Editing Passive solar building design (section)

==Passive solar heat transfer principles==

Personal [[thermal comfort]] is a function of personal health factors (medical, psychological, sociological and situational), ambient air temperature, [[mean radiant temperature]], air movement ([[wind chill]], [[turbulence]]) and [[relative humidity]] (affecting human [[evaporation|evaporative]] cooling). [[Heat transfer]] in buildings occurs through [[convection]], [[Heat conduction|conduction]], and [[thermal radiation]] through roof, walls, floor and windows.<ref name="autogenerated3">{{cite web|url=http://www.yourhome.gov.au/technical/fs47.html|title=Your Home Technical Manual - 4.7 Insulation|date=25 March 2012|archive-url=https://web.archive.org/web/20120325024055/http://www.yourhome.gov.au/technical/fs47.html|archive-date=2012-03-25}}</ref>

===Convective heat transfer===
[[Convection (heat transfer)|Convective heat transfer]] can be beneficial or detrimental. Uncontrolled air infiltration from poor [[weatherization]] / weatherstripping / draft-proofing can contribute up to 40% of heat loss during winter;<ref>{{cite web |url=http://www.ornl.gov/sci/roofs+walls/whole_wall/airtight.html |title=BERC – Airtightness |publisher=Ornl.gov |date=2004-05-26 |access-date=2010-03-16 |archive-url=https://web.archive.org/web/20100828100447/http://www.ornl.gov/sci/roofs%2Bwalls/whole_wall/airtight.html |archive-date=2010-08-28 |url-status=dead }}</ref> however, strategic placement of operable windows or vents can enhance convection, cross-ventilation, and summer cooling when the outside air is of a comfortable temperature and [[relative humidity]].<ref>{{cite web|url=http://yourhome.gov.au/technical/fs46.html|title=Your Home Technical Manual - 4.6 Passive Cooling|date=20 March 2012|archive-url=https://web.archive.org/web/20120320202939/http://yourhome.gov.au/technical/fs46.html|archive-date=2012-03-20}}</ref> Filtered [[energy recovery ventilation]] systems may be useful to eliminate undesirable humidity, dust, pollen, and microorganisms in unfiltered ventilation air.

Natural convection causing [[stack effect|rising]] warm air and falling cooler air can result in an uneven stratification of heat. This may cause uncomfortable variations in temperature in the upper and lower conditioned space, serve as a method of venting hot air, or be designed in as a natural-convection air-flow loop for passive solar heat distribution and temperature equalization. Natural human cooling by [[perspiration]] and [[evaporation]] may be facilitated through natural or forced convective air movement by fans, but ceiling fans can disturb the stratified insulating air layers at the top of a room, and accelerate heat transfer from a hot attic, or through nearby windows. In addition, high [[relative humidity]] inhibits evaporative cooling by humans.

===Radiative heat transfer===
The main source of [[heat transfer]] is [[radiant energy]], and the primary source is the sun. Solar radiation occurs predominantly through the roof and windows (but also through walls). [[Thermal radiation]] moves from a warmer surface to a cooler one. Roofs receive the majority of the solar radiation delivered to a house. A [[cool roof]], or [[green roof]] in addition to a [[radiant barrier]] can help prevent your attic from becoming hotter than the peak summer outdoor air temperature<ref>{{cite web|url=http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11680 |title=EERE Radiant Barriers |publisher=Eere.energy.gov |date=2009-05-28 |access-date=2010-03-16}}</ref> (see [[albedo]], [[Absorbance|absorptivity]], [[emissivity]], and [[reflectivity]]).

Windows are a ready and predictable site for [[thermal radiation]].<ref name=fs18a>{{cite web
 | url = http://www.yourhome.gov.au/technical/fs410.html
 | title = Glazing
 | access-date = 2011-11-03
|archive-url = https://web.archive.org/web/20071215075543/http://www.greenhouse.gov.au/yourhome/technical/fs18a.htm |archive-date = December 15, 2007}}</ref>
Energy from radiation can move into a window in the day time, and out of the same window at night. Radiation uses [[photon]]s to transmit [[electromagnetic waves]] through a vacuum, or translucent medium. Solar heat gain can be significant even on cold clear days. Solar heat gain through windows can be reduced by [[insulated glazing]], shading, and orientation. Windows are particularly difficult to insulate compared to roof and walls. [[Convection (heat transfer)|Convective heat transfer]] through and around [[window covering]]s also degrade its insulation properties.<ref name=fs18a/> When shading windows, external shading is more effective at reducing heat gain than internal [[window covering]]s.<ref name=fs18a/>

Western and eastern sun can provide warmth and lighting, but are vulnerable to overheating in summer if not shaded. In contrast, the low midday sun readily admits light and warmth during the winter, but can be easily shaded with appropriate length overhangs or angled louvres during summer and leaf bearing summer shade trees which shed their leaves in the fall. The amount of radiant heat received is related to the location [[latitude]], [[altitude]], [[cloud cover]], and seasonal / hourly [[angle of incidence (optics)|angle of incidence]] (see [[Sun path]] and [[Lambert's cosine law]]).

Another passive solar design principle is that thermal energy can be [[Thermal mass|stored]] in certain building materials and released again when heat gain eases to stabilize [[diurnal temperature variation|diurnal]] (day/night) temperature variations. The complex interaction of [[thermodynamic]] principles can be [[counterintuitive]] for first-time designers. Precise [[Building Information Modeling|computer modeling]] can help avoid costly construction experiments.