Sarah Lewis, the Research and Policy Director at the UK Passivhaus Trust and author of PHPP Illustrated, explains what the Passive House Planning Package (PHPP) is and how it is used during the design process.
Interview with Sarah Lewis
Sarah Lewis is the author of PHPP Illustrated and an architect with a wealth of experience when it comes to Passivhaus building. In 2004, whilst at university, Sarah wrote her thesis on the Passivhaus standard at a time when there were no Passivhauses in the UK. Her first job was working for a firm that were beginning to look at building to Passivhaus standard and it was here that she became a consultant on London's first Passivhaus. From then on, Sarah has worked on an array of Passivhaus projects and now with her current role as the Research and Policy Director at the UK Passivhaus Trust, it is safe to say her knowledge of Passivhaus and the Passive House Planning Package (PHPP) is extensive. In this chat, Sarah guides us through the PHPP, offering tips and tricks that will optimise the user experience and shares a bit about her own retrofit project.
PHPP is a part of the quality assurance process
Passivhaus is a whole building approach to construction that aims to deliver healthy, comfortable and energy-efficient buildings. It focuses on high quality construction and through an exacting quality assurance process, provides clear and measured targets. Although there are different targets for new build and retrofit, the process remains the same; it's about understanding and embracing building physics.
“It's a really good standard to look at if you just want to have good quality, low energy buildings.”
The Passivhaus Trust has useful resources for both professionals and self-builders
Targeting the Passivhaus standard can seem daunting if you are not familiar with the process, but the Passivhaus Trust has lots of excellent resources to help you:
- The Benefits Guide highlights the advantages of Passivhaus – from reducing our environmental impact, to reducing energy usage/bills and just generally providing a comfortable living environment.
- The How to Build a Passivhaus Guide is aimed at those who have already decided to embark on a Passivhaus project. The guide includes top tips from experts in the construction industry and helps you to avoid the mistakes that are often made on first projects.
- The Passivhaus Designer Course teaches construction professionals to deliver projects to the Passivhaus standard. It includes training in PHPP as well as considerations and methodologies for meeting the standard on site.
PHPP helps you make better predictions about the energy performance of a building
The Passivhaus Trust describe PHPP as: “PHPP’s level of sophistication enables it to consider a wide range of variable characteristics
The Passive House Planning Package (PHPP) is a modelling tool that is used in the design process of a Passivhaus. What makes it work so well is that it considers a wide range of variable characteristics which affect heat loss, energy use and internal comfort. This means that the predicted energy use is much, much closer to the real-world performance.
PHPP is an ‘energy balance' tool
PHPP is not a dynamic modelling tool. Dynamic models might use data points at 30-minute intervals, for example, but it doesn't mean it would be more accurate than a simplified model. Dynamic modelling's strength is when you were moving into unknown territory and don't know whether a simplified model will be accurate.
So PHPP is powerful because it has a simple way to do the an energy calculations. It's based on the average monthly data which allows us to get annual energy demand. All this is quick. The data entry points are reduced to the essentials.
Sarah's book: PHPP Illustrated is a comprehensive guide to PHPP that aims to break down the steps within the PHPP and provide clarity through her range of diagrams and flowcharts.
In every building there are heat gains and heat losses
All buildings will have heat gains and heat losses at various points throughout the year. In a Passivhaus, these losses are less noticeable because of the higher quality fabric but they will still occur at a lower rate through the walls, the roof, the windows, the ventilation system, etc. It is therefore essential to balance these losses with your heat gains. In a Passivhaus, there are three main categories of heat gain:
- Internal heat gains – This is the energy that is created when people move around the house or use equipment, such as an oven.
- Free heat gains – this is the solar gain, where sunlight passes through windows and creates a warming effect.
- Supplementary heat gain – Once you have calculated the internal heat gains and the free heat gains, whatever is still needed to balance the heat loss becomes the heating demand for the building.
