Editing Passive solar building design (section)

===Glazing selection===

====Equator-facing glass====
The requirement for vertical equator-facing glass is different from the other three sides of a building. [[Insulated glazing|Reflective window coatings]] and multiple panes of glass can reduce useful solar gain. However, direct-gain systems are more dependent on [[Insulated glazing|double or triple glazing]] or even [[quadruple glazing]] in higher geographic latitudes to reduce heat loss. Indirect-gain and isolated-gain configurations may still be able to function effectively with only single-pane glazing. Nevertheless, the optimal cost-effective solution is both location and system dependent.

====Roof-angle glass and skylights====

Skylights admit harsh direct overhead sunlight and glare<ref>{{cite web  | title = Florida Solar Energy Center – Skylights | url = http://www.fsec.ucf.edu/en/consumer/buildings/homes/windows/skylights.htm | access-date = 2011-03-29 }}</ref> either horizontally (a flat roof) or pitched at the same angle as the roof slope. In some cases, horizontal skylights are used with reflectors to increase the intensity of solar radiation (and harsh glare), depending on the roof [[angle of incidence (optics)|angle of incidence]]. When the winter sun is low on the horizon, most solar radiation reflects off of roof angled glass ( the [[angle of incidence (optics)|angle of incidence]] is nearly parallel to roof-angled glass morning and afternoon ). When the summer sun is high, it is nearly perpendicular to roof-angled glass, which maximizes solar gain at the wrong time of year, and acts like a solar furnace. Skylights should be covered and well-insulated to reduce [[natural convection]] ( warm air rising ) heat loss on cold winter nights, and intense solar heat gain during hot spring/summer/fall days.

The equator-facing side of a building is south in the northern hemisphere, and north in the southern hemisphere. Skylights on roofs that face away from the equator provide mostly indirect illumination, except for summer days when the sun may rise on the non-equator side of the building (at some [[latitudes]]). Skylights on east-facing roofs provide maximum direct light and solar heat gain in the summer morning. West-facing skylights provide afternoon sunlight and heat gain during the hottest part of the day.

Some skylights have expensive glazing that partially reduces summer solar heat gain, while still allowing some visible light transmission. However, if visible light can pass through it, so can some radiant heat gain (they are both [[electromagnetic radiation]] waves).

You can partially reduce some of the unwanted roof-angled-glazing summer solar heat gain by installing a skylight in the shade of [[deciduous]] (leaf-shedding) trees, or by adding a movable insulated opaque window covering on the inside or outside of the skylight. This would eliminate the daylight benefit in the summer. If tree limbs hang over a roof, they will increase problems with leaves in rain gutters, possibly cause roof-damaging [[Ice dam (roof)|ice dams]], shorten roof life, and provide an easier path for pests to enter your attic. Leaves and twigs on skylights are unappealing, difficult to clean, and can increase the glazing breakage risk in wind storms.

"Sawtooth roof glazing" with vertical-glass-only can bring some of the passive solar building design benefits into the core of a commercial or industrial building, without the need for any roof-angled glass or skylights.

Skylights provide daylight. The only view they provide is essentially straight up in most applications. Well-insulated [[light tube]]s can bring daylight into northern rooms, without using a skylight. A passive-solar greenhouse provides abundant daylight for the equator-side of the building.

Infrared [[thermography]] color thermal imaging cameras ( used in formal [[energy audit]]s ) can quickly document the negative thermal impact of roof-angled glass or a skylight on a cold winter night or hot summer day.

The U.S. Department of Energy states: "vertical glazing is the overall best option for sunspaces."<ref>{{cite web | title = U.S. Department of Energy – Energy Efficiency and Renewable Energy – Sunspace Orientation and Glazing Angles | url = http://www.energysavers.gov/your_home/designing_remodeling/index.cfm/mytopic=10320 | access-date = 2011-03-28 | archive-date = 2011-03-09 | archive-url = https://web.archive.org/web/20110309022331/http://www.energysavers.gov/your_home/designing_remodeling/index.cfm/mytopic=10320 | url-status = dead }}</ref> Roof-angled glass and sidewall glass are not recommended for passive solar sunspaces.

The U.S. DOE explains drawbacks to roof-angled glazing: Glass and plastic have little structural strength. When installed vertically, glass (or plastic) bears its own weight because only a small area (the top edge of the glazing) is subject to gravity. As the glass tilts off the vertical axis, however, an increased area (now the sloped cross-section) of the glazing has to bear the force of gravity. Glass is also brittle; it does not flex much before breaking. To counteract this, you usually must increase the thickness of the glazing or increase the number of structural supports to hold the glazing. Both increase overall cost, and the latter will reduce the amount of solar gain into the sunspace.

Another common problem with sloped glazing is its increased exposure to the weather. It is difficult to maintain a good seal on roof-angled glass in intense sunlight. Hail, sleet, snow, and wind may cause material failure. For occupant safety, regulatory agencies usually require sloped glass to be made of safety glass, laminated, or a combination thereof, which reduce solar gain potential. Most of the roof-angled glass on the Crowne Plaza Hotel Orlando Airport sunspace was destroyed in a single windstorm. Roof-angled glass increases construction cost, and can increase insurance premiums. Vertical glass is less susceptible to weather damage than roof-angled glass.

It is difficult to control solar heat gain in a sunspace with sloped glazing during the summer and even during the middle of a mild and sunny winter day. Skylights are the antithesis of [[zero energy building]] Passive Solar Cooling in climates with an air conditioning requirement.

====Angle of incident radiation====

The amount of solar gain transmitted through glass is also affected by the angle of the incident [[solar radiation]]. [[Sunlight]] striking a single sheet of glass within 45 degrees of [[perpendicular]] is mostly transmitted (less than 10% is [[Reflection (physics)|reflected]]), whereas for sunlight striking at 70 degrees from perpendicular over 20% of light is reflected, and above 70 degrees this percentage reflected rises sharply.<ref>{{cite web|url=http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd039_e.html|archive-url=https://web.archive.org/web/20090321191136/http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd039_e.html|archive-date=2009-03-21 |title=Solar Heat Gain Through Glass |publisher=Irc.nrc-cnrc.gc.ca |date=2010-03-08 |access-date=2010-03-16}}</ref>

All of these factors can be modeled more precisely with a photographic [[light meter]] and a heliodon or [[optical bench]], which can quantify the ratio of [[reflectivity]] to [[Transmittance|transmissivity]], based on [[angle of incidence (optics)|angle of incidence]].

Alternatively, passive solar computer software can determine the impact of [[sun path]], and cooling-and-heating [[degree day]]s on [[energy]] performance.