Editing Earth shelter (section)

====Passive heating and cooling====
[[File:Amplitude dampening phase shifting.jpg|thumb|left|Diagram showing effect of thermal mass and insulation in an earth sheltered structure. ''y''-axis represents temperature; ''x''-axis represents time. Blue line: external temperature fluctuations between day maximum and night minimum (could also represent Summer maximum and Winter minimum temperature fluctuation on a longer timescale). Red line: Internal temperature. 1: Phase shifting (delay between maximum/minimum external temperature and internal temperature). 2: Amplitude dampening (the reduction in maximum or minimum internal temperature relative to that of the external temperature).]]
Due to its density, compacted earth acts as [[thermal mass]],<ref name="roy2006" /> meaning that it stores heat and releases it again slowly. Compacted soil is more of a [[Thermal conduction|conductor of heat]] than an insulator. Soil is stated as having an [[R-value (insulation)|R-value]]<!-- LINK IS CORRECT FOR THIS CONTEXT DO NOT CHANGE TO [[R-value (soils)]] THANKS --> of about 0.65-R per centimeter (0.08-R per 1 inch),<ref name="Hait2013" /> or 0.25-R per 1 inch.<ref name="roy2006" /> Variations in R-value of soil may be attributed to different soil moisture levels, with lower R values as moisture level increases.<ref name="Hait2013" /> The most superficial layer of earth typically is less dense and contains the root systems of many different plants, thereby acting more like [[thermal insulation]],<ref name="roy2006" /> meaning, it reduces the rate of temperature flowing through it.

Approximately 50% of the heat from the Sun is absorbed at the surface.<ref name="BGS">{{cite web |title=BGS Reference and research reports - Ground source heat pumps |url=http://www.bgs.ac.uk/reference/gshp/gshp_report.html |website=www.bgs.ac.uk |publisher=British Geological Survey}}</ref> Consequently, the temperature at the surface may vary considerably according to the day / night cycle, according to weather and particularly according to season. Underground, these temperature changes are blunted and delayed, termed [[thermal lag]]. The thermal properties of earth therefore mean that in winter the temperature below the surface will be higher than the surface air temperature, and conversely in summer the earth temperature will be lower than the surface air temperature.

Indeed, at a deep enough point underground, the temperature remains constant year round, and this temperature is approximately the mean of summer and winter temperatures.<ref name=BGS /><ref name=Hait2013 /> Sources vary in their stated values for this deep earth constant temperature (also termed amplitude correction factor). Reported values include {{convert|5|-|6|m|ft|abbr=on}},<ref name="thorpe2018" /> {{convert|6|m|ft|abbr=on}},<ref name="Hait2013" /> {{convert|15|m|ft|abbr=on}},<ref name=BGS /> {{convert|4.25|m|ft|abbr=on}} for dry soil, and {{convert|6.7|m|ft|abbr=on}} for wet soil.<ref>Mechanical and Electrical Equipment for Buildings. Walter T. Grondzik, Alison G. Kwok 2014</ref> Below this level the temperature increases on average {{convert|2.6|C-change|0}} every {{convert|100|m|ft|abbr=on}} due to heat rising from the interior of the Earth.<ref name=BGS />

Diurnal temperature changes between maximum and minimum temperatures can be modelled as a wave, as can seasonal temperature changes (see diagram). In architecture, the relationship between the maximum fluctuations of external temperature compared to internal temperature is termed amplitude dampening (or temperature amplitude factor).<ref name="thorpe2018" /> [[Phase shifting]] is the time taken for the minimum external temperature to reach the interior.<ref name="thorpe2018" />

Partially covering a building with earth adds to the thermal mass of the structure.<ref name="thorpe2018" /> Combined with insulation, this results in both amplitude dampening and phase shifting. In other terms, earth sheltered structures receive both a degree of cooling in summer and heating in winter.<ref name="thorpe2018" /> This reduces need for other measures of heating and cooling, saving energy.<ref name="mcconkey2011" /> A potential disadvantage of a thermally massive building in cooler climates is that after a prolonged period of cold, when the external temperature increases again, the structures internal temperature tends to lag behind and take longer to warm up (assuming no other form of heating).

The reduction of air infiltration within an earth shelter can be advantageous. Because three walls of the structure are mainly surrounded by earth, very little surface area is exposed to the outside air. This alleviates the problem of warm air escaping the house through gaps around windows and door. Furthermore, the earth walls protect against cold winter winds which might otherwise penetrate these gaps. However, this can also become a potential indoor air quality problem. Healthy air circulation is key.

As a result of the increased thermal mass of the structure, the [[thermal lag]] of the earth, the protection against unwanted air infiltration and the combined use of passive solar techniques, the need for extra heating and cooling is minimal. Therefore, there is a drastic reduction in energy consumption required for the home compared to homes of typical construction.