“The number that you often hear in Passivhaus being quoted 15 kilowatt hours per metre squared per annum. So that's your space heating demand for the building. There's also an alternative metric, which is the peak heat load, which is 10 watts per metre squared. But the PHPP is constantly reworking that balance.”
It can be useful to visualise the heat loss and heat gain as a seesaw that needs to be balanced. The exact figures and components will be different for every building, so the solutions will need to be specific to your design.
“If you have a bungalow with a huge roof, you're going to have a lot more losses through the roof than if you had a tower block, which has a very small roof, but much more walls. So you can see how each building will end up with its own bespoke set of losses, and therefore its own bespoke set of gains as well.”
Lots of different ways to use the PHPP
The PHPP is a giant Excel book with many different sheets that you work your way through inputting the specific details of your project. The PHPP has built in assumptions that are based on general data and can help to provide an estimate without having to input into every single section. However, Sarah did highlight that these assumptions are often on the conservative side, and as you begin to input more of your own data, you will see your PHPP improve little by little.
“For example, there's a default assumption for how much the windows are shaded until you actually have your site set. And then you can work out how much those windows are actually shaded and put that information in.”
In the early stages of your planning using the PHPP you will want to provide information on the fabric of the house: how big the envelope of the building is, the orientation of that building and the size of the roof, walls and floor. From there you can play around adjusting elements, for example adjusting the orientation and seeing the impact that that has on the energy performance.
There are also a wide range of plugins that can supplement the PHPP and help to analyse certain aspects of your design:
- Design PH – A sketch up plugin that allows you to create a fast model which can be exported into the PHPP.
- The AECB PHribbon by Tim Martel – This helps model the embodied carbon of the building.
- Bim2PH – Allows you to export your BIM model into PHPP.
- PassivLink – Lets you export your Revit project data directly into PHPP.
Avoid design by ExCel spreadsheet!
Although PHPP is an excellent tool for predicting and testing the performance of a building, it should not become a replacement for actively making your own design choices.
“The PHPP, as an architect, allows you to go: okay, what if my building was orientated slightly differently? How does that affect the energy balance? Or if I have a big window here? Do I have a summer comfort issue? Or is that really good for my thermal performance of the building? What we don't want to do, is encourage design by Excel spreadsheet. People should be using their design intelligence, first and foremost, they need to make those intelligent design choices.”
Form factor, glazing ratios, complexity and orientation will all have a massive impact on a project and these can be optimised early in the design process.
“A common mistake is to use a model and then just tweak the entries. For example, window sizes, just to get the desired number to that 15. So while yes, of course, you could just increase your free gains, solar gains, in this case, by making your window bigger and bigger and bigger. You need to be smart and think about summer comfort, embodied carbon usability, and other things.”
As more Passivhaus projects have been delivered around the UK the key learning around window design is to focus on excellent daylighting, rather than maximum solar gains (which can lead to summer comfort issues).
PHPP gives us a ‘whole picture' of space heating, energy use and delivery
The bulk of the PHPP looks at the form factor, airtightness, insulation, windows, ventilation system and orientation of the building. All of this information combined will provide you with the space heating demand or cooling demand of the building. Then within this, the PHPP goes further than the general UK standards and metrics by looking at both regulated and unregulated energy. In the UK this is referred to as the EUI (Energy Use Intensity) which aims to look at the whole picture of energy use. This takes into account all the heating and cooling systems, the pumps, the lighting, the electrical equipment/plug loads, all the cooking that’s happening in the house and all the hot water being used. All of these combined make up the EUI, which provides the overall energy performance of house.
PHPP includes where the energy comes from
PHPP is not only concerned with the energy that is used in the building, but also the energy that is created and delivered to the building. This includes all the storage losses and distribution losses of transporting a kilowatt of energy from a wind turbine or solar panels, into your building (Primary Energy Renewable). All of these combined create a very comprehensive picture of how energy flows around the building, including where it is generated and where it is lost.
