Scott Wardlaw 3/11/21 Scott Wardlaw 3/11/21

Using PassivHaus to get to Net Zero Carbon

Designing to the Passivhaus standard can at times seem quite daunting and intimidating. Understanding the basic principles of the strategy is key, to ensure that you can develop your design to meet the standard without being succumbing to abortive work. This article outlines 10 broad design strategies to follow that will enable you to create a solid foundation from which your design can be developed, increasing your chances of meeting the required Passivhaus Standard.

The UK’s 2030 and 2050 Climate Change targets now look unlikely to be achieved with predictions showing that Buildings emissions will not reduce significantly given current use patterns and trajectories. To combat this, Scotland has indicated its intent to move towards a Zero Carbon building standard for all new buildings, using the Building Regulations as the primary mechanism. But what is a Zero Carbon building? In this article we explore this definition and present the Passivhaus Standard as a viable strategy in helping us achieve net zero carbon.

Embodied Carbon or Operational Carbon

The total carbon emissions of a building are made up of two types. Embodied Carbon describes the carbon emissions inherent in the creation of our construction materials. Operational Carbon, describes the carbon emissions generated in the use of our buildings through heating/ cooling, hot water, power and light. Our understanding of and ability to accurately calculate Embodied Carbon, whilst improving, is highly complex. Furthermore, the relationship between trading more Embodied Carbon to reduce Operational Carbon (for example, more insulation to reduce heating demand) is not yet fully understood or well established empirically. What is undeniable, is that we continue to build homes that are inefficient in operation, and unless this is addressed, high levels of Operational Carbon will continue to erode our ability to meet our Climate Change targets.

Carbon or Energy

In considering how best to reduce carbon emissions in the context of buildings, if we focus solely on carbon, it can be quite difficult to manage and propose strategies to reduce emissions. This is due to the complexities introduced by changing carbon factors associated with the energy mix of the grid over the life of the building. Furthermore, upgrading and replacement of different services over the life of the building can have significant carbon impacts that are difficult to estimate. It is far better to focus on the energy demand of the building. This is relatively consistent throughout the life of the building and is something that can be controlled and modelled at design stage. If the energy demand is minimised in the first place, then it follows that carbon emissions will be reduced. Design strategies such as Passivhaus, follow fabric first approaches to minimise the energy demand of the building. This energy use is focused on 4 principle areas; Heating & Cooling, Domestic Hot Water, Lighting and Auxiliary Electrical and finally, Appliances. To achieve net zero carbon emissions we need to offset actual energy use with energy derived from renewable energy sources.

How to Achieve Zero Carbon

The amount of carbon that we use is directly related to how we generate our energy. In the UK the aspiration is to 'decarbonise' our grid electricity by introducing renewable technologies into the energy mix. This has had a significant effect already, reducing the carbon emission factor from 0.519KgCO2/kWh down to 0.233KgCO2/kWh. Simply switching from gas to heat pumps to generate our heat and domestic hot water, would reduce this again by half. So, is it a simple case of installing heat pumps in all houses alongside solar pv to offset the electricity used? The chart below breaks down the annual energy demand for a typical new build UK home of 68m2 , firstly with a gas boiler, then with an air source heat pump. The chart indicates that 16 pv panels would be enough to offset the energy demand of the home with an air source heat pump. This, on the face of it, would appear to be easy! There are however, problems…….

The Performance Gap

The Performance Gap is a term that describes the difference between modelled energy use and actual energy use in our homes. There is increasing evidence that the energy performance of UK homes in use is actually 40% higher than predicted. This is due mainly to poor construction quality. Including the Performance Gap in our calculations pushes the energy demand of our average home from 4300 kWh/yr to 5400 kWh/yr. To offset this would increase your pv panels from 16 to 20. Introducing stringent construction site standards is imperative to give us a fighting chance of reducing the Performance Gap of our buildings and achieving Net Zero Carbon. In contrast the construction detailing and quality assurance procedures of the Passivhaus Standard ensure that there is little or no Performance Gap between the modelled and actual energy use of the building.

