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Passive solar buildings aim to maintain interior thermal comfort throughout the sun's daily and annual cycles whilst reducing the requirement for active heating and cooling systems.[1] Passive solar building design is one part of green building design, and does not include active systems such as mechanical ventilation or photovoltaics.

As a science Edit

The scientific basis for passive solar building design has been developed from a combination of climatology, thermodynamics (particularly heat transfer), and human thermal comfort (for buildings to be inhabited by humans). Specific attention is directed to the site and location of the dwelling, the prevailing climate, design and construction, solar orientation, placement of glazing-and-shading elements, and incorporation of thermal mass. While these considerations may be directed to any building, achieving an ideal solution requires careful integration of these principles. Modern refinements through computer modeling and application of other technology can achieve significant energy savings without necessarily sacrificing functionality or aesthetics.[2][3] In fact it is for this reason that this newly coined term, known as Architectural Science or Architectural Technology, has become an upcoming subject area in most schools of Architecture worldwide.

The solar path in passive designEdit

Solar altitude

Solar altitude over a year; latitude based on New York, New York

Main article: Sun path

The ability to achieve these goals simultaneously is fundamentally dependent on the seasonal variations in the sun's path throughout the day

This occurs as a result of the inclination of the Earth's axis of rotation in relation to its orbit. The sun path is unique for any given latitude. Generally the sun will appear to rise in the east and set in the west.

In Northern Hemisphere non-tropical latitudes farther than 23.5 degrees from the equator:

  • The sun will reach its highest point toward the South (in the direction of the equator)
  • As winter solstice approaches, the angle at which the sun rises and sets progressively moves further toward the South and the daylight hours will become shorter
  • The opposite is noted in summer where the sun will rise and set further toward the North and the daylight hours will lengthen[4]

The converse is observed in the Southern Hemisphere, but the sun rises to the east and sets toward the west regardless of which hemisphere you are in.

In equatorial regions at less than 23.5 degrees, the position of the sun at solar noon will oscillate from north to south and back again during the year.[5]

In regions closer than 23.5 degrees from either north-or-south pole, during summer the sun will trace a complete circle in the sky without setting whilst it will never appear above the horizon six months later, during the height of winter.[6]

The 47-degree difference in the altitude of the sun at solar noon between winter and summer forms the basis of passive solar design. This information is combined with local climatic data (degree day) heating and cooling requirements to determine at what time of the year solar gain will be beneficial for thermal comfort, and when it should be blocked with shading. By strategic placement of items such as glazing and shading devices, the percent of solar gain entering a building can be controlled throughout the year.

One passive solar sun path design problem is that although the sun is in the same relative position six weeks before, and six weeks after, the solstice, due to "thermal lag" from the thermal mass of the Earth, the temperature and solar gain requirements are quite different before and after the summer or winter solstice. Movable shutters, shades, shade screens, or window quilts can accommodate day-to-day and hour-to-hour solar gain and insulation requirements.

Careful arrangement of rooms completes the passive solar design. A common recommendation for residential dwellings is to place living areas facing solar noon and sleeping quarters on the opposite side.[7] A heliodon is a traditional movable light device used by architects and designers to help model sun path effects. In modern times, 3D computer graphics can visually simulate this data, and calculate performance predictions.[2]

Passive solar thermodynamic principles Edit

Laundromat-SolarCell

Solar panels are used in passive and active solar hot water systems

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 evaporative cooling). Heat transfer in buildings occurs through convection, conduction, and thermal radiation through roof, walls, floor and windows.[8]

Convective heat transferEdit

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,[9] 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.[10] Filtered energy recovery ventilation systems may be useful to eliminate undesirable humidity, dust, pollen, and microorganisms in unfiltered ventilation air.

Natural convection causing 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 near by windows. In addition, high relative humidity inhibits evaporative cooling by humans.

Radiative heat transferEdit

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[11] (see albedo, absorptivity, emissivity, and reflectivity).

Windows are a ready and predictable site for thermal radiation.[12] Energy from radiation can move into a window in the day time, and out of the same window at night. Radiation uses photons 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. Convective heat transfer through and around window coverings also degrade its insulation properties.[12] When shading windows, external shading is more effective at reducing heat gain than internal window coverings.[12]

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. The amount of radiant heat received is related to the location latitude, altitude, cloud cover, and seasonal / hourly angle of incidence (see Sun path and Lambert's cosine law).

