Heather Johnson from AD Practice explains what Passivhaus Plus is, how it differs from Passivhaus Classic accreditation and what benefits achieving the standard will afford. She also reveals why most of her clients target ‘Plus’ and outlines the steps to achieve this on a project.

Interview with Heather Johnson

Heather Johnson has worked in architecture since the age of 16, starting with a summer job as a CAD assistant. She studied architecture for four years at the University of Bath, with placements and work in Suffolk, where she learnt about barn conversions and rural issues. She then did her Part 2 at the Centre for Alternative Technology as her focus shifted to sustainability. A three-month temporary role at AD Practice back in Wheathampstead, where she grew up, turned into seven years and is where she still works today. Heather is now a Passivhaus Designer and has worked on numerous Passivhaus Plus and EnerPHit Plus projects over the years.

The performance criteria for Passivhaus ‘Classic’ must be met for ‘Plus’ and ‘Premium’ as well

Most people have heard of the Passivhaus standard, also known as ‘Classic’ but perhaps less well known are the other certifications ‘Plus’ and ‘Premium’. Before looking at the differences between them, these additional classes must meet the same stringent performance criteria laid out in ‘Classic’. For example the airtightness targets, energy demand, overheating risk and so on.

In this episode we’re going to focus on Passivhaus Plus and find out about Heather’s own project, a barn conversion, which is aiming for the EnerPHit Plus standard.

Passivhaus Plus sets a target for on-site renewable generation

Passivhaus Plus brings renewable energy generation into the certification process, setting a target per metre square of treated floor area (renewable energy generation ≥ 60 kWh/m².a).

It is a step towards self sufficiency and energy security, and a ‘Plus’ project must cover the home energy usage needs over the course of a year. This does not mean it is off-grid, but that the generation exceeds what gets consumed over a year. So there is likely to more generation at certain times of year according to which renewable is being used.

Solar PV is the most commonly used on Passivhaus Plus projects

In the UK, more often than not, solar PV will be chosen as the renewable energy source. The main reason is that most houses are in our towns and cities, where space is a premium and roof space is generally available. The technology has also advanced a great deal over the years. It’s cheaper, more effective and can be integrated seemlessly.

However, any renewable could be used on a Passivhaus Plus project. For example, if you are beside a watercourse then a water turbine might be appropriate.

Passivhaus Plus encourages a compact building form

When it comes to solar PV some key considerations are the roof pitch, orientation and surface area of the roof. If renewable energy generation was the starting point for the design of the house, it might lead us to build a bungalow with lots of roof space. However the surface area to volume ratio would not be good, and therefore we may not reach the other Passivhaus targets.

“The take home point is you still want to make the building as efficient as possible and then add on the generation.”

The Primary Energy Renewable factors relect the whole picture

There’s another characteristic the building must meet.

Primary Energy Renewable (PER) demand* ≤ 45 kWh/m².a

According to Passipedia: Renewable primary energy (PER) is the unit of energy generated from renewable resources. PER-factors reflect the primary renewable resources needed to cover the final energy demand of a building, including distribution and storage losses. In the case of a PER-factor of 1.5, a surplus of 50% renewable primary energy is needed to be able to meet the final energy demand at the building. The higher the PER-factor, the higher the required resources and therefore the more important the implementation of efficiency measures in order to avoid compensation from non-renewable sources.

Permission is required to export electricity to the grid

Although smaller scale renewable schemes (typically under 3.68kW per phase) do not require permission to export to the grid, a Passivhaus Plus project will need approval from the District Network Operator. Depending on the infrastructure of the local area there may be limitations.

However, Heather says that she has never had a project turned down, but it is always a possibility.

Additional constraints make achieving EnerPHit Plus more challenging

Retrofits can also achieve the ‘Plus’ certification. The main difference between a new build and a retrofit is the additional opportunities that building from scratch affords.

With an existing building the orientation is set and without significant work so is the roof space, orientation and pitch.

Amongst Heather’s design process she is keen not to break up the long parts of the roof, which are best for energy generation, with rooflights or other roof forms.

There may be other challenges, as well, such as planning restrictions if a property is in a conservation area. This was the case with Harpenden EnerPHit Plus but a whole roof pitch of solar PV was eventually accepted.

