complexity of a high concrete wall that is essentially a retaining wall, -- backfilled on one side and not supported on the other. It took Steve Rehagen, who drew our house plans and Mark Bachetti, our structural engineer, to educate me.  Prior to Steve and Mark, I went through a couple of budget-driven iterations that would never be stamped by a structural engineer as required by the Building Director.  

The wall is 92' long, 54' of which is 12' tall with the remainder 8' tall.  In order to maximize earth contact for the AGS system, the inside of the wall must remain open to air circulation. Consequently, a long, narrow storage area will abut the wall with all of the storage taking place on its south wall so as to leave the north wall unencumbered.   The disadvantage of this arrangement from a structural standpoint is that there will be no right-angle interior stem walls to brace the north wall.  Hence, its similarity to a retaining wall.

Dry-stacked Concrete Blocks
Originally, we envisioned more earth sheltering than we ended up with and the
First three courses of dry-stacked blocks
for our solar collector for the AGS system 
quintessential text on the subject is Rob Roy's "Earth-Sheltered Houses" which was an early acquisition and influencer.  He advocates using dry-stacked cinder blocks for earth contact walls -- primarily 12" thick rather than the usual 8" thick. Apparently, d
ry stacked (mortar-less) concrete blocks originated with the Corps of Engineers.and produces a wall that is not only stronger than a mortared wall but a wall that rivals a poured concrete. So our first vision was a 12" dry-stacked concrete wall.  Dry-stacking would be by far the cheapest approach and the most DIY-friendly but the amount of labor involved with stacking 60 lb blocks 12' high would be formidable. Also such a wall would be hard to insulate. Insulation appended to the outside tends to be disturbed by backfilling and insulation on the inside is not as effective because it is on the wrong side of the thermal mass.

At the time of this writing, we were constructing the walls for the solar collector for the AGS
Parging dry-stacked blocks with
 fiber-bonded cement
system using dry-stacked blocks. The photo above was taken while the first horizontal bond beam course was being filled with concrete and horizontal rebar.  A second bond beam coarse was similarly used higher up in the wall and many of the cores in the blocks were filled with concrete and vertical rebar.  As is typical, both sides of the walls will be coated with fiber bonded cement which makes the joints between blocks stronger than 3/8" mortar joints. Dry-stacking is perfect for this small project but I am glad that we cannot use it for the north wall because it is not as straightforward and easy as it might seem -- minor variations in the size of the blocks complicate stacking them level and plumb, particularly when half-blocks are mixed in with full-sized blocks.  

Complete Blocks 
Last year, I came upon a St Louis start-up making insulated blocks for house walls (Complete Block Company).  After several visits with Herb Walters, the inventor, I become convinced that Complete Blocks were exactly what we needed for our project. With proper equipment, the 200 pound blocks could be dry-stacked to form our long wall in
Dry-stacking the blocks with lifting equipment; notice the
 stamped concrete exterior; the mating surfaces are sealed
with an elastomeric material as the blocks are seated
one or two days.  They would be poured off-site with or without insulation in them. As can be seen in the lower photo, they fit together in tongue and groove fashion then vertical rebar is added after stacking.  It is threaded and epoxied into holes in the slab or footing then tensioned from above.to pull the blocks into tight contact with with each other and with the slab or footing.  Our project would require insulated blocks only at the periphery then solid blocks for most of the wall in order to give thermal mass for the AGS system.  As mentioned above, the intrinsic insulation would be more effective if it were on the exterior side like with insulated concrete forms (post on insulated concrete forms).

Despite the perfect match between Complete Blocks and our needs and a generous offer
Notice the tongue and groove mating and the horizontal
rebar; note also the intrinsic insulation and furring strips
 (for attaching interior finish materials such as drywall);
the vertical rebar on 16" centers is not apparent here
from Herb to help with installation for essentially the cost of the blocks alone, we decided to go a different route.  The system was so new that there were no structural data available that a structural engineer could use to stamp our plans. We are still a couple of months away from constructing the north wall so I recently contacted the company to see if data now existed and any engineers stood ready to stamp our plans.  The answer is still essentially "no".

Poured Concrete Wall
Meanwhile, the default position was to pour concrete in order to keep the project moving. The wall that Mark designed is 1' thick and rests on a monolithic footing (poured at the same time as the slab) that is 1' thick and 8' wide.  The amount of rebar extending from the footing into the wall and interlaced within the wall is mind-boggling -- literally tons of number 4, 5 and 6 rebar.   Presently, I am vetting potential vendors and do not know yet the cost of pouring such a tall wall.

As for insulating, a poured wall would be as problematic as a dry-stacked cinder block wall.

*     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *
A month or so after this posting was published, I made the decision definitely to go with a poured wall and worry later about some creative way(s) to insulate it to at least R-20.