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

Site specific considerations during design Edit

Design elements for residential buildings in temperate climates Edit

  • Placement of room-types, internal doors & walls, & equipment in the house.
  • Orienting the building to face the equator (or a few degrees to the East to capture the morning sun)[7]
  • Extending the building dimension along the east/west axis
  • Adequately sizing windows to face the midday sun in the winter, and be shaded in the summer.
  • Minimising windows on other sides, especially western windows[12]
  • Erecting correctly sized, latitude-specific roof overhangs[13], or shading elements (shrubbery, trees, trellises, fences, shutters, etc.)[14]
  • Using the appropriate amount and type of insulation including radiant barriers and bulk insulation to minimise seasonal excessive heat gain or loss
  • Using thermal mass to store excess solar energy during the winter day (which is then re-radiated during the night)[15]

The precise amount of equator-facing glass and thermal mass should be based on careful consideration of latitude, altitude, climatic conditions, and heating/cooling degree day requirements.

Factors that can degrade thermal performance:

  • Deviation from ideal orientation and north/south/east/west aspect ratio
  • Excessive glass area ('over-glazing') resulting in overheating (also resulting in glare and fading of soft furnishings) and heat loss when ambient air temperatures fall
  • Installing glazing where solar gain during the day and thermal losses during the night cannot be controlled easily e.g. West-facing, angled glazing, skylights[16]
  • Thermal losses through non-insulated or unprotected glazing
  • Lack of adequate shading during seasonal periods of high solar gain (especially on the West wall)
  • Incorrect application of thermal mass to modulate daily temperature variations
  • Open staircases leading to unequal distribution of warm air between upper and lower floors as warm air rises
  • High building surface area to volume - Too many corners
  • Inadequate weatherization leading to high air infiltration
  • Lack of, or incorrectly installed, radiant barriers during the hot season. (See also cool roof and green roof)
  • Insulation materials that are not matched to the main mode of heat transfer (e.g. undesirable convective/conductive/radiant heat transfer)

Key passive solar building design conceptsEdit

There are four primary passive solar energy configurations:[17]

Direct solar gainEdit

Illust passive solar d1

Elements of passive solar design, shown in a direct gain application

Direct gain attempts to control the amount of direct solar radiation reaching the living space. This direct solar gain is a critical part of passive solar house designation as it imparts to a direct gain.

A Passive Solar Sunbed is currently being designed at UCD.

The cost effectiveness of these configurations are currently being investigated in great detail and are demonstrating promising results.[18]

Indirect solar gainEdit

Indirect gain attempts to control solar radiation reaching an area adjacent but not part of the living space. Heat enters the building through windows and is captured and stored in thermal mass (e.g. water tank, masonry wall) and slowly transmitted indirectly to the building through conduction and convection. Efficiency can suffer from slow response (thermal lag) and heat losses at night. Other issues include the cost of insulated glazing and developing effective systems to redistribute heat throughout the living area.

Isolated solar gainEdit

Isolated gain involves utilizing solar energy to passively move heat from or to the living space using a fluid, such as water or air by natural convection or forced convection. Heat gain can occur through a sunspace, solarium or solar closet. These areas may also be employed usefully as a greenhouse or drying cabinet. An equator-side sun room may have its exterior windows higher than the windows between the sun room and the interior living space, to allow the low winter sun to penetrate to the cold side of adjacent rooms. Glass placement and overhangs prevent solar gain during the summer. Earth cooling tubes or other passive cooling techniques can keep a solarium cool in the summer.

Measures should be taken to reduce heat loss at night e.g. window coverings or movable window insulation

Examples:

Heat StorageEdit

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. These include a Trombe wall, a ventilated concrete floor, a cistern, water wall or roof pond.

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 3m under a house, and insulate the ground with a 6m waterproof skirt.[20]

Insulation Edit

Main article: Building insulation

Thermal insulation or superinsulation (type, placement and amount) reduces unwanted leakage of heat.[8] Some passive buildings are actually constructed of insulation.

Special glazing systems and window coverings Edit

Main article: Insulated glazing

The effectiveness of direct solar gain systems is significantly enhanced by insulative (e.g. double glazing), spectrally selective glazing (low-e), or movable window insulation (window quilts, bifold interior insulation shutters, shades, etc.).[21]

Generally, Equator-facing windows should not employ glazing coatings that inhibit solar gain.

There is extensive use of super-insulated windows in the German Passive House standard. Selection of different spectrally selective window coating depends on the ratio of heating versus cooling degree days for the design location.

Glazing selection Edit

Equator-facing glass Edit

The requirement for vertical equator-facing glass is different from the other three sides of a building. Reflective window coatings and multiple panes of glass can reduce useful solar gain. However, direct-gain systems are more dependent on double or triple glazing 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 / Skylights Edit

Skylights admit sunlight either horizontally (a flat roof) or pitched at the same angle as the roof slope. In most cases, horizontal skylights are used with reflectors to increase the intensity of solar radiation depending on the angle of incidence. Large skylights should be provided with shading devices to prevent heat loss at night and heat gain during the summer months.