Heather makes a comparison between the Harpenden EnerPHit Plus which spent £95 on electricity for a year, and her office which typically spends around £200 a month, so it is a huge saving!

On another retrofit project it made sense to rotate the roof by 90 degrees. This not only increased solar generation but created a mezzanine with a view of the surrounding countryside, which the client didn’t have before.

Solar PV has come a long way and can now look fantastic

There can be a double win from maximising generation because a whole roof pitch of solar PV can become the roof finish as well, saving money on tiles, etc.

Heather often uses the GB-Sol, Infinity system, which is completely seamless.

On a Passivhaus Plus project in Wheathampstead there is a more conventional in-roof solar array, which does not cover the whole roof. The solar panels are inset so they’re flush with the concrete tiles but it’s not a seamless finish. Interestingly, the clients wished they had put more solar PV on the roof.

Heather say the cost of solar PV has come down dramatically from when they were first being installed. Unfortunately, the feed-in tariffs for exporting have also gone down, but there is still a much shorter payback period.

There are also schemes around, such as Solar Together in Hertfordshire, that give discounts for solar installations. It’s becoming a much more attractive offering, especially with more people opting for electric cars.

Solar PV does not just have to be on south-facing roofs

South facing roofs tend to work best for solar PV but that doesn’t mean it ends there. When Heather speaks to the suppliers and specialists, they’re quite keen on southeast and southwest as well. This gives a longer period of time generating power, so can be better for the homeowner and takes pressure off the grid at peak export times. North facing won’t give as much generation, but a mixture of orientations will give more consistent energy generation throughout the day.

Use as much of the energy generated on site, before exporting

There’s normally a hierarchy of uses for the energy that is generated on site.

For example, it may be running a heat pump, topping up the hot water, charging a battery or an electric car. So exporting to the grid would be the last option.

Different suppliers also pay different rates for exported electricity and these rates will probably vary during the day. So if you have a home battery you can maximise your return by importing and exporting at specific times. For example, if you need to draw from the grid to charge your battery it makes sense to do it overnight when the tariffs are low.

Heather even wants to make her ‘long, thin barn’ an EnerPHit Plus

Heather and her husband fell in love with a barn in rural Hertfordshire, which was ripe for modernising. It’s a long thin brick building that is not really conducive to a Passivhaus project, but this has not deterred them!

The first air test achieved 0.7 air changes per hour at 50 Pascals so it’s well under what’s needed for the EnerPHit standard.

As they are targeting EnerPHit Plus, the large roof area makes it easy to achieve the required generation and they are looking to install around 50 solar PV panels.

Heather’s barn is being converted in two phases

Heather is converting the building in two phases. As it is long and thin (around 25 metres) the barn will have two MVHR units (to limit the length of duct runs). Phase one is the old traditional barn, the taller area, with a side utility room and shower room that’s run off one system. They plan to put up a temporary wall up at the end, so they can live in that part of the house, studio style, whilst they carry on fitting out the rest of the house.

A new timber structure is being built within the old barn

Heather is taking a similar approach to this barn conversion as they did at Stirley Farm. They are building a timber box within the old brick structure.

So there’s an insulated slab (the Isoquick system) and I-stud walls which are filled with cellulose insulation. The new extension will be finished with timber cladding, whilst the existing barn will be the existing brick, retaining the historic fabric.

The design does not add any loads to the original building as that would have required underpinning. The existing timber trusses have been retained and now function as tie beams within the new timber frame structure. Heather explains that because the building is essentially a long tube, it needs the beams to stop it just squishing and falling flat.

Dealing with ventilation and moisture across the old and new structures

Heather outlines that where they’ve built against the brick wall, they have a nominal 50 to 100mm cavity, which is much wider in places because the brick walls are not straight. With the new timber clad structure there is a rain screen system, which is battened and counter battened out before the cladding is put on. The timber frame is also wrapped in a membrane before the cladding goes on, so any moisture would hit the membrane and trickle out the bottom, as with many similar systems.

The bigger risk with phasing the work is that they don’t ventilate the uninhabited internal spaces correctly. As it is airtight and insulated, they need to make sure air is circulating by using fans and opening windows. Otherwise condensation could occur which would eventually lead to mould.

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