Keys to the Passivhaus standard: what is it and how to achieve it?

The Passivhaus standard is a voluntary energy certification standard for new and retrofitted buildings, aimed at providing maximum comfort for occupants, excellent indoor air quality and near-zero energy consumption. It was developed in the 90’s by the Passivhaus Institute in Darmstadt Germany, and it has since been expanding across the globe. The current climate crisis is increasingly bringing the standard into the limelight due to its radical no-nonsense approach, rooted in buildings physics, a rigorous design process and an emphasis on proper site supervision and commissioning. The objective is to close the so-called “performance gap” (where buildings fail to perform as predicted), making them fit for purpose, comfortable, healthy, and resilient. A Passivhaus building typically consumes up to 90 % less energy than a conventional building.

This article, written by Oliver Style, explains some of the key aspects of the Passivhaus and what you need to do, to achieve certification.

Passivhaus: the Basic Principles

It is often said that the standard is based on the following 5 principles:

• High levels of thermal insulation
• High performance windows
• Efficient mechanical ventilation with heat recovery
• Air tightness
• Absence of thermal bridges

Although this simplification can make it easier to understand what Passivhaus is all about, a Passivhaus building requires a holistic design process where the whole is more than the sum of the parts… and there are many parts. So, beyond the 5 basic principles, there are several other factors that are important for achieving certification and making sure a building does what it says on the tin, especially in warm climates, namely:

• External shading devices to reduce solar gains and avoid summer overheating 
• Efficient Domestic Hot Water (DHW) systems, equipment and lighting: to reduce primary energy consumption and reduce internal heat gains in summer (helping avoid summer overheating).
• Efficient heating and cooling installations
• In climates with sufficiently low minimum night time temperatures: Natural night ventilation combined with thermal inertia, to remove heat from the building without relying only on active cooling.

A building designed to have very low heat losses in winter, will also keep heat out in the summer: if you fill a vacuum flask with cold water in the summer, the water will stay cool for longer than if you just left it in outside in ambient warm air.  However, once the heat is inside, it will logically dissipate more slowly due to the high level of thermal protection. That is why it’s particularly important to go beyond the “5 principles” if you want to avoid overheating problems, which will increasingly become an issue as summers get warmer due to global warming (Figure 2).

PHPP: “Passive House Planning Package”

For the design of a Passivhaus building, the PHPP (“Passive House Planning Package”) tool is used, a quasi-steady-state single zone energy modelling program, based on a series of spreadsheets in Excel, that provides monthly and annual energy balances. The algorithms in the tool are based on a number of ISO standards, mainly in the monthly method of EN ISO 13790, now replaced by ISO 52016 0.

The PHPP has been calibrated with thermodynamic simulations carried on with the DYNBIL tool, developed by the Passivhaus Institute, itself calibrated by extensive validations with monitored data.

PHPP stands out for its simplicity compared to dynamic tools and can be used to model (albeit in a simplified way) a wide variety of active and passive systems, at a very affordable price. The results indicate the energy balance of the building in both summer and winter, yielding results of heating and cooling demands and total and final and primary energy consumption. Although a dynamic tool is- in principle- more accurate, the large number of parameters and input data increases the possibility of modelling errors and requires experience and time for accurate use. In energy simulation, it is sometimes better to be approximately correct than precisely wrong…

Passivhaus for new build

The Passivhaus standard for new buildings is performance based: i.e., it doesn’t limit thermal transmittance values ​​of the different construction elements, but establishes maximum energy demands and energy consumption, calculated with the PHPP. The air infiltration level cannot exceed 0,6 air changes per hour (ACH) at a pressure difference of 50 Pascals, measured with an on-site test, known as the “Blower Door” test.

The limiting values for heating and cooling demands and total primary energy consumption, are shown in Figure 3.

There are 3 classes of certification: Classic, Plus and Premium. Classic does not have renewable energy generation. To get to Plus, you have to generate ≥ 60 kWh/m2·a of renewable energy (references to the building’s footprint)- typically at least as much as what the building consumes. To reach Premium, the building must generate ≥ 120 kWh/m2·a (or 4-5 times more than what the building consumes). This is shown in Figure 4. The advantage of this approach is that energy demands are reduced first, before considering on-site generation of renewable energy. This differs from the conventional net-zero energy approach, which inevitably favours buildings with very large roofs on which a large solar PV generator can be installed, leading to buildings with a poor form factor (the ratio of enclosed envelope area to useful floor area), higher losses and where on-site renewable energy generation is prioritised over reducing energy use.

PHI Low Energy Building

If the above requirements are not met, a building can be certified as PHI Low Energy Building, complying with less stringent requirements, shown below in Figure 5.

Passivhaus for retrofitting: EnerPHit

For the retrofitting of existing buildings, there is the EnerPHit seal, which offers two ways to achieve certification:

• EnerPHit, Demand Method: performance based, with the requirements shown in Figure 6
• EnerPHit, Component Method: prescriptive, with the requirements shown in Figure 7

For both methods, an air tightness test result of N50 ≤ 1.0 ach must be obtained. The Classic, Plus and Premium classes are also applicable to the EnerPHit standard.