The PHPP looks at components in more detail than standard UK metrics
Take the example of windows. Manufacturers will provide the U value of their windows which measures how effective the overall product is as an insulator. This measurement is not enough for the PHPP because windows are made from a range of different materials. In order to get an accurate understanding of the window’s overall performance, it needs to know the performance of each material used in the window. Manufacturers who sell Passivhaus rated windows will often have this data available, even for products which aren't certified, but it can sometimes to be difficult to get the necessary data from suppliers who are not used to working on Passivhaus projects. Heat pumps can also be difficult because you need to input a variety of test points both externally and internally, it has been challenging to get the enough information for the PHPP to accept it. However, Sarah believes this is changing and that the more Passivhauses that are built in the UK, the more accessible this information will become.
PHPP can be used for a quick check
People who are experienced with PHPP can make a mock-up in a couple of hours, especially if they are also familiar with the project. One way to upskill is on the Passivhaus Designer Course where you will learn to use the PHPP tool over a couple of days. Because the system is based in Excel, which is a familiar software for most of us, once you have learnt how to input data, it isn’t that difficult to use. It is very common for designers to tailor the process and create their own base PHPP, which has all the values of the construction systems they usually work with already pre-programmed.
“It makes it a bit quicker the next time around, maybe your favourite Windows already input, or the air source heat pump you like working with. All these things you can put into the PHPP and then use for future projects”
Your PHPP can provide a lot of useful information that you can take away
Because there are a lot of people who use the PHPP, there have been number of different plugins developed to help you get the most out of the software and analyse the data it provides.
- The Passivhaus Trust Summer Comfort plug in assesses your PHPP and creates a summary of anything that could be a point of risk, for example, a large temperature swing over the course of a single day. The plug in would then ask for a justification of why this is happening and if there is an adequate control strategy. By highlighting these areas, it helps to minimise an oversight in design that could effect the performance of the building.
- The DeltaQ plug in analyses your data and determines whether your building will comply with the RIBA 2030 challenge and the LETI climate emergency design guide targets. This therefore aligns the building performance with UK metrics rather than just the Passivhaus specific ones.
- There is also a section of the PHPP which can analyse the data to calculate an estimate of the running costs for your Passivhaus.
PHPP is a core principle of Passivhaus design
Completing the PHPP is an essential step in creating a Passivhaus.
Sarah says that there is a common misconception that Passivhaus principles only refer to the fabric characteristics of the building such as super insulation, triple-glazed windows, MVHR and so forth. However, these alone do not create a Passivhaus. The quality assurance comes from the certification process and PHPP is a big part of this.
There must also be attention to detail on site, otherwise we may not close the performance gap as much as we expect to.
PHPP can be used by anyone!
You don’t necessarily need to work with an architecture firm that is experienced in Passivhaus design. The PHPP should be easy enough for any architect to use and its always great to get more consultants to come on board.
“I would certainly say that if you've got an architecture team or consultant team who haven't worked with it before, it's something where the Passivhaus certifiers can bring a lot of value. So we always recommend that the certifiers are appointed somewhere around RIBA stage two or maybe the beginning of RIBA stage three once you really understand what the building is going to be so the certifiers have something concrete to price. At that point, they can come on board, they can do design checks on the PHPP to really highlight if there's been any early errors or omissions and then they can do construction assistance… And then obviously, they're doing the certification at the end as well. So they can really help if a project team are new to Passivhaus.”
However, Sarah also highlights that the PHPP isn’t just exclusive to architects and that if you are keen to give it a go yourself, the manual and wealth of other supporting documents can help you along in that process.
Looking back on one of our previous episodes, Mark Tiramani undertook the PHPP process himself.
