Passivhaus
Passivhaus is a low energy building standard developed in Germany. It uses a scientific approach to building design where all the building data is entered in the Passivhaus software, Passive House Planning Package (PHPP) to model how it will perform based on it’s detailed design, climate, and surrounding context. To achieve the Passivhaus standard a building must be designed and built to require a very low level of heating (and cooling in hotter climates), and provide a high level of thermal comfort and internal air quality.
In the UK climate, this requires a highly efficient mechanical ventilation system with heat recovery (MVHR), a very high level of airtightness, high levels of insulation with almost no thermal bridges (interruptions to the insulation), high performance triple glazed windows and doors, and a very high level of quality control during design and construction to ensure the building is built as designed. Passivhaus strongly encourages an efficient building shape and optimised glazing design as these make it much easier to achieve the standard. If the site or brief require a less optimal shape or glazing then the standard can still be achieved, but it will require improvements to be made, such as further increasing the level of insulation.
Existing Buildings & EnerPHit
Although the Passivhaus standard has been achieved on retrofit projects, it is much harder to reach when dealing with an existing building so the Passivhaus Institute has created a lower standard for existing buildings called EnerPHit. EnerPHit projects require all the same information as Passivhaus, and although not as low energy as Passivhaus, an EnerPHit project is still significantly better than a house that only meets the Building Regulations requirements for a new house in the UK.
Passivhaus Building Principles
From the Passivhaus Trust:
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High performance insulation which is optimised for comfort, energy demand and the climate zone.
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Continuous, high performance airtight layer to minimise heat loss, eliminate draughts and maximise fabric longevity.
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Thermal bridges minimised to prevent cold spots and mould.
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High performance window specification & shading to provide optimum comfort and efficiency in summer and winter.
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Quiet and efficient mechanical ventilation with heat recovery (MVHR) delivering high indoor air quality.
Passivhaus/EnerPHit Criteria
Airtightness (measured to n50 standard):
Passivhaus: ≤ 0.6 Air Changes/hour @ 50 Pa
EnerPHit: ≤ 1 Air Changes/hour @ 50 Pa
Space Heating Demand:
Passivhaus: ≤ 15 kWh/(m².yr)
EnePHit: ≤ 25 kWh/(m².yr) (or component limits when using componenet method)
OR max. Heating Load:
Passivhaus: ≤ 10 W/m² (n/a for EnerPHit)
Primary Energy Renewable (renewable energy needed to meet building energy demand):
Passivhaus: ≤ 60 kWh/(m².yr) (Classic) | ≤ 45 kWh/(m².yr) (Plus) | ≤ 30 kWh/(m².yr) (Premium)
Renewable Energy generation:
Passivhaus: Plus: ≥ 60 kWh/(m².yr) | Premium: ≥ 120 kWh/(m².yr) (n/a for Passivhaus Classic or EnerPHit)
Summer Overheating:
≤ 10% exceeding 25°C (best practice: ≤ 2%)
Minimum surface temperature:
Typically ≥ 17°C
Ventilation:
Typically ≥ 30 m³/(hr.person) with ≥ 75 % Heat Recovery in UK
Airtightness, "Breathability" & Ventilation
To achieve the high performance of Passivhaus, buildings need to be highly airtight to reduce heat lost via leaks in the walls, roof, gaps around windows & services etc.
Being airtight does not mean the same thing as vapour tight. A Passivhaus can be built from vapour open and hygroscopic materials that help manage moisture in the building, sometimes referred to as allowing a building to "breathe" however this is an unhelpful term as it is often miss-used and implies the movement of air as well as moisture. Instead breathability in the building context ashould be thought of in the same was as a high performance waterproof coat can be waterproof but breathable i.e. allow moisture (sweat) to escape.
In order to get enough fresh air to maintain a healthy internal environment Building Regulations requires that houses should replace all the air inside at a rate of 0.3 air changes an hour, (or around every 3 hours all the air in the house is replaced with fresh air).
If this air change rate is achieved by standard extract fans or a passive ventilation system all the heat the air carries is lost to the outside which contributes greatly to heat loss in winter. To avoid this heat loss Passivhaus and low energy buildings in the UK climate recover heat that would otherise be lost to the outside using a mechanicle ventilation system with heat recovery, or MVHR system.
Whole House MVHR System
Extract terminals take stale air from wet rooms (kitchens, bathrooms & utilites) and provide fresh (but room temperature) air in living & bed rooms. A single ventilation unit connects to the outside with high quality filters, and contains a heat exchangerwhich saves at least 80% of the heat that would otherwise be lost.
AECB CarbonLite Standards
It is not allways posible or affordable to target Passivhaus or EnerPHit on your project, but even if not targetting Passivhaus Certification, following Passivhaus design methadology and principles can still result in a high performance building. However, there are benefits to getting a project certified; having a specific target, beyond what is required to achieve Building Regulations Part L approval, pushes the level of attention to detail in design and construction to achieve a higher standard than is typically reached without having an external standard to measure against.
One alternative is to target a different standard than Passivhaus or EnerPHit. The AECB CarbonLite Standards for new-build and retrofit have been created to be a more achievable standard than Pasivhaus/EnerPHit but the standards are still mean a building is significantly higher performance than Building Regulations requires. The standards also give a greater degree of oversight and external verification than Building Regulations, whihc gives a higher degree of cofidence that your building will actually be built as it is designed.
Andy is an AECB CarbonLite Approved Certifier & Modeller for New Buildings, so can assist with the design and certification of AECB New-Builds.