Construction - The Next-to-Last Major Section of the Insulation/Watershed Umbrella

For once Mother Nature cooperated despite it is the rainy season.  With help from a few volunteers we were just able to carry the installation of the umbrella behind the house near enough to completion that a pair of spring rains did not affect it.  A few days later, we completed it and moved on to final backfilling, grading and planting.  (Reminder:  clicking on any picture enlarges it for better viewing.)

Water Control
There are two considerations for water control behind the house -- surface water and water that soaks through the soil to reach the level of the umbrella.  The former would comprise most of the water from rainfall were the grade sloped enough to encourage quick runoff.  But the fall of the final grade northward from the house will be only 2 - 3' over a distance of about 100' -- a very gradual slope that will nicely handle runoff without erosion but one that will give the runoff more time to soak into the soil.  At a distance from the house, the quicker soak time will be less consequential but, over the umbrella, it means that the surface of the umbrella will handle more water.

Hiat's Design
The umbrella in front of the house sloped much less than 

the one behind the house. The rain that soaks through the soil overlying it slowly finds its way off the umbrella and down the hill away from the house.  By contrast, the rain that will soak through the soil covering the umbrella in back of the house needed to be handled by a French drain so as to direct it down-slope around the east and west sides of the house instead of flowing northward away from the house where it might pivot and undermine the umbrella.  Therefore, the umbrella slopes more severely and has a French drain at its periphery as Hiat describes in his book, Passive Annual Heat Storage.*  The idea is get the water away from the house before it can soak into the soil beyond the umbrella and flow back under the umbrella (which was its natural tendency before the lot was excavated).

As for the composition of the umbrella, it is exactly the same as described in a prior post for the front umbrella except that it was complicated by the conduits for the AGS system sticking out of the ground.  As in previous sections of the umbrella, the extruded polystyrene foam board insulation is thicker near the house and thinner towards the periphery of the umbrella.  For the first 8' from the house, the thickness is 4" (R-20); for the next 4' the thickness is 3" (R-15) and for the last 8' it is 2" (R-10).

Installation
The umbrella is bracketed on the east and west sides by

West retaining wall underway thanks to several volunteers

substantial retaining walls made from recycled limestone foundation stones.  The east wall was installed last fall; the west wall had to be done before work on the umbrella behind the house could begin. 

In addition to the layer of insulation, there were three layers of 6 mil plastic sheeting.  Our initial experience with the first layer and the foam board over it taught us that both the sheeting and foam board slid away from the house as we worked.  So, special precautions were necessary to anchor all layers until anchored with backfill.  We used a couple of dozen of 12" galvanized spike nails driven through the foam and into the ground in

First layer of plastic and the insulation in place with layers
of sand to serve as a drainage plane in case water leaks
through the top two layers of sheet plastic

strategic locations to hold the insulation tight against the house.  Of course, this meant poking holes in the plastic sheeting under the foam but, since it will be protected by two additional layers of plastic, the holes were a good trade-off for stable and tight insulation.  

In order to keep the two top sheets of plastic from creeping down-slope during installation, we turned their edges upwards at the wall and attached them to the bottom edge of the wall sheathing using batten boards.  Also we spread a layer of sand between the sheets so it served as a counterweight against the wind for the first sheet until the second

Recycled carpet in place; sand had not yet been added
to the edges of all of the carpets (as seen in the
background for some)

sheet could be installed and more sand on top of it held it down until the recycled carpet could be laid over it.  The primary reason for the sand, however, was to provide a drainage plane for any water that makes its way into the lower layers of the umbrella.  Despite all of the effort, varying amounts of space existed between the horizontal foam and the wall that we discovered after it was too late to fix.  If I were to do it over again, I would anchor the foam boards with twice as many spikes before covering them with plastic rather than after the sheeting had been attached to house then bunched up against the house to get it out of the way of anchoring the foam after the fact. The plastic prevented us from knowing for sure that the foam was tight against the wall when spiked.

