Passive Houses

I have noticed more and more news about something called a "passive house." I hadn't heard this term until recently. I know a lot about passive solar design, an ancient and simple concept utilized by the Anasazi in southwestern United States and by many others.

On December 26, 2008 the New York Times published an article about Passive Houses in Germany and a retrofit in Berkeley, California, to "Passive House standards." The basic design principles involve a super-insulated and tight building shell, basic passive solar design (south facing windows to soak up the winter sun with overhangs to keep out the summer sun,) and a ventilation system with an air-to-air heat exchanger. These houses capture heat from lights, people, appliances, and the sun and don't allow it to escape. Therefore they require very little additional heat. The heat exchanger is also used to keep the heat out when it is hot outside. Shading the windows and super-insulating also reduces cooling needs. Some passive houses also have labyrinths of concrete to act as thermal mass to store heat or cold or underground tunnels to bring entering air close to ground temperature before it enters the house. This works to warm the air in winter or cool it in summer.

This is a checklist for creating passive solar, superinsulated, tight buildings with an air to air heat exchangers and minimal heating and cooling requirements. To qualify as an official "Passive House" there are certain minimal requirements for heating and cooling loads that must be met.

Site Selection & Design:

• Proximity to public transportation. (a good long term investment as well)
• Ability to site the building with a sizeable clear area to the south. (exact dimensions of clearing needed depends on latitude and solar requirements) Trees and other buildings are the obvious problems.
• Planting that will not cause unwanted shade in the future.
• Is the site zoned to allow row or multi-story buildings? (Inherently more efficient)

Schematic Design of Building:

• Use a compact building shell, and take advantage of opportunities to combine buildings. (make it a duplex and benefit from a neighbor's heat.
• Keep the shape simple if possible, without dormers, returns, etc. The simpler the shape the easier to achieve continuous insulation
• Glazed surfaces facing south are optimal; keep east, north and west windows small. (In the summer the sun comes up in the northeast and sets in the northwest. If you don't want the heat, minimize the direct sunlight getting in.)
• Provide overhangs on south facing windows to minimize direct solar gain in the summer and maximize direct solar gain in the winter. (In the winter the sun spends most of its time in the south and low.)
• It is easy to precisely calculate the sun angles and heat gain through glazing as it varies seasonally and diurnally. Eaves can be carefully sized to block only unwanted sun.
• Plan for changing uses of spaces: seasonally, over a lifetime, with changing owners and numbers of occupants.

Construction Documents:

• Determine insulation thickness of building envelope. (based on climatic conditions of the site.)
• Avoid heat bridges.
• Well insulated and tight windows and doors (so they don't leak away all your heat or cool) But make sure you don't install standard low e glazing that will keep out heat from the winter sun!
  
• Calculate optimal amount of thermal mass

Ventilation System:

• Ductwork: Keep cold air ducts outside, and warm air ducts inside the insulated building envelope, unless they are well insulated and kept very short. Use short runs, smooth inside surfaces and good airflow velocities. Include measurement and adjustment capability, sound attenuation, and fire protection measures.
• Air vents: Provide adjustability and do not remove stale air over radiators.
• Ventilation: Place central unit and air-to-air heat exchanger near the thermal envelope (good locations are in the envelope, or on the lower floor). Place the air heater inside the insulated envelope, and provide additional insulation of the central unit and air heater as appropriate. It should be adjustable, with a housing that is well insulated acoustically and thermally.
• Ventilation controls: Provide user-operated settings for "low", "normal" and "high", and consider additional controls in kitchen and baths/toilets.
• Stove hoods: Use a unit with a high capture rate at low airflow rate and velocity. Unit must include an efficient grease filter.
• Underground earth-to-air heat exchanger (optional & more desirable in desert climates): System should be airtight, with colder parts away from the house, and a bypass (for periods when heating or cooling air to earth temperature is not desired).

Plumbing & Electrical:

• Sewage and water lines should be short, and should be well insulated. Insulation must prevent sewer and cold water lines from sweating.
• Insulate water and heating system valves and other accessories.
• Use water-saving plumbing fixtures and provide hot water connections for washing machine and dishwasher.
• Sewer system should preferably have only one stack, and be properly vented. Vent pipes must be insulated within the building envelope.
• Assure that electrical and plumbing penetrations of the airtight, insulated building envelope are well insulated and airtight.
• Use low energy household appliances

Execution:

• Inspect for thermal bridges
• Insure that insulation layers are contiuous
• Check joint details for airtightness while they are accessible
• Have a building shell pressure test performed before interior is finished.
• Assure that ducts are neatly installed and carefully sealed
• Check central air exchange unit for acoustical insulation and ease of changing filters
• Adjust ventilation in normal service; measure and balance supply and exhaust air volumes; balance supply and exhaust air distribution; measure the systems electrical power consumption
• Check for proper insulation of plumbing pipes and check for airtightness of plumbing penetrations through the envelope.