Editing Passive solar building design (section)

===Heat storage===
The sun doesn't shine all the time. Heat storage, or [[thermal mass]], keeps the building warm when the sun can't heat it.

In diurnal solar houses, the storage is designed for one or a few days. The usual method is a custom-constructed thermal mass. This includes a [[Trombe wall]], a ventilated concrete floor,{{Sfn|Kachadorian|2006|pp=26-43|loc=§3. The Solar Slab and Basic Solar Design}} a cistern, water wall or roof pond.<ref>{{cite journal|last1=Sharifi|first1=Ayyoob|last2=Yamagata|first2=Yoshiki|title=Roof ponds as passive heating and cooling systems: A systematic review|journal=Applied Energy|date=December 2015|volume=160|pages=336–357|doi=10.1016/j.apenergy.2015.09.061|bibcode=2015ApEn..160..336S }}</ref> It is also feasible to use the thermal mass of the earth itself, either as-is or by incorporation into the structure by banking or using rammed earth as a structural medium.<ref>{{cite web|url=http://earthship.com/|title=Earthships|website=earthship.com}}</ref>

In subarctic areas, or areas that have long terms without solar gain (e.g. weeks of freezing fog), purpose-built thermal mass is very expensive. Don Stephens pioneered an experimental technique to use the ground as thermal mass large enough for annualized heat storage. His designs run an isolated thermosiphon 3&nbsp;m under a house, and insulate the ground with a 6&nbsp;m waterproof skirt.<ref>[http://greenershelter.org/TokyoPaper.pdf Annualized Geo-Solar Heating, Don Stephens]- Accessed 2009-02-05</ref>