Sealing the Pipe Holes
We had to cut holes through all three layers

of plastic sheeting and through the 2" thick portion of the insulation to fit around the conduits.  Those in the sheeting were invariably stretched when the plastic was lifted and slid down over the pipes, such that they did not even come close to sealing out water.  So, just before installation of the carpet, each conduit was fitted with a thermoplastic roof vent flashing for 4" pipes.  The space under each flashing was made as smooth and clean as possible so that the weight of the backfill would press the rubber-like flashing tight against the sheeting to keep out water.  The holes in the carpet were cut with a hole saw turning backwards on a cutting board that had a hole in it to accommodate the centering twist drill. 

French Drain
The depression for the French drain at the northern periphery of the umbrella was a V-shaped ditch that drained both ways from the

Final grade for the umbrella including the ditch for the
French drain

center, i.e., it was deeper at the ends than in the middle.  The bottom layer of sheet plastic completely lined the ditch and its loose edge was pinned to the soil beyond the ditch so that water will be confined in it until it can find and escape via the drain.  Unfortunately, the ditch undermined the last course of foam board to varying degrees.

The design of the French drain itself was similar to that described in a prior post on French drain fabrication for the drains we installed early on 10' below the floor of the house.  Luckily, I still had a roll of the geotextile material used for the original drains, one that is unique in filtering out the kind of fine

Perforated pipe laying on geotextile material and partially
embedded in sand ready for closure

silt characteristic of  the wind-blown loess here in the bluffs of the Mississippi River.  We rolled it out along the length of the ditch and laid a 4" perforated drain pipe on top of it.  We then shoveled sand alongside the pipe, pulled the edges of the fabric together, rolled them three times and clipped them together with hog rings as described in detail in a prior post. In doing so, we made sure that the pipe was tight against the ground side of the fabric and the sand loosely filled the space on either side and on top of the pipe.

Since the last course of foam board partially bridged over the ditch, it had to be supported somehow before it could be covered.  Accordingly, we filled the ditch with

French drain closed with hog rings

sand to the level necessary to support the foam's cantilevered edges, which, at the same time, improved the permeability of the medium around the fabric-covered French drain.  With the foam supported, we could extend the sheet plastic over the insulation, add the carpet and begin backfilling in earnest.  In order to be sure any water coming off the umbrella found the sand around the drain before it could be dammed or slowed down by the backfill soil, we made sure the edges of both the plastic sheeting and the recycled carpet (that forms the top layer of the umbrella) were amply covered with sand.

Unique Insulation Design
The prior to the AGS concept, insulation for earth contact homes was fastened to the concrete wall.  Hiat was the first to suggest taking it off of the wall and laying it horizontally in order to create a much bigger conditioned thermal mass for the house.  He was a purist to the extend that the roof of the house would be earth sheltered and the umbrella would be merely an outward extension of the insulation on the roof.  In our instance, the roof is conventional so the relationship of the wall insulation and the umbrella had be handled differently.

Accordingly, as detailed in a prior post, the top 30" of the concrete wall situated under the stick-built portion of the wall is insulated to an R-14.  (In time, the top 30" inside will be similarly insulated.)  The insulation in the umbrella (R-20) butts up against the wall insulation (eventually R-28) at about midway of the latter's vertical dimension, such that about half of the wall insulation lies above the umbrella and about half lies below. 

Ideally perhaps, the umbrella insulation should 

Spreading topsoil

be R-30 near the wall instead of R-20 but I accepted Hiat's word that any additional loss of heat through the umbrella next to the wall will be offset by the heat saved by extending the umbrella a full 20' out from the wall. 

Planting
 As soon as the final grading was done for the large area north of the house, topsoil was distributed and planted with grass seed as a means of controlling erosion in the short run. The original intention was to plant several dozen bare-rooted hardwood tree seedlings in the newly graded area but the subsoil under the topsoil ended up being largely hardpan (glacial till) in which trees do not grow well.  The turf grass will control erosion until it can be replaced with native prairie species that are not at all picky about soil quality.  Gradually the turf over most of the rest of the property will also be natively landscaped in the manner that Doug Tallemy crusades for in his book, Bringing Nature Home; How You Can Sustain Nature with Native Plants, which I referenced in an earlier post on native gardening.  
___________________
*  Hiat's book is out of print but a few copies are still available on online.  For an understanding of his design for the umbrella, go to my prior post that discusses it in detail.  The same post is part of the trilogy on Annualized GeoSolar conditioning that can also be accessed indirectly by clicking on the "Featured Post" in the left column above.