Sunday, April 10, 2016

Timeline - Deciding on the Amount of Earth Sheltering

Past Six Years

Earth Contact Walls and Roof
Concrete walls are included in the design of our passive solar home even though their extent is limited to the north wall and about half of the west wall, technically making the design "earth-bermed" instead of "earth sheltered".  The classical earth sheltered dwelling   
had only the south wall exposed.  The north, east and west walls would have earth contact and so would the roof to varying degrees. It took me a while to realize that so much earth contact is not necessary in order to eliminate conventional heating and air conditioning.  As you can see in the nearby photo of the house model that was built nearly five years ago and in the prior post on the house model, the original design called for half of the roof to be earth covered.

Going overboard with earth sheltering at that stage was the result of my confusion about the role of that part of the house envelope not facing south.  I bought in on the idea that maximizing earth contact seals off the hostile external environment and makes the surrounding ground temperature the default temperature for attenuating living space temperatures -- all of
which is true.  It took awhile and considerable research, however,  to realize that the story doesn't end there.  Conventional walls and ceilings can be air-sealed and super-insulated (R-50 range) to ward off the outside environment at least a well as contacting earth does. Then, Hiat's "Passive Annual Heat Storage" and Stephens' "Annualize GeoSolar System, made me realize that earth contact can be severely reduced then intentionally manipulated so as to raise the default ground temperature to a comfortable level for living spaces without any help from conventional heating or air conditioning.  

In addition, there were three practical reasons for downsizing the amount of earth contact.  First was the realization that building a two-story concrete wall using something like dry-stacked concrete blocks (like Rob Roy showed in his book, Earth-Sheltered Houses) was beyond my skill and endurance levels and beyond what any structural engineer would be willing to stamp, especially when half of the wall would be two stories
tall with no bracing from internal T-walls.  And not to mention that our budget made it necessary to limit the amount of concrete work we delegated to professionals. Secondly, there are two options for a roof that is strong enough to support earth, neither of which we could afford. One is concrete supported by concrete or steel (wood is not allowable).  Another is like Roy advocates -- 2-by tongue and groove decking supported by timber framing.  But for our rather large project, the timber frame would have to be professionally done -- again a budget buster. The third reason for avoiding an earthen roof is that, since the roof would not support earth moving equipment, the amount of the labor involved in wheel-barrowing enough soil to cover a 2,800 sq ft roof to an appropriate depth would have been formidable.  

According to printed and on-line resources, water leaks have always been a problem with earth contact roofs, at least in non-desert climates.  But this negative did not influence my decision against earth covering for the simple reason that I think Hiat is dead-on in saying that the insulation/watershed umbrella covering the roof and extending laterally from the house, as opposed to conventional waterproofing applied directly to the deck, is very unlikely to allow roof leaks.  And, since the insulation/watershed umbrella is critical for our AGS system, it would have been carried onto the roof if we had opted for earth contact.

The advent of "super-insulated" walls and roofs tipped the scales.  I am betting that our iteration of the AGS system will generate and store as many BTUs as the house will ever need.  So the only issue is to make sure as many BTUs stay in the house during cold weather (and out of the house in hot weather) as possible.  And I am betting that our meticulously air-sealed and super-insulated walls and ceilings will do a better insulating job than the earth contact walls and ceilings they replaced.  
The concrete wall is limited to the entire north side and
half of the west side of the house.  The north wall will
be backfilled to nearly a two-story height over more than
 half its length.

In our case, earth contact is limited to the north wall and half of the west wall. The walls have two functions. One function is to become part of the thermal mass but, in this regard, they are only a small part of a larger thermal mass comprising the soil under a 2,800 sq ft floor plus the soil extending 20' outward from the house in all directions under the insulation/watershed umbrella. The large volume of earth thus encompassed by the system is the primary thermal mass; the concrete walls play a minor role.  Actually, the concrete floor plays a far bigger roll than do the walls by a ratio of 2:1 in terms of earth contact area.  The two thirds of the concrete area that the floor represents lies just a few feet above the AGS conduits and thereby is in position to heat-sink more solar energy than the walls.

The second and most important function for the concrete walls is to serve as a heat transfer medium between the soil behind and west of the house and the interior environment.  For this reason, the inside of the walls will always remain unobstructed and accessible to air movement.  Situated just inside the north wall and part of the west wall of the house will be the utility area (aka, "vertical basement" by Stephens) which means that storage shelves will be attached to the stick built walls opposite the concrete walls in order to keep the latter completely exposed for transferring heat.