Seasonality of Renewables

When it is dark we need light. When it is cold days are shorter and we require more heat. When it is sunny with long days, we require less heat and less light. This is the classic conundrum of renewable energy sources - they are at their highest availability when we do not need them. This is known as Seasonality. In short, our energy demand is at its highest when the availability of renewable energy sources is at its lowest. We can store the energy, but there is significant storage losses incurred when doing this. The result is that ultimately, the effective energy demand of the standard home increases, as does the amount of renewables required to offset it (7700 kWh/yr and 28 pv panels!). This is clearly unrealistic as a viable strategy. The key is to reduce energy demand and close the Performance Gap.

Reducing Demand and Closing the Performance Gap

Emissions associated with heating and cooling a building can be significantly reduced by employing a fabric first design strategy and improving quality of construction. This is the primary focus of the Passivhaus standard, reducing thermal losses by improving the building thermal envelope and aligning this with ventilation heat recovery. Undertaken alongside efficient domestic hot water design and educating occupants in energy efficiency behaviour can drastically reduce the energy demand of the home to a level where energy use in the average 68m2 home with an ASHP could be held as low as 3700 kWh/yr, offset by just 14 pv panels.

Comparing Zero Carbon Strategies

The Passivhaus Trust, in their publication Passivhaus: the route to zero carbon?, presents data of net emissions from 5 Zero Carbon scenarios along with the required area of pv panel required to offset regulated energy (heating/ cooling, hot water, lighting + aux elec) and achieve net zero carbon. It demonstrates that a 68m2 Passivhaus fitted with an ASHP and sufficient pv generation capacity to offset energy used has significantly lower net emissions (8kgCO2/m2.yr) than any of the approved building regulations scenarios, in some cases in excess of a 50% improvement. However, none of these scenarios achieve true net zero carbon as unregulated energy use (appliances, etc) and storage losses have not been added when calculating the required renewables to offset. This time, the results show that the proposed move to a notional net zero building (using an ASHP and reducing the TER by 19%) will still have emissions of 3 kgCO2/m2 .year, which would require 32m2 of solar panels to offset. In contrast, the Passivhaus fitted with 22m2 of solar panels lowers emissions to actual zero. Of all the scenarios modelled, this is the only actual zero carbon building.

Conclusion

The Passivhaus Trust's study demonstrates that the current Zero Carbon targets won't necessarily realise zero emission buildings. The notional zero carbon buildings that the building regulations demand will in fact generate around 18kgCO2/m2.yr, requiring massive renewables expansion that will not be achievable on every home or flat as there is simply not enough area to accommodate it. Initiating design and build strategies aligned with Passivhaus principles is really our only way to achieve a truly zero carbon building with a manageable level of renewables to offset the energy used. Furthermore, consideration will have to be given to offsite implementation of renewables as it will not be possible on every site. Governments and Developers will have to consider something akin to a Developer's Renewables Contribution scheme to fund offsite renewable generation where onsite capacity cannot be achieved in order to ensure that net zero carbon emissions targets can be met.

In closing we would encourage readers to refer to the detailed report issued by the Passivhaus Trust Passivhaus: the route to zero carbon?

Scott Wardlaw 2/11/21 Scott Wardlaw 2/11/21

PassivHaus - 10 Design Strategies

01 Compact Building Massing

Keeping the surface area of your building to a minimum means that you will minimise heat loss, reducing energy consumption associated with space heating. This is known as the Form Factor; the ratio of surface area to internal floor area. The lower the Factor Factor, the more thermally efficient your building will be. The key point here, is to ensure that the thermal envelope is minimised. Simple vernacular forms work best. This does not mean that you have to sacrifice architectural expression however, as this can be articulated via other means out-with the thermal envelope.

02 Location of Unheated Spaces

Co-locate unheated spaces such as stores, bin stores, bike stores. Better yet, separate them from the thermal envelope altogether. This way they do not act as a heat sink, drawing energy from the heated building and complicating your Form Factor. Draw your thermal line and airtightness line around the building at an early stage to manage strategy and detailing, and continue to do so throughout the design process.

03 Optimise solar gain in the Winter through Orientation

In the northern hemisphere, prioritise dual aspect, south facing facades to optimise solar gain during the winter months. Anything beyond +/- 30 degrees is no longer considered a south facing façade by the PassivHaus Planning Package (PHPP). Optimise fenestration and living patterns to take advantage of the path of the sun throughout the day. Avoid overshadowing from neighbouring buildings and vegetation. Consider species of vegetation as well as location. Deciduous trees provide welcome shade in the Summer, whilst allowing solar gain from low angle sun in the Winter.