Skylights on roofs that face north provide fairly constant but cool illumination. Those on east-facing roofs provide maximum light and solar heat gain in the morning. West-facing skylights provide afternoon sunlight and heat gain. South-facing skylights provide the greatest potential for desirable winter passive solar heat gain than any other location, but often allow unwanted heat gain in the summer. You can prevent unwanted solar heat gain by installing the skylight in the shade of deciduous (leaf-shedding) trees or adding a movable window covering on the inside or outside of the skylight. Some modern designs have special glazing that can help control solar heat gain while still allowing high levels of visible light transmittance. Skylights are often the only method to bring passive solar into the core of a commercial or industrial application.

Angle of incident radiation Edit

The amount of solar gain transmitted through glass is also affected by the angle of the incident solar radiation. Sunlight striking glass within 20 degrees of perpendicular is mostly transmitted through the glass, whereas sunlight at more than 35 degrees from perpendicular is mostly reflected[22]

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 transmissivity, based on angle of incidence.

Alternatively, passive solar computer software can determine the impact of sun path, and cooling-and-heating degree days on energy performance. Regional climatic conditions are often available from local weather services.

Operable shading and insulation devicesEdit

A design with too much equator-facing glass can result in excessive winter, spring, or fall day heating, uncomfortably bright living spaces at certain times of the year, and excessive heat transfer on winter nights and summer days.

Although the sun is at the same altitude 6-weeks before and after the solstice, the heating and cooling requirements before and after the solstice are significantly different. Heat storage on the Earth's surface causes "thermal lag." Variable cloud cover influences solar gain potential. This means that latitude-specific fixed window overhangs, while important, are not a complete seasonal solar gain control solution.

Control mechanisms (such as manual-or-motorized interior insulated drapes, shutters, exterior roll-down shade screens, or retractable awnings) can compensate for differences caused by thermal lag or cloud cover, and help control daily / hourly solar gain requirement variations.

Home automation systems that monitor temperature, sunlight, time of day, and room occupancy can precisely control motorized window-shading-and-insulation devices.

Exterior colors reflecting - absorbingEdit

Materials and colors can be chosen to reflect or absorb solar thermal energy. Using information on a Color for electromagnetic radiation to determine its thermal radiation properties of reflection or absorption can assist the choices.
See Lawrence Berkeley National Laboratory and Oak Ridge National Laboratory: "Cool Colors"

Landscaping and gardensEdit

Main article: Energy-efficient landscaping

Energy-efficient landscaping materials for careful passive solar choices include hardscape building material and "softscape" plants. The use of landscape design principles for selection of trees, hedges, and trellis-pergola features with vines; all can be used to create summer shading. For winter solar gain it is desirable to use deciduous plants that drop their leaves in the autumn gives year round passive solar benefits. Non-deciduous evergreen shrubs and trees can be windbreaks, at variable heights and distances, to create protection and shelter from winter wind chill. Xeriscaping with 'mature size appropriate' native species of-and drought tolerant plants, drip irrigation, mulching, and organic gardening practices reduce or eliminate the need for energy-and-water-intensive irrigation, gas powered garden equipment, and reduces the landfill waste footprint. Solar powered landscape lighting and fountain pumps, and covered swimming pools and plunge pools with solar water heaters can reduce the impact of such amenities.

Other passive solar principlesEdit

Passive solar lightingEdit

Main article: Passive solar lighting

Passive solar lighting techniques enhance taking advantage of natural illumination for interiors, and so reduce reliance on artificial lighting systems.

This can be achieved by careful building design, orientation, and placement of window sections to collect light. Other creative solutions involve the use of reflecting surfaces to admit daylight into the interior of a building. Window sections should be adequately sized, and to avoid over-illumination can be shielded with a Brise soleil, awnings, well placed trees, glass coatings, and other passive and active devices. [17]

Another major issue for many window systems is that they can be potentially vulnerable sites of excessive thermal gain or heat loss. Whilst high mounted clerestory window and traditional skylights can introduce daylight in poorly orientated sections of a building, unwanted heat transfer may be hard to control.[23][24] Thus, energy that is saved by reducing artificial lighting is often more than offset by the energy required for operating HVAC systems to maintain thermal comfort.

Various methods can be employed to address this including but not limited to window coverings, insulated glazing and novel materials such as aerogel semi-transparent insulation, optical fiber embedded in walls or roof, or hybrid solar lighting at Oak Ridge National Laboratory.

