New to Passivhaus? Here’s what you need to know

Earlier this year, Scotland’s government announced plans to introduce Scottish Passivhaus-equivalent standards to all new-build homes.

The news was most welcome, and I can only hope that similar legislation will become more prevalent around the globe.

Because the more I’ve learned about Passivhaus, and how it can make homes up to 90% more energy efficient, the more I wonder why we don’t apply this standard industry-wide already.

There are design challenges and budgets to consider, but the benefits, not just for our clients but the environment as a whole, far outweigh them.

But before we get into said benefits, let’s cover the basics.

What is Passivhaus?

Passivhaus is a performance-based building standard.

You design and build to a certain set of requirements that help reduce how much energy a building needs to keep warm.

Some of the key principles within the standard are:

• insulation
• air tightness
• ventilation
• free from insulation gaps (also known as thermal bridges)
• high performance windows

Passivhaus buildings are wrapped in insulation, what we call the ‘envelope’. This retains the heat in the structure. In order to do so successfully, you must deal with any gaps, or thermal/cold bridges, in the insulation.

Then, the level of air tightness is carefully considered, aiming for 0.6 air changes per hour (ACH). ACH measures how many times the total air volume in a space is completely removed or replaced within the hour.

Whenever warm air leaves your building, you have to heat it again – so keeping it airtight means less heating.

For ventilation, a mechanical system with heat recovery (MVHR) is often used, which allows for fresh air to come in, as well as heat recovery.

That way, the building is set to a comfortable ambient temperature that’s maintained throughout the envelope.

How do Passivhaus buildings stay cool during the hot summer months?

With Passivhaus, we often talk about retaining heat as much as possible, but with increasingly hotter summers, overheating could become a problem – however, this is factored into the design criteria.

Using bespoke software as part of the passive house planning package (PHPP), you can calculate the risk of overheating the home.

That’s part of the designer’s job. We make sure the risk is less than 10% of ambient temperatures rising more than 25 degrees Celsius, though we often aim for less than 5%.

There are a few things you can do to prevent overheating, like opening your windows to purge hot air. Heat exchange mechanisms can also be turned off during the summer.

Another design solution is to use shading smartly. You can gain passive solar heat from the winter sun, which hits at a lower angle, and you can also block it in the summer, when the sun is high in the sky.

Trees in the surrounding area can also provide shade if the placement is favourable.

The role of engineers

The Passivhaus standard is purely about performance, independent of what materials are used in the structure.

When applying Passivhaus, as designers, we’re thinking about all these performance requirements, but also about embodied energy, and a holistic way of reducing the emissions that a building produces.

The challenge then comes to engineers – how do we apply the design, balancing the structural needs with the thermal and low embodied carbon requirements?

Some engineers might worry that by applying Passivhaus principles they may be crossing over into architect’s domain, however having the knowledge of these principles allows for better communication within the design team.

Firstly, these two spaces need to work together, but also, the role of the engineer is incredibly important, and unique.

It’s about fabric engineering. You’re choosing materials, thinking carefully about thermal conductivity as well as strength and durability.

We’re also often involved in covering any thermal bridges, thus making sure that there are no breaks in the insulation.

And building physics also plays a big part. You’ll be using heat equations and calculations – that's our thing!

Why apply Passivhaus?

There are several benefits to gain from building to the Passivhaus standard.

Something my clients have constantly fed back is the level of comfort they experience. You can’t really quantify comfort and wellbeing, but it adds to a better quality of life.

There are also health and wellbeing benefits because there's more fresh air in the house.

The mechanical ventilation filters pollution from the home, resulting in better air quality indoors. This means less allergens enter the home as well.

What you have is a home that has the same ambient temperature everywhere, with no cold or hot spots.

You don’t have to play around with heating, in fact you may not even have to put heating on at all. This would considerably reduce your energy bills.

And by consuming less energy, you’d also do your part for the environment by producing less carbon emissions and having a lower demand on the grid.

Finally, since Passivhaus homes have breathable structures, there’s also less maintenance and better structural longevity that reduces condensation, and concerns like black mould.

With all these benefits, Passivhaus would be an excellent option for social housing. It’s a no-brainer.

Alternative standards

Passivhaus can be expensive. Applied using best practice, on average it comes at 9% extra cost.

Luckily, there are other standards and guidance around that can be useful when there are constraints on your budget:

• Association for Environment Conscious Building (AECB) building standard: to create high-performance, low-carbon and low-energy buildings using widely available technology.
• AECB retrofit standard: to retrofit buildings to a high standard, improving the building fabric and providing access to low-carbon energy.
• Low Energy Transformation Initiative (LETI) guide: for new buildings, helping to set clear, achievable targets across operational energy, embodied carbon, the future of heat, demand response and data disclosure.
• Royal Institute of British Architects’ 2030 Climate Challenge: sets targets to reduce operational energy, embodied carbon, potable water and health and wellbeing.
• BREEAM standards: help to holistically achieve environmental, social and corporate governance (ESG), health, and net zero goals at every stage of building.
• LEED standards: helps create efficient, low-carbon and cost-saving buildings by addressing carbon, energy, water, waste, transportation, materials, health and indoor environment quality.

Whichever standard you choose, you’ll be doing your part to help us all reach net zero!

Find out more about Passivhaus