Construction - Insulation/Watershed Umbrella for the Annualized GeoSolar System

The insulation/watershed umbrella is a concept advanced by John Hiat in his self-published book, Passive Annual Heat Storage:  Improving the Design of Earth Shelters".

The book is mandatory reading for anyone contemplating a project like ours but it is out of print and apparently available only secondhand online. Don Stephens later improved (in my view) Hiat's PAHS --  including Hiat's umbrella concept, -- and called his iteration "Annualized GeoSolar" (AGS) (also check out  three prior posts on AGS:  first post, second post, third post (the third post zeros in on the insulation/watershed umbrella)).

I began excavating for the umbrella at Thanksgiving-time so it is uncertain how much of it I can do before winter weather interferes.  The umbrella adjacent to the concrete walls will have to wait several months until the tops of the walls between the umbrella and the mudsills are insulated on the outside then backfilled to the level of the umbrella. 

Priority Dilemma
My concern is that all of the footings are protected from freezing this winter by completing the umbrella (highly unlikely) or burying the footings under at least 3' of dirt (behind the north wall) or with some other stopgap measure.  As it turns out, I resorted to covering the exposed footings with EPS foam insulation board with the intention of using the boards later to insulate the concrete north wall in the area of the garage.  Unfortunately, the umbrella will have to wait until after the Spring rainy season.  

Actually, a higher priority for what good weather we have left before the ground freezes is to remove the superfluous dirt from in front of the house in order provisionally to restore the pre-construction contour of the slope.  The grading will be finished in conjunction with installing the AGS umbrella in late Spring.

Most of the superfluous dirt came from the final grading for the floor of the
house but considerably more came from excavating for the AGS umbrella.
(Click on the image to enlarge it.)

Superfluous dirt has been moved behind the wall as backfill.

The backfill against the wall is +/- 6' deep in the middle and sloped
towards both ends for surface water drainage; notice  the AGS
conduits protruding upwards; after backfilling against the wall,
the remainder of the dirt from in the front of the house was stacked
to the right of the conduits to a height approximating the final
 backfill level when it is extended all of the way to the wall;
 eventually, the conduits will have to be extended even more in
order to reach daylight through the taller backfill.

Composition of the Insulation/Watershed Umbrella
The umbrella will be merely a "sandwich" made up of plastic sheeting, rigid foam insulation board and sand.  Starting at the bottom in the order in which the layers will be built up, the sequence is as follows:

• Sand over the soil to create a smooth bed on which to lay the umbrella
• 6 mil plastic
• Thin layer of sand
• Mostly expanded polystyrene insulation board; some extruded polystyrene board
• Thin layer of sand 
• 6 mil plastic
• Thick layer of sand
• 6 mil plastic
• Thin layer of sand
• Two layers of recycled synthetic carpet upside down
• Topsoil

I will be layering the plastic and insulation precisely as Hiat recommends except, of course, we have no earth sheltering on the roof.  Stephens' influence will be represented by the carpet overlay to protect the sandwich from

Illustration from Hiat's book (click on picture to magnify the details)

mechanical damage from such things as burrowing critters or thoughtless use of shovels and other tools.  It will also protect somewhat against the penetration of plant roots although the latter will mostly be inhibited by the fact that the only area that is not too dry to support them will be above the first layer of plastic.

As recommended by Hiat, the insulation will be thickest near the house and thinnest at the periphery in five steps corresponding to the width of 4' x 8' foam board --   4" thick for 8 ft,, 3" for 4 ft,  2" for 4 ft and 1" for 4 ft -- making the umbrella 20' wide. Where a single sheet of plastic is not at least 20' wide abutting sheets will be shingled with a large overlap. The sand is important for a couple of reasons.  First, to provide smooth interfaces between the layers and eliminate air spaces that might allow the umbrella to be damaged by heavy equipment or vehicular traffic, particularly punctures of multiple layers of plastic at the same place allowing through-and-through leakage. Second, to hold the plastic sheeting apart enough that any water finding its way through one layer is sure to flow downhill through the porous sand to the periphery.  Without the sand, the backfill over the sandwich would compress the plastic layers together and trap water, particularly where the plastic extends beyond the insulation.  The umbrella will slope away from the house in all directions and, in some places, end in a mini-French drain for faster drainage ("drainage gravel" in the illustration).

The white expanded polystyrene (EPS) foam board will suffice for all areas except under the driveway and the garage floor where pink 250 psi extruded polystyrene (XPS) will be a better choice for supporting vehicles.  The sand in the sandwich will go a long way towards stabilizing and supporting the weaker EPS. The two reasons for selecting EPS for most of the umbrella insulation are price and EPS's long history of use in wet environments (think insulated concrete forms and flotation for boat docks). I trust that the plastic sheets will keep the foam pretty dry but there will be a few small leaks inadvertently created during installation that will admit trickles of water through a given layer that will have to travel in association with the foam and sand layers to an exit at the periphery of the next layer of plastic, so it is prudent to anticipate some exposure to moisture.

The reason for the plastic is to keep the soil under the umbrella dry.  Doing so lowers its thermal conductivity and inhibits transfer of heat from the thermal mass to the outside environment.  Through dry soil, it takes 6 mos for a unit of heat to move 20' which is the basis for making the umbrella 20' wide.  As Hiat explains, even if some moisture would somehow find its way through three layers of plastic, the negative impact on the ground below would be inconsequential, that the saturation would be so spotty and limited that it would not be enough to compromise the efficacy of the AGS system.

Garage Serves As Part of the Umbrella

Insulation of the wide footing is completed by the addition
of  XPS foam board horizontally; it was covered with sand
before the backfilling; EPS would have worked as well

The entire floor of the garage will be insulated for a couple of reasons. One is to complement the other measures that will make the garage warmer, viz., the insulated foundation, the thick walls and ceiling filled with insulation and the insulated overhead doors. The other is to serve as the umbrella for the north half of the east end of the house.  

In order to fulfill the requirements for a frost-protected shallow foundation, the garage foundation footing below the insulated concrete forms will have to be insulated where it is not protected by the house and garage floors or the umbrella next to the south half of the east end of the house. Likewise, the wide footing for the east end of the concrete wall was poured against vertical insulation but it needed to be insulated on top, which was simply a matter of fitting foam board over it, sandwiching it between two thin layers of sand (to carry any water from behind the damp proofing membrane to the footing drain) and backfilling over it. Eventually, a retaining wall will be resting on it.  

Horizontal insulation in place and the backfilling started

I insulated the narrow footing under the garage foundation in conjunction with extending the footing drain to daylight downhill. I enlarged the overdig next to the footing so that it would accomodate foam board 3' wide and embedded the drain in sand, much like along the backside of the wall, screeded the sand level with the top of the footing and laid in the insulation. I did the same procedure for that part of the front footing for the garage that will not be protected by the umbrella, except there was no drain to worry about and the EPS was only 2' wide since it faced south and direct sunshine should warm the backfill enough to make for a warmer footing anyhow.

The footing drain at the other end of the house merely needed to be extended downhill to daylight, bedded in sand and backfilled.  

The extension of the footing drain for the west concrete wall.

With respect to the exposed footing inside the garage that will be covered by the floor eventually, I used soil to fill the overdig flush with the floor grade and laid foam boards against the north wall and the insulated concrete forms for the other three walls and weighted them down with stones. That should protect the footing this winter; the floor will go in next summer.

For some reason, the follow-up post on construction of the insulation/watershead umbrella was posted out of sequence and is dated 11/28/15.  Please drop down four or five posts to find it.