Saturday 1 September 2012

Earth Sheltering

As I continued to research further into Passive Solar Design and Thermal Mass (See previous post) I became fascinated with the concept of earth sheltering.

Earth's big advantage is as a thermal mass NOT as insulation. The first 3 metres below grade is a giant thermal mass which is very slow to change temperature. This characteristic (thermal lag) explains why the coldest earth temperature ( in this first 3 metres - the normal range for earth sheltered houses) lags about 6 weeks behind the surface climate in both summer and winter.

Existing earth sheltered "dwelling"
In order to control the temperature of the mass fabric of the house insulation must be placed between the home's mass and the earth. In northern climes it is necessary to completely wrap the below grade portion of the house (concrete, concrete block, stone etc.) with a substantial layer of insulation. Using this approach it is possible to use our internal heat source (wood burning stove, solar etc.) to "charge-up" the mass fabric of the building itself to comfort level.

Without such insulation conduction through the dense massive concrete footings , for example, causes the wall and floor surface near the footings to be at about the same temperature as the earth at this depth. Any moist air created in the house would then condense on the cold surface at the base of the external walls causing condensation.

As build further south the insulation strategy should be adjusted since the further south we go the more important cooling becomes in terms of both comfort and energy cost. By lowering the level of insulation the moderating effect of the earth's temperature performs more effectively as a means of natural cooling.

The correct order of events for protecting the thermal mass in an earth sheltered dwelling is waterproof first, then insulation then drainage. In the "fully bermed" or "cave" building (ie east, west and north walls as well as the roof are earth sheltered) I was envisioning the wall protection system melds with the roof protection system as a continuous unit.
Location of cave

There seems general agreement within the literature that the advantages of earth sheltered roofs are:

- Drainage - The earth roof drainage, particularly where the roof drains from a single pitch directly into the berm, is slow and natural and does not require gutters, downpipes, storm drains etc.

- Aesthetics - Undoubtedly the most beautiful of roofs particularly one of wild flowers and herbs. This is a very important factor for my build as the Crete Archaeology Department, who have to give their approval to my earth sheltered design before the plans pass to the Planning Department for the granting of a Building Permit, have been raising issues with my Engineers for over nine months!
It seems there is either an ancient Minoean Tomb under an existing shepherds hut on the site or there is one 800 metres to the north of the site and the issue is that they, understandably, do not want a typical new development white box jutting out of the mountain side that can be seen from either site. I am assured that the Archaeology Department now understand that the earth sheltered dwelling I am proposing will be covered in earth and rocks to blend with the mountainside and planted with local herbs, shrubs and wild flowers.

Site of Minoan Tomb?
- Cooling - Particularly important in my case in southern Crete since the sun beating down on most roofing causes high surface temperatures. A living roof, however, stays nice and cool because of the shading effect of plants, the mass of earth and the evaporative cooling effect of stored water.

- Longevity - Built properly the roof will require little or no maintenance. Sun, wind and frost never get to the roofing surface, so protected by earth from these adverse weather conditions, the waterproofing membrane is virtually non-biodegradable.

- Ecology - While not the right place to grow trees and root vegetables the earthroof can support a wide selection of plants and wildlife.

- Protection - Just a few inches of earth protects from fire (particularly important as a recent serious out of control bush fire destroyed thousands of hectares of olive grove and came within a kilometre of the plot!), radiation and sound. With suitable berm  the "cave" type design I am proposing will also contribute to tornado, hurricane and earthquake protection as well. The latter being particularly appropriate in Crete.


"Earth Sheltered Houses" - Rob Roy