Interior reflectingEdit

Reflecting elements, from active and passive daylighting collectors, such as light shelves, lighter wall and floor colors, mirrored wall sections, interior walls with upper glass panels, and clear or translucent glassed hinged doors and sliding glass doors take the captured light and passively reflect it further inside. The light can be from passive windows or skylights and solar light tubes or from active daylighting sources. In traditional Japanese architecture the Shōji sliding panel doors, with translucent Washi screens, are an original precedent. International style, Modernist and Mid-century modern architecture were earlier innovators of this passive penetration and reflection in industrial, commercial, and residential applications.

Passive solar water heatingEdit

Main article: Solar hot water

There are many ways to use solar thermal energy to heat water for domestic use. Different active-and-passive solar hot water technologies have different location-specific economic cost benefit analysis implications.

Fundamental passive solar hot water heating involves no pumps or anything electrical. It is very cost effective in climates that do not have lengthy sub-freezing, or very-cloudy, weather conditions. Other active solar water heating technologies, etc. may be more appropriate for some locations.

It is possible to have active solar hot water which is also capable of being "off grid" and qualifies as sustainable. This is done by the use of a photovoltaic cell which uses energy from the sun to power the pumps.[citation needed]

Comparison to the Passive House standard in EuropeEdit

Main article: Passive house

There is growing momentum in Europe for the approach espoused by the Passive House Institute in Germany. Rather than relying solely on traditional passive solar design techniques, this approach seeks to make use of all passive sources of heat, minimises energy usage, and emphasises the need for high levels of insulation reinforced by meticulous attention to detail in order to address thermal bridging and cold air infiltration. Most of the buildings built to the Passive House standard also incorporate an active heat recovery ventilation unit with or without a small (typically 1 kW) incorporated heating component.

The energy design of Passive House buildings is developed using a spreadsheet-based modeling tool called the Passive House Planning Package (PHPP) which is updated periodically. The current version is PHPP2007, where 2007 is the year of issue. A building may be certified as a 'Passive House' when it can be shown that it meets certain criteria, the most important being that the annual specific heat demand for the house should not exceed 15kWh/m2a.

Design tools Edit

Traditionally a heliodon was used to simulate the altitude and azimuth of the sun shining on a model building at any time of any day of the year.[25] In modern times, computer programs can model this phenomenon and integrate local climate data (including site impacts such as overshadowing and physical obstructions) to predict the solar gain potential for a particular building design over the course of a year. This provides the designer the ability to evaluate design elements and orientation prior to building works commencing. Energy performance optimization normally requires an iterative-refinement design-and-evaluate process.

Levels of applicationEdit

Pragmatic Edit

Many detached suburban houses can achieve reductions in heating expense without obvious changes to their appearance, comfort or usability.[26] This is done using good siting and window positioning, small amounts of thermal mass, with good-but-conventional insulation, weatherization, and an occasional supplementary heat source, such as a central radiator connected to a (solar) water heater. Sunrays may fall on a wall during the daytime and raise the temperature of its thermal mass. This will then radiate heat into the building in the evening. This can be a problem in the summer, especially on western walls in areas with high degree day cooling requirements. External shading, or a radiant barrier plus air gap, may be used to reduce undesirable summer solar gain.

Annualised Edit

An extension of the "passive solar" approach to seasonal solar capture and storage of heat and cooling. These designs attempt to capture warm-season solar heat, and convey it to a seasonal thermal store for use months later during the cold season ("annualised passive solar.") Increased storage is achieved by employing large amounts of thermal mass or earth coupling. Anecdotal reports suggest they can be effective but no formal study has been conducted to demonstrate their superiority. The approach also can move cooling into the warm season.

Examples:

Minimum machinery Edit

A "purely passive" solar-heated house would have no mechanical furnace unit, relying instead on energy captured from sunshine, only supplemented by "incidental" heat energy given off by lights, computers, and other task-specific appliances (such as those for cooking, entertainment, etc.), showering, people and pets. The use of natural convection air currents (rather than mechanical devices such as fans) to circulate air is related, though not strictly solar design.

Passive solar building design sometimes uses limited electrical and mechanical controls to operate dampers, insulating shutters, shades, awnings, or reflectors. Some systems enlist small fans or solar-heated chimneys to improve convective air-flow. A reasonable way to analyse these systems is by measuring their coefficient of performance. A heat pump might use 1 J for every 4 J it delivers giving a COP of 4. A system that only uses a 30 W fan to more-evenly distribute 10 kW of solar heat through an entire house would have a COP of 300.