“I think anyone can do it. The very first PHPP I ever did, I had to just use the manual and fill it out because there was no English courses at that point. I was self taught in that sense. I would say it took me a bit longer to fill out. You have to be quite experienced to be able to do in a couple of hours. And I think the main thing is orientating yourself. There's a lot of spreadsheets to work through. But the manual comes with a flow diagram. And you can follow through the sheet by sheet and you can fill out everything. But it is definitely helpful to have gone on a course before trying to delve in. I didn't have that luxury when I did my first PHPP, which is why I ended up writing the book.”
The PHPP has had a few changes over the years
In 2016, the formula for how the Primary Energy was calculated was changed, which also allowed the Passivhaus Institute to bring in different classifications: Classic, Plus and Premium. These classifications are based on both the usage and generation of primary energy.
“So rather than just standard Passivhaus, it brought in Plus and Premium, which were ways to acknowledge projects which are generating energy, so renewables as well as being very energy efficient.”
Primary energy hadn’t been a massive focus in UK Passivhaus practices and this change to the PHPP helped formalise a shift in mentality towards renewable energy generation and promoted including these aspects in the designs. Before, it could have been considered acceptable to fit a gas boiler into a Passivhaus, but this change shifted the intention of Passivhaus to also work towards a future grid supplied primarily by renewable energy. Apart from this, the majority of other changes have only been minor tweaks or improvements to the interface, to make it more user friendly.
Sarah's Retrofit Project
In 2018, Sarah bought an ex-local authority, 1950s terraced house in East Lothian, Scotland. She has since been renovating the house with the aim of ultimately getting it to reach EnerPHit standard.
“I would probably say, we had exchanged and had the keys for about a day before I got into the house, measured everything and started inputting it into PHPP. The main reason for that is I wanted to see what the heat loss from the building was, so that I could plan what we're going to do to improve the building. So that's one of the ways that you can use PHPP in a retrofit, you model the existing building and then you can see where all the heat loss is, and you can see where you should focus your money to save that energy.”
Sarah’s retrofit has not quite reached EnerPHit standard yet as it does not meet the airtightness requirement. This is one of the largest challenges in a retrofit project and Sarah says that it is the roof of her project that is not currently up to spec.
“There's some air leakage in the roof, which is just really difficult. We didn't replace the roof so it was just really difficult for us to address all of that. I think when the roof comes up for its replacement, so it's a 1950s house, but might have another 20 years in it, at that point, I hope we can get the airtightness down.”
For Sarah, retrofits can really challenge you to think creatively with your design because there are so many constraints for you to work within. A challenge arose in her project when it came to insulate the floor, as to add insulation to the floor would reduce the clearance height of the room. Sarah worked around this by taking the ceiling right back to gain some extra headroom, which in turn exposed the beams, adding a lovely design feature to the room. This shows that with a creative mindset, constraints can be turned into intentional design choices that add character and detail to a home.
“You can plan out a retrofit and you can do that over a few years or you could do over a few decades”
Create a ‘long term' retrofit plan for your home
So long as we have a comprehensive, well thought through retrofit plan it doesn't all have to be completed all at once. PHPP could be used to help us break down a retrofit into steps.
“The EnerPHit retrofit plan encourages you to design all of that upfront, and it almost sits as a kind of retrofit passport with your building, which can be passed on to future owners if you move or if it's an asset, which is owned by like a local authority, or a council, or housing association or someone, they can also sit with that asset as people in the company change. And then people always know what's the next plan for any particular building.”
This procedure could be revolutionary in the UK and PHPP will be a really useful tool to analyse the existing housing stock and work out the best way to get as many houses as possible to higher standards.
“Going for a full EnerPHit retrofit is probably not the right solution for all buildings. We've got a huge job in the UK to retrofit our existing stock and we need to take the easy buildings and go as far as we can. And then we need to recognise that some buildings will never get to go that far and planning out a lesser retrofit design for those projects is also really important and can also be done in a really robust tool like the PHPP.”
Find out more
Visit the website of the Passivhaus Trust
RIBA Books – PHPP Illustrated
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