Design - Earth Sheltering For East and West Slopes

Traditionally, Annualized GeoSolar (AGS) houses*, or for that matter, nearly all earth sheltered or partially earth sheltered houses, are nestled into south facing hills.  Like most of Illinois, our topography is glaciated flat land that was once tall-grass prairie.  But after a long search for a suitable location for passive solar, we were lucky to find a south facing slope adjacent to the prairie in the hilly Mississippi River "bluffs" across the river from downtown St Louis.  

Probably most AGS enthusiasts are not lucky enough to find a south facing hill so the paradigm needs to change.  And our experience with AGS makes me confident that it can be modified for east and west facing slopes when, for other earth sheltered designs that have less control over as much thermal mass, it would be a stretch.

Designs for energy neutral or near-neutral housing that do not require earth sheltering continue to evolve.  However, it seems that AGS is still the best alternative for cost-conscience DIYers and, for that reason, we need to find ways to utilize east and west facing hills as well as flat land (the subject of my next blog post).

I now believe that the type of solar collector we have in front of our house is unnecessary for AGS.  If it were replaced with a inflow chimney and the conduits ended at an outflow chimney with a fan in it, the system would be more efficient at a considerable reduction in construction costs and would lend itself to east and west facing houses.

The following discussion assumes that the reader already understands the basics of AGS passive solar, especially its advantages over 1970's earth sheltering designs.**

East and West Facing Slopes

The following discussion attempts to take what we have learned from our house and adapt it to houses identical to ours but not facing south.  It is only an example and not intended as a ridged blueprint for other creative DIYers.  Here are some parameters to guide the discussion:

--   the hillside faces east and the house faces south

--   the layout of the living quarters is identical to our house

--   a "vertical basement' wraps around the end of the house (instead of being restricted to the north wall as in our case)

--  the insulation/watershed umbrella replicates what we have now, viz., extending below grade outward 20' from the living quarters on all sides irrespective of grade changes and retaining walls 

--  all non-earth-sheltered exterior walls and ceilings are super-insulated to an R-50 or more

--  the house has no soil on the roof

A drawing of our south facing house that has been modified for an east facing hill.  Since the second floor remains unchanged, it is not included.  Concrete walls are dotted, stick-built exterior walls are black.

The drawing represents our house built into an east-facing hill and is a mirror image of a design for a west facing hill.  Note that the vertical basement wraps around the west (uphill) end of the house instead of being confined to the north side like previous designs for AGS.

The structures running at angles from the north and south sides are retaining walls that preserve full-depth earth sheltering even as the hillside slopes more rapidly downward from west to east.  The red line identifies the outer edge of the insulation/watershed umbrella, including that portion under the garage.  The green lines depict north-south conduits and the dotted blue lines identify alternative east-west conduits.  

Two Options for Conduit Configuration

The only way the conduits shown here in green ink differ from our build is the lack of a solar collector in front of the house and, to simplify the drawing, fewer in number.  The inflow chimney captures hot summer air.  Running under the floor of the house are conduits connecting it to another chimney located just north of the umbrella.  The latter houses a small furnace fan that pulls hot air through the system.

A second option for the layout of an AGS system -- the dotted blue lines -- are running east and west for which the inflow chimney is located just east of the garage and the fan-outfitted outflow chimney penetrates the umbrella next to the west concrete house wall.  The chimney could also be positioned westward beyond the umbrella but at the expense of making the long conduits even longer.

North-South Conduits

Our original passive solar design has north-south conduits running from a solar collector in front of the house to daylight behind the house. The collector was supposed to generate heat that would passively rise through the conduits but it didn't work that way.  The air falling out of the cold thermal mass overwhelmed the hot air from the collector, necessitating the addition of a solar chimney containing a fan to pull air through the conduits.

I now believe that it would be smart to replace the collector with a simple inflow chimney.  The question then is where to locate it?  One option would be in the area vacated by the collector whereby a large insulated pipe would connect it to the conduits under the house as shown in the drawing. 

A better option would be to eliminate the pipe by siting the chimney next to the foundation.  Care would have to be taken for an accurate fit of the umbrella against the chimney in order not to compromise the water seal or the integrity of the insulation. And it would be best to insulate the walls of the chimney down to its connection with the conduits, at least 6 feet below floor level.  The north outflow chimney should be left at the outer edge of the umbrella so that any residual heat in the airflow warms the soil under the umbrella.  And it remains uninsulated.

A considerable advantage to the north-south configuration is that the distance between the two chimneys would be much shorter than for east and west chimneys -- about 58' in the N-S direction (with the intake chimney next to the foundation) versus 93' in the E-W direction.  When air exits our outflow chimney, it is cool to the touch, even in the hottest part of the summer, which means it has done its job heating the soil.  However, there is no way to know when it gave up its last bit of heat.  Was it early on or near the north side or somewhere in between?  Since E-W conduits would be about two-thirds longer, I think the extra length would cause the heat to be distributed to the mass long before the airflow reached the outflow chimney at the west end of the house, taking an indeterminate amount of the thermal mass out of play for heat storage.

Arguably, a conduit with a direct path to the outflow chimney moves more air than a conduit with a more convoluted pathway.  If so it is also likely to distribute heat throughout its travel, including nearer the outflow chimney.  The corollary is that a more roundabout and branching conduit likely distributes heat in the early part of its journey and leaves thermal mass cooler further on.  Presently, our conduits do not branch until they are within a few feet of the outflow chimney.  Their minor looping to reach the ends of the house does not seem to impair a uniform comfort level throughout.  And the vertical basement with its tall north concrete wall, backed up by earth covered by the insulation/waterproof umbrella and running the full length of the house, provides thermal mass directly in the pathway of all of the conduits.  Unfortunately, the long vertical basement on the north would not work for an east or west facing hill.  