Zero Energy Building Edit

Passive solar building design is often a foundational element of a cost-effective zero energy building.[27][28][29] Although a ZEB uses multiple passive solar building design concepts, a ZEB is usually not purely passive, having active mechanical renewable energy generation systems such as: wind turbine, photovoltaics, micro hydro, geothermal, and other emerging alternative energy sources.

See alsoEdit

Energy Rating systems

ReferencesEdit

  1. "Passive design - Introduction". http://www.greenhouse.gov.au/yourhome/technical/fs10.htm. Retrieved 2008-01-14. 
  2. 2.0 2.1 "Rating tools". http://www.greenhouse.gov.au/yourhome/technical/fs110.htm. Retrieved 2008-01-14. 
  3. Your Home Design Guide - Home Page[dead link]
  4. http://www.srrb.noaa.gov/highlights/sunrise/fig5_40n.gif
  5. http://www.srrb.noaa.gov/highlights/sunrise/fig5_0n.gif
  6. http://www.srrb.noaa.gov/highlights/sunrise/fig5_90n.gif
  7. 7.0 7.1 Your Home Design Guide - Home Page[dead link]
  8. 8.0 8.1 Your Home Design Guide - Home Page[dead link]
  9. "BERC - Airtightness". Ornl.gov. 2004-05-26. http://www.ornl.gov/sci/roofs+walls/whole_wall/airtight.html. Retrieved 2010-03-16. 
  10. Your Home Design Guide - Home Page[dead link]
  11. "EERE Radiant Barriers". Eere.energy.gov. 2009-05-28. http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11680. Retrieved 2010-03-16. 
  12. 12.0 12.1 12.2 12.3 "Glazing - Overview". http://www.greenhouse.gov.au/yourhome/technical/fs18a.htm. Retrieved 2008-01-14. 
  13. Springer, John L. (December 1954). "The 'Big Piece' Way to Build". Popular Science 165 (6): 157. http://www.popsci.com/archive-viewer?id=1yADAAAAMBAJ&pg=157&query=1955. 
  14. Your Home Design Guide - Home Page[dead link]
  15. Your Home Design Guide - Home Page[dead link]
  16. "Introductory Passive Solar Energy Technology Overview". U.S. DOE - ORNL Passive Solar Workshop. http://www.passivesolarenergy.info/#S1. Retrieved 2007-12-23. 
  17. 17.0 17.1 Chiras, D. The Solar House: Passive Heating and Cooling. Chelsea Green Publishing Company; 2002.
  18. "Zero Energy Buildings". Fsec.ucf.edu. http://www.fsec.ucf.edu/en/research/buildings/zero_energy/index.htm. Retrieved 2010-03-16. 
  19. "Two Small Delta Ts Are Better Than One Large Delta T". Zero Energy Design. http://www.zeroenergydesign.com/Passive%20Solar%20Cooling.html#c22. Retrieved 2007-12-23. 
  20. Annualized Geo-Solar Heating, Don Stephens- Accessed 2009-02-05
  21. William A. Shurcliff. Thermal Shutters & Shades - Over 100 Schemes for Reducing Heat Loss through Windows 1980. ISBN 0-931790-14-X. 
  22. "Solar Heat Gain Through Glass". Irc.nrc-cnrc.gc.ca. 2010-03-08. http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd039_e.html. Retrieved 2010-03-16. 
  23. "[ARCHIVED CONTENT] Insulating and heating your home efficiently : Directgov - Environment and greener living". Direct.gov.uk. http://www.direct.gov.uk/en/Environmentandgreenerliving/Greenerhome/DG_064374. Retrieved 2010-03-16. 
  24. "Reduce Your Heating Bills This Winter - Overlooked Sources of Heat Loss in the Home". Allwoodwork.com. 2003-02-14. http://www.allwoodwork.com/article/homeimprovement/reduce_your_heating_bills.html. Retrieved 2010-03-16. 
  25. [1][dead link]
  26. "Industrial Technologies Program: Industrial Distributed Energy". Eere.energy.gov. http://www.eere.energy.gov/de/passive_solar_design.html. Retrieved 2010-03-16. 
  27. "Cold-Climate Case Study for Affordable Zero Energy Homes: Preprint" (PDF). http://www.nrel.gov/docs/fy06osti/39678.pdf. Retrieved 2010-03-16. 
  28. "Zero Energy Homes: A Brief Primer" (PDF). http://www.toolbase.org/PDF/CaseStudies/ZEHPrimer.pdf. Retrieved 2010-03-16. 
  29. Moving Toward Zero Energy Homes (Fact Sheet)[dead link]

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