Some Passive Design Basics
Before designing a passive home, or making changes to your home to allow for more passive design, it is important to know how heat transfers through different materials. Heat always transfers from warmer materials until cooler ones until there is no difference between the two materials, unless something else interacts with the system. Details of the different ways heat transfers can be found in the Science section.
There are three different ways that heat can be introduced to the interior of a building:
- Direct gain is when sunlight enters a building through a transparent section of a wall
- Indirect gain is when heat enters a building from a heat transfer or storage source that was itself heated by the sun
- Isolated gain is when only a particular part of the building is heated, and that part of the building is separate climactically from the rest of the building.
Elements Passive Design
Below are the important elements of a passive solar home design:
- Aperture (Collector)
- Any glass through which sunlight enters the building. In order to maximize solar heat gain, an aperture should face within 30 degrees of true south and not be shaded by any other buildings or trees from 9 am to 3 pm each day during the heating season.
- Absorber
- Any element that receives heat. The best absorbers are hard dark surfaces, sit in the direct path of the sunlight, and are able to transfer their heat to a thermal mass.
- Thermal Mass
- Materials that store heat produced by the sun. The difference between thermal mass and absorber are that the absorber is the exposed surface the sunlight hits, and the thermal mass is the material below that actually stores the heat.
- Distribution
- The method by which solar heat circulates from the collection and storage points to different areas of the building. A passive home will distribute heat through conduction, convection, and radiation, which are described above. In active systems, fans, ducts, and blowers will assist with distribution.
- Window location and glazing type
- Windows that face south are going to have more solar gain during the day than those on other faces, and those on the east side of a building are going to heat up during the early part of the day, while those in the west will heat up towards the end of the day. The glazing type determines the insulation qualities of a window. Each window is an opening in the building that is the opportunity for heat to easily move between the interior and exterior of the building.
- Roof-angled glass (sun roofs) are not a good idea in any climate. In the summer, when the sun is nearly perpendicular to it, it creates a solar furnace, whereas in the winter, the low angle of the sun bounces off the low-angled glass. A vertical window in the wall is much more efficient. Sunroofs also have the problem of releasing hot air that has risen to the roof because the window has a lower insulation value than the roof. For many good thermal reasons, roof-angled glass is not a great option in any building in any climate. In the summer, it creates a solar furnace, with the sun nearly perpendicular to it. On cold winter days, the low angle of the sun mostly reflects off of roof-angled glass. Warm air rises by natural convection, touches the roof angled glass, and then conducts and radiates heat outside. Vertical equator-facing glass is far superior for solar gain, blocking summer heat, and daylighting throughout a well-designed passive solar building.
- Insulation and air sealing:
- This is the ability of a building to separate the interior and exterior environments. The more well-sealed the house, the less it takes to heat and cool because it maintains the goal climate for a longer period of time and has lower fluctuations in interior climate.
When incorporating these design elements and techniques, you want to design for summer comfort, not just for winter heating, and most importantly, always design with your particular climate in mind.