Calculating the Depth of the Overhang
Fixed architectural overhangs are designed based on the solar geometry for your latitude. The width of the overhang is determine by the solar angle of the sun on June 21 and the height of window relative to the width of the soffit. It should totally shade the window in June and allow full solar penetration through the window on December 21.
Passive solar seems simple enough conceptually; it is the balance of all components that make it work year round. It is the balance of glass to mass and appropriate shading with natural ventilation strategies that makes the house work as a system.
The following list of principles should help guide the passive solar design process.
- Orient the house within 30 degrees of due south
- Use design software to optimize passive solar heating
- Super-insulate the house
- Design the size of south glazing to meet the heat loss of the house
- Configure thermal mass to absorb sunlight
- Determine appropriate overhangs for all south glass
- Limit east, west, and north glass while providing for cross-ventilation
- Design appropriate shading strategies for east and west glass
- Calculate the backup heating and cooling required
Due south is the equivalent of 100% potential solar heat gain through windows. Rotating the house to within 30 degrees of due south still provides about 90% of the potential solar gain and allows latitude for adjusting the house to lot limitations. Much further than 30 degrees starts to make architectural shading difficult and can lead to overheating.
Several software tools are available to help optimize passive solar designs. Foremost among those is Energy-10®, developed by Dr. Doug Balcomb at National Renewable Energy Laboratory. In its latest edition, it also assists in sizing solar hot-water systems and photovoltaics.
Super-insulation and air sealing are the perfect context for passive solar heating. The lower the heating and cooling loads of the house, the less solar heat is necessary to provide comfort through much of the year.
The size of south-facing glass is determined by location, the potential amount of sunshine, degree days of the local climate, the heat loss of the house, the internal architecture of the building, and intended thermal mass. In an ideal design, south windows would be distributed across the south wall to provide heat gain to as many areas of the house as possible
Concrete, concrete masonry units (CMUs), brick, stone, and tile are typical materials used as thermal mass inside the building.The amount of glazing and insulation helps determine the amount of mass required to keep the house from overheating during the day and reduces the backup heat required during the night.Thermal mass that is directly irradiated is much more effective than thermal mass that receives only reflected light. Thermal mass requires that solar geometry be used as a design criterion for the placement of the mass.
All south-facing glass needs to be shaded in the summer to prevent overheating.
East and west windows lose as much heat as they gain. They are particularly problematic in regions with hot summers because they face the low solar angles in the mornings and afternoons. But they also provide an important function in creating cross-ventilation in rooms or for the entire house. Glazing becomes an important architectural features, so the type of glass is important when placing east-west windows. Low solar heat gain glass can reduce the potential over overheating.
The easiest way to limit summer heat gain through east and west windows is through strategic planting outside. Trees and bushes that leaf out in summer and drop their leaves in fall provide for light and ventilation while shading the window when it is hottest outside. Be careful that the species selected doesn't grow too tall and lose its lower branches, thereby reducing its shading potential.
In a house insulated to current codes, thoughtful passive solar design can reduce the cost of heating and cooling by as much as 50%. In a superinsulated house, passive solar can replace even more fossil fuel than that. Good design is the key. Overheating is one of the main reasons that builders are skeptical of passive solar. Design software has to be adjusted to take into account the passive solar contribution before sizing HVAC equipment.