Overheating risk in summer

For certification, overheating risk is assessed through one of the two following methods:

• With active cooling: the limiting total cooling demand must be met (sensible + dehumidification), calculated with the PHPP, with mechanical systems capable of maintaining thermal comfort at all times (according to ISO 7730), with an operating temperature ≤ 25 ºC and a maximum of 10% of the hours in the year with an absolute indoor humidity ≥ 12 g/kg dry air.
• With passive cooling:  the maximum overheating frequency must be met, calculated with the PHPP, with a maximum of 10% of the hours in the year with an indoor operating temperature > 25 ºC.

In the case of passive cooling, it is important to stress that 10% of the hours of the year are a total of 876 hours with an indoor operative temperature above 25 °C (the whole month of August, for example). Therefore, the recommendation is not to exceed 5%, with a good design objective being 2 – 5%, shown in Figure 8.

To reduce the risk of overheating where no active cooling is available, it is important to perform stress tests of extreme climate situations with the PHPP. For this purpose, Jessica Grove-Smith of the Passivhaus Institute has developed theSummer temperature tool[2], which adjusts the summer temperatures in the PHPP climate file, taking into account the urban heat island effect and the increase in temperatures according to IPCC’s predictions (Figure 9). This feature will be integrated directly into PHPP version 10.

For tertiary buildings and/or buildings with areas exposed to very different indoor and outdoor conditions, it is advisable to accompany the PHPP calculation (a single zone tool) with a dynamic multi-zone calculation, to analyse specific zones that may be more susceptible to overheating (for example, the top floors in a tall building with west-facing glazing).

Additionally, if the building does not have an active cooling system, it is important to test assumptions regarding user behaviour, for example, in relation to closing blinds and opening windows at night for natural ventilation. Is it realistic to think that a person will sleep all night with the blinds open, all windows fully open, and all interior doors open? It is more likely that occupants will close the blinds before going to sleep so the sun light doesn’t wake them up in the morning. If you live in an urban area, you might want to close all windows facing the street to avoid noise. All this reduces natural night ventilation flow rates and the amount of heat that can be extracted from the building.

Although the goal is always to reduce dependence on active systems to maintain comfort, in most cases, some active cooling in summer is advisable. Fortunately, the summer is also the time when there is the most solar radiation, which means active cooling energy consumption can be directly off set with on-site renewable energy production.

Air tightness: the “Blower Door” test

Central to the Passivhaus standard is the “Blower Door” airtightness test, that measures air infiltration through a piece of equipment that pressurizes and depressurizes the building. Preliminary tests must be carried out before the interior finishes are completed (to detect and correct leaks, if found), together with a final test, according to the EN 13829 [3].

The “Blower Door” test is a clear indicator of build quality. What are the advantages of reducing air leaks?

• Reduces energy losses and heating bills in the winter
• Reduces the entrance of moisture in warm-humid climates, thereby reducing dehumidification cooling consumption and cooling energy bills
• Improves comfort, eliminating air drafts
• Improves people’s health by preventing the entry of radon gas, suspended particles and other pollutants from outside

An air tightness strategy must always be accompanied by controlled mechanical ventilation, to ensure good air quality and the elimination of moisture and pollutants generated inside the building.

An existing building will typically have an infiltration level of n50 ~10 ach. A certified Passivhaus has an n50 ≤ 0.6 ach. The Spanish CTE Building Regulations, in the 2019 update, establishes a limit for air infiltration of n50 3 ach and 6 ach according to the compacity of the building (Figure 10).

Audits and certification: quality assurance from design to completed building

The certification process and auditing begin in the design phase and conclude when on-site construction is finished. Certification can only be carried out by the Passivhaus Institute or an approved Building Certifier. As an external agent to the project, the certifier verifies that the project complies with the standard and that construction has been completed as planned, accurately reflected in a the PHPP model. The client needs to supply detailed as-built architectural and service drawings, photographic documentation showing the execution of all elements related to energy and air tightness, the final certificate of the Blower Door test, the ventilation system commissioning results, together with a letter signed by the Site Supervisor, indicating that the project has been built as designed.

Passivhaus and health

Although the standard does not specify which materials you use in a building, the PHPP manual explicitly recommends the use of low-emission materials, to reduce VOC’s (volatile organic compounds) in indoor air.

To ensure good air quality, the correct sizing of the ventilation system at design stage is checked. Once installation is completed, the commissioning of the system and the measurement and adjustment of flow rates in all supply and return valves is mandatory.

The goal is to create healthy, comfortable, efficient and resilient buildings, closing the performance gap between projected and real-life performance.

Bibliography

• [1] EN ISO 13790, Energy performance of buildings – Calculation of energy use for space heating and cooling. This standard has been revised by ISO 52016-1:2017
• [2] Criteria for the Passivhaus, EnerPHit and PHI Low Energy Building Standard, version 9f. 15.08.2016 1/30. 2016 Passive House Institute.
• [3] Passive House Institute Summer Temperature Tool, Available at: https://passiv.de/en/05_service/02_tools/02_tools.htm
• [4] DIN-EN 13829, Thermal performance buildings – Determination of air permeability of buildings – Fan pressurization method. (ISO 9972:1996, modified).

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