Since reasonably straight conduits are probably best, it makes sense to move the first pair of chimneys and their conduits eastward and add another pair of chimneys with dedicated conduits westward, say, under bedroom #2 in the drawing.  Then, with minor looping of the conduits, it would be easy to heat the vertical basement from below.  One or two conduits could even be swung behind the tall earth sheltered west wall, perhaps in parallel at two heights, to mainline heat into the thermal mass adjacent to the largest area of the vertical basement.

East-West Conduits

Another way to warm the thermal mass would be with conduits running east to west.  But fewer could be squeezed into the narrower dimension of the house and their longer pathways almost certainly would cause them to exhaust their heat long before reaching the vertical basement and the thermal mass beyond it.  And, since this arrangement is less likely to support pure passive solar, conventional heating in some form would be necessary.

Southeast or Southwest Slopes

 AGS being based on heat from the summer sun with less dependency on solar gain in winter provides more latitude for siting the house.  While a south facing house built into a northeast or northwest facing slope would leave too much of the back of the house sticking out of the hill, a house in a southeast or southwest facing hill would work just fine -- even better than strictly east or west facing hills because it would increase the amount of earth sheltered north wall.

Also, without compromising its thermal performance, a south facing house built into a east or west facing hill could be turned slightly eastward (east facing hill) or westward (west facing hill) to bring more of the back of the house into earth contact.  We rotated our "south facing" house slightly westward for a different reason -- to pick up more winter sunlight during the afternoon.  

French Drains

If French drains are necessary to prevent the water table from encroaching on the conduits, it might be well for the designer to review my posts on planning, fabricating and laying the French drains which detail methods and materials that are unique.  Otherwise, expect the excavation contractor to ruin most of the undisturbed soil that would support the house with the wide trenches that are necessary to accommodate the trench shoring cages that keep workers safe in deep trenches.

Respect the Umbrella

The amount of time and expense that goes into the insulation/watershed umbrella might tempt the designer to compromise on its size but, in my view, at great peril. The umbrella should extend outward from the house at least 20' on all sides 

Extending the umbrella (orange line) 20' minimizes heat loss
from the thermal mass under the house because it takes
  6 months for heat to travel 20 feet through dry soil.
Spring arrives before significant heat loss occurs.

to make sure that the temperature of the thermal mass under the house reaches a steady state.  The large umbrella requires an oversized excavation and running it behind retaining walls is an added hassle.  The cost of multiple layers of 6 mm plastic and several of foam board insulation is not inconsequential.  And backfilling over the umbrella without crushing the foam or puncturing the plastic takes ingenuity.  But the payback is the time not spent running furnace ducts and writing checks for HVAC installation and, best of all, to the utility company ad nauseum.

Fewer Windows

One minor disadvantage when our floor plan is adapted to a east or west facing hill is that the wrap-around vertical basement obliterates the south facing windows for the master bedroom.  But it would still have the second story windows adjacent to the catwalk,  And an energy efficient skylight could be added to the north-sloping ceiling without compromising the overall thermal performance of the house.

Underperformance?

I am not privy to AGS for slopes other than south facing which makes this post mostly informed guessing.  If it misses the mark, it is probably with regard to its optimism about winter, rather than summer, comfort, particularly in more northern climates. If so, before rushing to the nearest HVAC contractor or dismissing AGS altogether, consider the green solution we employ.

There are times in the early spring*** when our house, in a south facing slope in a southern-ish climate, is a little chilly but it is something we easily handle with a couple of portable infrared space heaters during the daytime.  Even with the heaters, our minimalist PV array produces more electricity than we use 8 months of the year.  And there is the possibility that the thermal performance of the house may still improve over the next few years as the umbrella-protected thermal mass absorbs and retains more heat.  Therefore, early adopters utilizing an east or west slope for AGS might consider living in the house, using infrared space heaters -- either fixed or portable -- and tracking energy consumption for a year or so then adding just enough PV to zero out consumption.  If we had done so, rather than trying to predict our needs ahead of time when buying the PV array, we would be energy neutral today.

The total amount of earth contact wall of the vertical basement in the above drawing, is about the same as for our house but it is not as well distributed relative to the living space, being bunched to one end instead of running all the way across the back the house.  And, where we have an abundance of through-and-through ventilators at the top and bottom of the wall separating the living quarters from the basement, skewing the basement towards the west end of the house potentially limits the the number of ventilators.

But for the house under discussion here, the problem might be resolvable.  The cathedral ceiling in bedroom #2 could become a flat ceiling, creating space above it for air passage from the basement to the dining room.  Ventilators across the bottom of the bedroom wall and an open bedroom door would suffice for bottom ventilation.  A ventilator in the bottom of the door would do the job when the door was closed.  Similarly, the door from the hall to the vertical basement could house a ventilator to pair with ventilators high in the wall above the door.  And, with the basement wrapping around the master bedroom, ventilators at the top and bottom of its north wall would duplicate what we have.  

If the hill sheltering the house ran southeast instead of due east, the back concrete wall could be extended eastward quite a bit and, with it, the vertical basement enough to allow ventilators at the top and bottom of the living room wall, pretty much duplicating what we have now.

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* For a refresher on AGS, check out the "Featured Post in the left column labeled "Timeline -- Annualized GeoSolar".

**  The best source on early earth sheltering is Rob Rob's book, Earth-Sheltered Houses, How to Build An Affordable Underground House.

***  Our low temperatures in the living space and thermal mass occur during the late winter and early spring rather than in the dead of winter.  By then, the heat stored in the thermal mass during the summer has been depleted enough to compromise comfort somewhat, at least for those of us who look for sweaters when the thermometer drops below the mid-seventies. For details on the thermal performance of our house, consult the recent post.