04 Design Glazing to balance heat gain, heat loss and daylight

Optimise your window design to consider orientation, daylight and summer comfort. Poorly located, excessively large windows are the biggest cause of summer overheating and excessive heat loss in winter months. On East/ West orientations, vertical solar shading devices are best, to the South horizontal solar shading devices should be used to control solar heat gain in the summer months. Minimise heat loss to the north by having smaller windows, whilst taking advantage of solar heat gains via larger windows to the south orientation. Designed in tandem with appropriate solar shading, this strategy will go a long way to minimise the energy requirement of the building.

05 Detailing of Windows

Optimise glazing to frame ratios to increase the thermal performance of the window by reducing the number of mullions and transoms. Triple glazing is almost mandatory in the northern hemisphere to achieve the required U-value standards, so budget accordingly. Windows should be located within the thermal line of the façade to minimise cold bridging and optimise detailing.

06 Natural Ventilation

Try to design dual aspect rooms and homes allowing rooms to benefit from natural cross ventilation. Also, consider night time purge ventilation strategies, particularly in bedroom spaces located on upper storeys. This improves indoor air quality and well-being by optimising user control over their environment.

07 Mechanical Ventilation with Heat Recovery (MVHR)

The core plant in any Passivhaus, a suitably designed and installed MVHR system is fundamental to the successful operation of the building.

MVHR units provide background ventilation by extracting moist warm air from kitchens and bathrooms, exchanging the heat to incoming cold fresh air, and then supplying the air to the other rooms in the home. The MVHR unit should be located in a suitably soundproofed room, not more than 2m from the façade. Design of ductwork and distribution is very important to ensure optimal efficiency of the system. Designed correctly and in tandem with a highly insulated fabric, the MVHR system can be sufficient to provide the entire space heating requirement for the home, whilst ensuring a high indoor air quality.

08 Fabric First

The Passivhaus standard requires a highly thermally efficient envelope. It is not prescriptive in how this is met however, allowing designers to consider a multitude of technical options to achieve the desired operational standards. Accepted ranges of thermal performance to aim for are as follows;

Ground Floor - 0.08 to 0.10W/m2k

Walls - 0.13 to 0.15W/m2k

Roofs - 0.10 to 0.12W/m2k

Windows - 0.80 W/m2k

External Doors - 1.0 W/m2k

These are stringent requirements and how you intend to achieve these should be carefully considered against design and budget constraints. Aligned with this strategy is controlling thermal bridging interfaces in the design of the external fabric. Once again, use your 'thermal line' to identify key interfaces that require attention in design to minimise the effects of thermal bridging.

09 Airtightness

Ensuring an airtight building envelope will drive improvements in construction, reduce energy demand caused by air infiltration and draughts, and protect the fabric of your building from premature decay. There are several ways of creating an airtight building, from application of airtight membranes and tapes, to high performance lining boards. The key design strategy is to draw an 'airtight line' on plan and in section around the façade, then focus and manage interface details between different systems and penetrations appropriately. This must be undertaken at an early stage in the design process and followed rigorously through each stage to construction of the building. Achieving an air change rate of 0.6 ACH is required to meet the Passivhaus standard and to ensure optimisation of the MVHR system. This is a rigorous level, when compared to current building regulation requirement of 7.0 ACH.

10 Renewables - getting to net zero carbon

By designing the building to meet the Passivhaus standard, you have taken a significant step towards achieving a net zero carbon building. This is because the building envelope is already so energy efficient that only very low levels of space heating are required. Following a Fabric First approach to achieve the Passivhaus Standard leaves only two more strategic steps to achieve Net Zero Carbon;

Addressing Domestic hot water demand via solar thermal and/ or heat pump.
Addressing electrical demand via implementation of a Photovoltaic array.

Following these 10 simple broad steps will ensure that you stand the best chance of achieving the Passivhaus Standard. You might even get to a Net Zero Carbon home, thus helping in the country's drive towards meeting our 2035 Net Zero Carbon energy targets. At the very least, if you implement some of the strategies, you will have a building or home that is far more energy efficient than the majority of buildings under construction today. All that is required is the implementation of a few simple steps. At Novo, we are happy to assist clients in whatever capacity they require to develop the strategies required to realise an energy efficient building. Please reach out to us by clicking on the button below if you feel we could be of help.