Jae Cotterell, founder of Passivhaus Homes, explains the importance of the choices we make when considering our construction approach. It is possible to create a high performance home with low embodied carbon that doesn’t cost the earth if we understand the process, where the impacts lie and where optimisation can make a huge difference.

Interview with Jae Cotterell

Jae Cotterell has extensive experience in designing Passivhaus buildings. She is a co-author of The Passivhaus Handbook and a Director at Passivhaus Homes, where she specialises in the PH15 Construction System. In this chat, Jae explains the importance of considering both embodied carbon and energy efficiency when undertaking new build projects.

What is the PH15 Construction System?

The PH15 Construction System is a complete construction solution that enables you to achieve Passivhaus performance whilst using low embodied carbon materials, with a primary focus on timber. Passivhaus Homes use a range of timber variations in their PH15 Construction System, from Laminated Veneer Lumber (LVL) to Low carbon, wood fibre insulations. This combination and variety of materials allows them to use the timber in the most efficient way and ultimately provide a finished product that is incredibly energy efficient with low embodied carbon.

“I don’t think that you go into these areas to make a lot of money. You do it out of interest because you want your work to be meaningful and to contribute in some way. And, I hope that we will have done that”

The story of the three little pigs says a lot about our current cultural attitudes towards timber construction

The UK has a cultural nervousness towards timber which combined with a long history as a brick block country has created a construction culture that is very different to the timber-centric approaches of Canada, Australia and many European countries. This cultural mindset means that there is a resistance to change in large scale construction, but for Jae, it is important that this preconception is challenged. Smaller projects and self builds allow for the freedom to challenge the status quo, which can hopefully pave the way for a timber industry to be built in the UK.

Timber is a precious resource and it is important how we use it

Due to its low embodied carbon, timber is a much better resource to be using in Passivhaus construction than alternative brick block methods. That being said, timber is a precious resource, and we must be looking at contemporary solutions to ensure that we are intelligently engineering timber in order to use it as effectively as possible.

Laminated Veneer Lumber (LVL) allows you to use timber more efficiently

You can use 35% to 50% more of the log when you create an LVL I-joist than when you construct with solid section timber. Furthermore, Cross Laminated Timber (CLT) uses 44% more timber than an LVL I-joist. At present, there is no production of LVL in the UK but there is hope that this production could be introduced with the regeneration of the timber industry.

For a new and successful timber industry to grow, the biodiversity of forests must be at the centre

Instead of a timber industry that favours monoculture planting with low biodiversity, we need to be looking at a timber industry that is planting with a wider range of trees and considering biodiversity gain. Not only will this improve the ecology of these areas but it will also provide more biodiverse sources of wood.

Timber structures do not increase the risk of fire

Timber in its natural state does burn, however there are many ways to treat the wood to ensure that it is incredibly safe to use in construction. There are a large variety of fire related products on the market, including fire resistant membranes that can be wrapped around the framework and the PH15 Construction System doesn’t include any voids within the internal structure, reducing the ability for fire to spread. In fact, timber construction is significantly safer than many plastic or oil based materials on the market, the likes of which were a primary component in the Grenfell Tower Fire.

Embodied carbon is important to consider, but so is energy efficiency

Embodied carbon looks at the present carbon cost of a new build structure but ensuring that this structure operates efficiently will lower the overall carbon footprint of building throughout its life. Jae highlights that the process of decarbonising energy will limit the national supply and put pressure on the grid. It is therefore crucial that we find ways to use less energy and reduce this demand; the efficiency of Passivhaus structures will play a large role in this.

It’s not about building a large amount of new housing, but instead about ensuring that the ones we do build are of high quality and efficient to operate

Passivhaus Homes’ most recent social housing development, Hook Hollow, has emphasised that the PH15 construction system can deliver both high quality and affordable homes. Jae also highlights that a return to modest terrace housing over large stand-alone homes will be an incredibly efficient approach to building Passivhaus structures because there is a smaller envelope to be considered. Not only does this reduce the number of materials required, but will also reduce the overall costs, allowing for more affordable housing to be built

“I feel the disparity between the have nots and the haves is so big, and having a high-quality home, that also has low bills and energy, and is also very healthy. Those are massive benefits to someone who is under pressure.”

Visualisation of Hook Hollow social housing scheme

An all electric system allows for simplicity

Passivhaus Homes have committed to a no more gas campaign and will not put gas in to any of their new homes. This means all of their new builds will be all electric, allowing for low maintenance and low-cost solutions. The focus is on getting the fabric of the house to a really high quality with very simple hot water and heating solutions. The Hollow Hook social housing development does not include any heat pumps either, reducing the money spent on technology and the cost of future maintenance.

Sacrificing your Passivhaus certification is a slippery slope

By focusing on achieving low embodied carbon but reducing the standard of energy efficiency in your new build, you are likely to lose a lot of the benefits that a Passivhaus house can provide. By opting to forgo triple glazing and MVHR systems, you will end up doubling your energy demand. These systems are also what allows you to maintain a constant temperature, ensure you have high indoor air quality and prevent mould from developing.

“You drop the standard, and you drop a whole load! A little a basket of benefits just get thrown away. I think it will be an absolute error to not actually do it properly.”

It is important to be aware of the greenwashing within the construction industry

There are many products that claim to decarbonise certain areas of construction, but if these products are also limited resources, then they are not sustainable solutions. Instead, they are a solution for a small amount of people and in many cases, the fundamental method may need to be reconsidered. For example, a transition from concrete foundations to above ground timber shell cassettes.

Size and materials matter

A Passivhaus certification does not guarantee that a house is sustainable. As Jae points out, a Passivhaus with a large footprint and built from high embodied carbon materials will use more energy and have a larger carbon footprint than a modest sized standard house. Therefore, it is crucial that scale and materials are considered in combination with energy efficiency.

Overall, we need to shift our cultural mindset to value the quality of our homes

Our current cultural psyche places value on the size and the features of our homes, yet it’s not quite reached the point where we value the benefits of a Passivhaus. It will take time for this to shift, but hopefully, in the not-so-distant future, our cultural mindset will begin to value constant temperatures, high indoor air quality and comfort over the number of bedrooms and wood burning stoves.

“I come across people all the time, they’re not willing to compromise on size, but they’re very willing to compromise on quality. And we need that flipping as a culture… I’d love to see that change, because I think it would make a big, big difference and be much better for us. A much more insightful and wiser way of viewing things.”

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