It winter it doesn't take much heating to bring the inside air temperature up to a comfortable 22 degrees C - sunlight might even be enough.
In summer the inside is likely to stay around 20-22 most of the time, climbing to a peak of 25-26 after several weeks of 40 degrees outside.
Thermal Stability of Earth Sheltered Houses
How and Why they Work
The temperature of the earth at 2 to 3 metres below the surface remains relatively constant, at a temperature equal to the annual average air temperature directly above the surface. This is because that is the depth where the daily, seasonally and yearly temperature fluctuations even themselves out.
Many ground based animals have known about this for as long as they have been on the earth...and early man was no exception.
In most places in Australia the annual average air temperature is around 14 to 19 degrees.
An Earth Sheltered House is built into this stable temperature environment, so a large part of the external structure is subjected to this constant temperature which in turn influences the inside air temperature. This constant ground temperature effects the inside air temperature by conduction through the structure which in turn creates convection currents, distributing the heat around the home.
The interior temperature of the home is also modified by direct radiant heat transfer. The ground temperature (sometimes referred to as "Coolth") is constantly being radiated into the interior of the house, transmitting it's energy directly to any solid objects, which in turn modifies the air temperature by convection.
Another big factor influencing the thermal stability of an Earth Sheltered House is the fact that the side and rear walls are not subjected to the cold winter winds and rains that an above-ground house does.
Radiant Heat Effect
The human body detects changes in air temperature that surrounds it and also the radiant heat that lands on it. The greater the mass in contact with the structure, the more the radiant heat effect...and the mass of the earth against the house is huge and at a constant temperature.
The radiant heat falling on the skin plays a big part in human comfort...even if the air temperature is different.
The nett effect of this radiant heat transfer is to add to year-round human comfort... cool in summer and warm in winter. If it is 40 outside, but the rear walls are radiating a constant 18...then you will feel cooler than the actual 23 degree air temperature.
All homes in Australia are required by the Australian Building Code to have specific amounts of airflow, and this is especially so in an earth sheltered home.
We at ShelterSpace have spent many years devising a system that maximises the air-flow through our homes and make sure that every room has the appropriate amount of cross ventilation.
Our designs utilise small internal courts that light the rear of the home and achieve maximum air-flow by the updraft principal. Airflow can be achieved through the house even on a completely still day.
Tropical climates obviously require special attention.
The most efficient way of capturing passive solar heat energy in a home in winter is to face the windows in a Northerly direction and provide them with appropriate overhanging eaves to control sunlight entering in summer.
An earth sheltered home is not so critical in this regard as it derives a large proportion of it's thermal stability from the constant temperature of the earth mass surrounding it. In most cases the orientation is governed by the site slope direction, but it is still best to orientate to the North if possible...so bear this in mind when selecting a site.
It is essential to understand the effect of the micro-climate on the design of the earth sheltered house. One side of a hill can have a vastly different requirement to the other side. Knowing the details of the micro-climate makes a difference to the final outcome...that's what 'design' is all about!
The area of the exposed windows needs to be in proportion to the floor area - dictated to a large degree by the climatic region.
The glass type (single, double glazing and E glass, etc) can play a big part of the window area decision.