Saturday, April 28, 2012

What is a Passive House?

Here’s another crack at a very brief summary of what a Passive House is.  There’s a ton of information on this subject available on the internet.  At the end of the post are links to a bunch of sites that are very informative if you’re interested in reading further.

What is a Passive House?

A Passive House (PH) is an ultra-energy efficient building.  The PH standard was developed in Germany in the 1980s and 1990s.  A building built to the PH standard will require a small fraction of the energy to heat and cool compared to a building constructed in a more standard manner.  A common boast about a PH is that it only requires a couple of hair dryers for heating in the dead of winter in a cold climate.  By capturing heat from the sun and effectively keeping that heat inside the building, a PH is able to accomplish this seemingly magical feat.  The PH standard is one of the most rigorous building standards in the world.

The PH standard requires that a building’s energy usage not exceed very strict limits.  Additionally, there is one prescriptive requirement to meet the PH standard, i.e. that there is very little air leakage in the building.  The table below illustrates the PH requirements.

Requirement in kBTU/ft2
Requirement in kWh/m2
Annual Heat Demand
≤ 4.75 kBTU/ft2
≤ 15 kWh/m2
Peak Heat Load
≤ 3.17 kBTU/(ft2hr)
≤ 10 kWh/(m2hr)
Annual Cooling Demand
≤ 4.75 kBTU/ft2
≤ 15 kWh/m2
Total Annual Source (Primary) Energy
≤ 38.1 kBTU/ft2
≤ 120 kWh/m2
Air Leakage
≤ 0.6 ACH @ 50 Pascals

Here’s an example of a comparison of a Passive House to a traditional house.  The current 2100 ft2 condo we live in, built in the 1970s, is heated by 525 gallons of kerosene per year.  At 135,000 BTU/gallon of kerosene, this equates to an annual heat demand of 71,000 kBTU.  To meet the PH standard, a 2100 ft2 condo would need to have an annual heat demand of no greater than 10,000 kBTU (4.75 times 2100).  A PH the same size as our current home would need to consume at most 14% of the energy we currently use to heat our home.  For our condo to meet the PH standard, we’d need to be able to heat our home with 74 gallons of kerosene instead of 525 gallons.

Preliminary indications are that our new house with a floor area of 2800 ft2 will have an annual heating demand of 3.6 kBTU/ ft2, exceeding the PH standard by 25%.  A very small 9000 BTU air source heat pump will be able to heat the entire house using only 1250 kWh per year of electricity.  At $0.14/kWh this will cost $175 per year which is considerably cheaper than the $2500 a year we spend to heat our smaller condo.

The analysis above is a bit of a simplification.  The hypothetical 2100 ft2 condo described wouldn’t need to actually use 74 gallons of kerosene to heat, rather, the PH modeling software would need to think that it would use the energy equivalent of 74 gallons of kerosene to heat.  We’re taking the leap of faith that, in our new home, the heating requirements projected by the PH modeling software will be close to the actual heating requirements.  We’ll report back to you on this after the winter.

What techniques are employed in building a Passive House?

The combination of the following 3 strategies can result in a building meeting PH certification requirements.

  1. Passive Solar Building Design
    1. The vast majority of windows face south.
    2. High efficiency windows with very high solar heat gain coefficients are used, i.e. windows that are able to capture heat from the sun inside the house rather than reflect the heat back outside.
    3. Roof overhangs shade windows when the sun is high in the sky during the summer months to prevent overheating.
  2. Air Tightness
    1. No air gets in or out of the building except through a mechanical ventilation system that transfers energy from the indoor air being exhausted to the incoming outdoor air.  Very efficient Energy Recovery Ventilation systems are able to retain up to 90% of the indoor air’s energy.
  3. Super Insulation
    1. Framing with techniques to reduce Thermal Bridging, i.e. energy leaking in and out of the building through studs in the wall
    2. Thick walls that contain very effective insulation
    3. High efficiency windows that reduce heat losses overnight

None of these techniques are very difficult or expensive to implement.  They just require thought in the design process to achieve the goal of energy efficiency.

What are the benefits of living in a Passive House?

  • A planet friendly lower carbon footprint.
  • Reduced costs for building heating and cooling.
  • More comfortable indoor environment.
    • The entire house stays one temperature.  No cold or hot rooms.
    • There are no drafts. 
    • Wall and window temperatures remain very close to room temperature.
  • Healthier indoor environment.
    • The mechanical ventilation system ensures that fresh air is constantly circulating through the building. 
    • Ventilation system filters reduce the amount of particles in the indoor air. 
    • Moisture levels are carefully controlled to eliminate the risk of mold.

Places to read more about Passive Houses?

There are tons of sites with information about PH.  Below are some sites that I’ve found useful.

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