Passive House Definition

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1. What is a ‘Passive House’ building?

A ‘Passive House’ is a building designed and constructed to meet three rigorous building performance benchmarks. The result is a building with very small space conditioning systems, exceptional thermal comfort, exceptional indoor air quality, very durable thermal envelope assemblies and such a low total energy demand that they resiliently coast through potential power disruptions or system malfunctions. Achieving such performance requires passive building envelope strategies that maximize reduction of heating, cooling and dehumidification loads/demand while minimizing potential long term moisture damage – hence the term ‘Passive’.

A ‘Passive House’ building, simply stated, is one that meets the following 3 pillar benchmark criteria:

  1. Annual space heating/cooling demand and hourly peak heating/cooling load maximum. For the Central European climate, the values were originally established at </=4.75 kBtu maximum (1.39 kWh) per ft2 per year (15 kWh per m2 per year) [OR peak load heating </=3.17 Btu/h (0.93 W) and cooling </=2.54 Btu/h (0.74 W) per ft2] (10 W heating, 7.96 W cooling/m2). The PHIUS+ 2015 Passive Building Standard – North America adjusts the value to the specific North American climates (annual temperature, humidity and solar radiation). For example, the climate zone in Southern Wisconsin/Northern Illinois, the values are:
    • </=6.3 kBtu (1.85 kW) per ft2 (19.9 kWh per m2) maximum annual heating demand
    • </=3.6 kBtu (1.06 kW) per ft2 (11.4 kWh per m2) maximum annual cooling demand
    • </=7.3 Btu (2.14 W) per ft2 (23 W per m2) maximum hourly heating load, and
    • </=6.3 Btu (1.85 W) per ft2 (19.9 W per m2) maximum hourly cooling load
      • Current U.S. codes do not currently establish a maximum.
      • This benchmark causes a thicker, better insulated, building envelope and very high performance glazed windows and doors
  2. Building envelope air leakage maximum. </=0.5 cfm/ft2 of exterior surface of total building envelope (including slabs and below grade walls) when tested at 0.2 w.g. (50 Pascals) [1 lb./ft2] pressure differential, OR </= 0.08 cfm/ft2 (0.04 L/s * m2) when tested at 0.3 w.g. (75 Pascals) [1.5 lb./ft2]
    • The definition of an air impermeable material is one that leaks </= 0.04 cfm/ft2 (0.02 L/s m2) of material surface area when tested at 0.3 w.g. (75 Pascals) [1.5 lb/ft2]
      • The 2012 and 2015 ICC International Energy Conservation Code (IECC) currently adopted in much of U.S. requires testing only for residential buildings at up to 5.0 interior volume Air Changes per Hour (ACH) at 0.2 w.g (50 Pascals (1 AC/12 minutes). If buildings other than residential chose to test, code allows up to 0.4 cfm/ft2 (2.0 L/s m2) of building surface area when tested at 0.3 w.g. (75 Pascals) [1.5 lb/ft2].
      • This benchmark causes the minimization of air and humidity flowing through the building envelope regardless of the changes in weather. As a result, the risk of vapor laden air moving into the envelope and cooling whereby the vapor condenses into water trapped within the assembly.
  3. Total Source (Primary) energy demand maximum. </= 6200 kWh per person per year for dwelling units (presuming the number of occupants = 2 persons in 1st bedroom plus 1 person in each additional bedroom) and </=38.1 kBtu (11.2 kW) per ft2 per year (120 kWh per m2 per year) for other occupancies/building types. Demand is inclusive of space heating/cooling/dehumidification, domestic (service) hot water production, household electricity and auxiliary electricity.
    • Source (Primary) energy, unlike Site (Final) energy includes the amount of energy it took to harvest fuel source, transport it to energy conversion plant (i.e. electrical plant or fuel refinery) and distribute or transmit it to site. Site (Final)) site energy is only that consumed at the project site. In the US, the factors for converting site (final) energy to Source (Primary) are between 2.894 and 4.022 (3.16 average) depending on which of the 4 electric grids the site/building is connected for electricity and 1.1 for all other fuel sources. (Source – US EIA and 2012 ICC IECC)
      • U.S. codes do not currently establish a maximum.
      • This benchmark limits the total CO2e emissions per person or building floor area unit causing the need to maximize efficiency of energy using systems (some of which are already smaller in size due to 1st pillar benchmark above) and conscious efficient use of systems by the building occupants.

Pillar 1 causes need for: Glazed windows and doors of superior air tightness and thermal performance such that interior side maintains a temperature high enough for comfort and condensation prevention and to prevent air and humidity from moving through these ‘weakest links’ in the building envelope.

Strategic design of shading of glazing prevents undesired solar radiation gains in climates or seasons where they could cause overheating of building interior.

Pillar 2 causes need for: Though not required by definition, Passive House recommends maintaining an average intentional ‘balanced’ ventilation rate between 0.3 and 0.4 Air Changes per Hour for air volume within 2.5 m (8.2 ft) of floor in residential dwellings (higher or lower in other building use types based upon occupant load density). ‘Balanced’ ventilation means exhaust rate equals supply rate which helps insure air and humidity leakage through the building envelope is not increased due to pressure differential caused by ‘unbalanced’ ventilation.

Ventilation may be ‘natural’ through open windows and doors when the weather outside is conducive to that. When ‘natural’ ventilation is not available or adequate, ‘mechanical’ ventilation must be provided to adequately introduce ‘fresh’ outdoor air and exhaust ‘stale’ indoor air for the purposes of human health and comfort.

Passive House does not prevent ventilation rates from being lower during no or low occupant load times and higher rates during times of high occupant load, elevated indoor humidity or elevated indoor odor, so long as the average daily rate is adequate to maintain health and comfort.

Current U.S. codes require a central ventilation system with a rate based upon a formula that considers both floor area and number of occupants. The formula does not give consideration to the number of air changes per hour and does not require the system to be balanced.

 

Passive House criteria establish the lowest building energy demand framework in on Earth and results in permanently comfortable, healthy, durable and affordable buildings. The Passive House mantra – First reduce demand so dramatically and permanently that they may maintain comfort with little to no added energy, then, harvest a small amount of energy on site to satisfy the remaining demand for current day conveniences.

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Passive House Definition