Passive House Key Design Principles

Passive House is a holistic construction certification standard, allowing Certified Passive House professionals flexibility to determine the most suitable building geometry based on usage and location. Passive buildings are thus comprised of a set of design principles used to attain a quantifiable and rigorous level of energy efficiency within a specific quantifiable comfort level under a “fabric first” design philosophy. To that end, a passive house building is designed and built in accordance with five building-science principles:

Thermal Insulation
Sufficient insulation is what’s needed within the building’s envelope, providing enough thermal separation between the heated or cooled conditioned inside environment and the outdoors. This improves thermal comfort and reduces the risk of condensation (no more cold internal surfaces in winter!).

Passive House (High Performance) Windows
It’s not just the solid areas of your building envelope that need to have good levels of insulation but your windows too. No more single glazing, but instead low-emissivity double or triple glazing with thermally broken or non-metal frames. The size of the windows should be appropriate to each orientation, to allow solar radiation to penetrate during the winter months (free heating!) but not result in too much solar radiation during the summer. Watch out for how well they’re sealed too, as leaky windows just won’t do.

Mechanical Ventilation Heat Recovery
Now this doesn’t mean that you can’t open your windows! The incorporation of a mechanical ventilation unit means that you simply don’t need to rely on opening them to achieve good indoor air quality. The unit effectively recovers heat and coolth that would otherwise be wasted whilst also filtering the air that’s coming into the building. This leads to fewer pollutants in the air and a lower risk of condensation meaning a healthier indoors.

An essential part of every Passive House is an air tight building envelope (see the requirement in the certification criteria). This ensures that there are only a very limited amount of gaps and cracks within your envelope, giving you full control over your internal environment and significantly improving thermal comfort – no more draughts!

Thermal Bridge Free Construction
The insulation not only needs to be sufficient in thickness but also needs to be continuous. This means keeping penetrations through the insulation to an absolute minimum, and if not avoidable then using materials that are less conductive to heat (i.e. timber in place of metal) and/or incorporating thermal breaks (whereby a material that doesn’t conduct heat well separates the two conductive elements). Otherwise your wonderfully insulated building will have a number of thermal highways that will cause increased energy consumption and increased condensation risk whilst impacting thermal comfort.

Passive House Criteria

Passive House design principles are met via quantifiable Passive House Criteria, a series of minimum performance requirements to achieve certification.

Annual Space Heating Demand / Heating Load
Annual space heating demand is set at ≤ 15kWh/m2.yr. Alternatively space heating load is set at 10W/m2. This represents the demand on the heating system to maintain a consistent comfortable temperature throughout the year. It is a function of the building fabric thermal performance, the air permeability and the outside air ventilation required for maintaining indoor air quality. It is independent of the proposed mechanical plant efficiencies.

Annual Space Cooling/Dehumidification Demand / Load
Annual space cooling/dehumidification demand is set at ≤ 15kWh/m2.yr. Alternatively, space cooling load is set at 10W/m2. This represents the demand on the cooling system to maintain a consistent comfortable temperature throughout the year. It is a function of the building fabric thermal performance, the air permeability and the outside air ventilation required for maintaining indoor air quality. It is also independent of the proposed mechanical plant efficiencies.

Airtightness is set at  ≤ 0.6 ach (+/- 50 Pascals). Alternatively, air permeability is set at  ≤ 0.6m3/hr.m2 (+/- 50 Pascals) for larger buildings. This is a minimum requirement for all Passive Houses, and represents an extremely air tight building with only minimal gaps in the envelope. An air tight building allows more closely controlled environments, with significantly improved thermal comfort (no more draughts!).

Annual Primary Energy Demand
Annual primary energy demand is set at ≤ 120kWh/m2.yr. Alternatively with renewables, it is set at ≤ 60kWh/m2.yr. This is the predicted total energy demand of the building, including heating, cooling, hot water generation, ventilation, lighting and equipment loads. It takes into consideration the efficiencies of mechanical plant and any renewable generation (if proposed).

Occupant Comfort
The air temperature must not exceed 25°C for more than 10% of the occupied time to ensure that comfortable temperatures are achieved during the hot summer months.

Indoor Air Quality
The absolute indoor air humidity levels do not exceed 12 g/kg for more than 20% of the occupied time. Dehumidification allowance if no cooling plant is proposed.

Passive House Planning Package (PHPP) Software

In order to determine a building’s performance to the Passive House building standard, you must use the Passive House Planning Package (PHPP) software.

PHPP allows a building to be defined based on its individual elements. This allows architects and engineers to study the impact of design changes whilst continuously monitoring the likelihood of meeting the Passive House criteria.

Thoroughly validated against measured performance data of real buildings in various climates, the energy demand predicted reflects actual energy consumption to address ‘The Performance Gap’ commonly experience in predicted and operational energy usage.

To assist in ensuring an accurate incorporation of the proposed building geometry, and for early stage concept analysis, a Sketch-up plugin, designpH is available. Here, you can draw your building in sketch-up and then import the geometry into PHPP, saving time and allowing the building to be visually represented in the modelling process.

The Passive House Certification process

Buildings meeting the strict Passive House certification criteria can be certified as Passive House buildings by any of the Passive House Institute accredited Building Certifiers operative worldwide. Locate your local Passive House Certifiers and other professionals here.

APHA recommend contacting a Passive House Certifier early on in the planning process, where any problems identified can be easily corrected at this point in time. In principle, certification can also be applied for after completion of the building.

As a rule, all energy-relevant planning documents and technical data of the construction products are submitted before the start of construction work. After careful checking and comparison with the energy balance calculation, the certifier will provide information about any necessary corrections.

After completion of the construction, any changes in the planning will be updated and documents relating to construction will be checked during the final inspection.


The Passive House Certification process

As an independent authority, the Passive House Institute tests and certifies products based on their suitability for use in Passive Houses. Products that carry the “Certified Passive House Components” certificate have been tested according to uniform criteria; they are comparable in terms of their specific values, and are of excellent quality regarding energy efficiency. Their use facilitates the designer’s task and significantly contributes to ensuring the faultless functioning of the resulting Passive House.

The PHI has set up a database listing certified products for the following building components:

  • wall and construction systems
  • floor slab insulation systems
  • roof parapet and balcony connections
  • façade anchors
  • window frames
  • window connections
  • front doors and sliding doors
  • post and rail façades
  • roof windows and skylights
  • roller shutters and external blinds
  • glazing
  • mechanical ventilation systems
  • compact heat pump units
  • exhaust systems