What is Passivhaus? And is it the future of self builds?

September 2020
Passivhaus

As we become more eco-conscious, more of us are considering self-building a home to Passivhaus (or Passive House as it’s often called in the UK) standards. This entirely voluntary building performance standard is one way to build a low-energy home.

 

The focus of Passivhaus is significantly reducing heating demanding rather than choosing to rely on renewable energy sources. Designers use a software package to model the likely future energy demand of the property.

 

The Passivhaus standards far exceed the current Building Regulations and can be met in multiple different ways. Two projects might use completely different routes yet meet or exceed these standards. As a designer, this represents a unique and exciting challenge!

 

 

What’s unique about Passivhaus?

Many previous attempts at building low-energy house failed to deliver on their promises and so Passivhaus started life as a research project to examine these failings.

 

The study draws many conclusions from the research which were then distilled into a set of guidelines and thus Passivhaus came into being. This standard provides a very thorough and exacting way to build. Unlike previous attempts, numerous studies have shown that it works.

 

 

How do the standards work?

As a specific set of guidelines, a Passivhaus is designed from the beginning around these principals. Unlike solar panels or adding additional insulation, for example, which can be done after the build, to adhere to the standards they must be central to the design from the start.

 

To conform to the standards, the relevant construction details are input into a complex spreadsheet known as the Passivhaus Planning Package (PHPP). Thee include:

  • Insulation depths
  • Window sizes and orientations
  • Junction details

 

The result is a predicted space heating demand (expressed in kWh/m²/yr). Typically, after several adjustments to the design, you arrive at a configuration that meets the standard.

 

What makes building a Passivhaus so interesting, is that it can be built using almost any construction method. That said, there are some universal features, including:

  • Massive insulation (average depth 300mm)
  • Triple glazing with insulated frames
  • Fantastic airtightness levels (must score better than 0.6 air changes per hour)
  • Mechanical ventilation with heat recovery

 

 

Passivhaus section

 

 

Key principles

The key principles of Passivhaus construction can be summarised as:

  • Uses 90% less energy to heat than an average home
  • Minimal micro-renewables
  • Mechanical Ventilation with Heat Recovery (MVHR) provides constant fresh air and retain heat inside the house
  • Triple glazing with insulated frame
  • The Tea Cosy Effect (or ‘minimising thermal bridging’)
  • Super-insulation
  • House retains heat from sun and occupants’ activities
  • 20 times more airtight than a standard build

 

 

So no heating?

The standards aim to reduce the amount of space that requires heating to such a minimum that your property doesn’t need a conventional heating system.

 

The current Passivhaus standard is set at 15kWh/m²/yr.

 

Putting this into context, on a 160m² house to Passivhaus standards, you would need just 2,400kWh of energy throughout the year. This figure is roughly a tenth of the number used by the typical British home of the same size. It’s also about a third of the consumption of most eco-home.

 

So again, there some heating involved. If no heating was required, then the figure would be 0kWh/m²/yr.

 

 

Heating a Passivhaus

As the need for heating is greatly reduced, there’s no need for a combi-boiler. Instead, you’re free to pick from several options including adding a small heating element to the ventilation system. This pumps warm air around the property when needed.

 

Many of these devices are rated around 3kW. They only start to work when the external temperature is close to zero.

 

As you’ll still require hot water, you will need some form of boiler to fulfil this need.

 

 

Passivhaus example

 

 

Should my design be certified?

As a set of principals rather than a licensed term, most Passivhauses are built to the standards without an external body certifying that their dwelling officially meets them.

 

That said, the Passivhaus Trust acts as a training and a certification centre, which seeks to uphold and develop the standards. It charges around £1,500 to certify a house.

 

Having your home certified as a Passivhaus ensures that it does conform to the standards and maintains a high level of quality throughout. It, therefore, makes the property slightly more valuable as you can prove the design actually does conform to these high standards.

 

 

How do they compare to zero-carbon properties?

A zero-carbon property conforms to a different mixture of standards. The average detached zero-carbon house can use up to 46kWh/m²/yr, which still is a demanding target but nowhere near the Passivhaus standard of 15kWh/m²/yr.

 

The designer uses on-site renewables and carbon offsetting to create a property that’s zero carbon. Comparing these to the Passivhaus standards, the overall energy consumption is far lower and the design centres around saving energy rather than generating it.

 

 

Are there any downsides?

As a unique concept, some will be delighted to live in a Passivhaus, while others will struggle to know how to live within it. The owners must operate in harmony with the building.

 

For example, you’ll need to learn when’s best to open windows or boost the ventilation system. You’ll also need to maintain the filters. It’s a bit like learning to drive an electric car after driving a manual one for years. There is a steep learning curve, but it’s easy to be accustomed to its differences and quirks.

 

What you can’t do is start adding or taking elements away as the moment you do, as it will no longer be a Passivhaus. Even something as simple as adding cables for a satellite dish is discouraged. Nor can you sleep with the windows open.

 

Of course, you’re free to live in your home as you see fit but you’ll want to respect the integrity of the design and principles as otherwise, you’ve gone to a lot of trouble for little.

 

 

 

 

Does it cost more to build?

As the building’s specification is higher, it does cost more to build. However, as running costs are significantly reduced, there is an overall cost-benefit to building to Passivhaus standards.

 

As there’s no single build method, calculating the cost of building a Passivhaus is tricky. Building to this higher standard could increase the build cost by 10 to 25%, this figure will be slightly higher if you choose to get your build certified.

 

Another factor is your plot and the surrounding buildings. It will generally cost more to build a single house to a higher standard than a small development of multiple Passivhaus being developed by an expert team.

 

Typically the extra costs are spent on:

  • Triple-glazed windows and doors that reach the exacting airtightness ratings required. This amounts to around 35% of the extra spend
  • Another 30% is spent on achieving the required airtightness.
  • The rest of the cost is spread across insulation, mechanical ventilation, shading and design costs.

 

 

Is there an overall cost-benefit?

So while it will cost you far more upfront, are the saving how worth it?

 

Let’s say we’re building a 200m2 property. Building regulations standard specify that you’ll need at least 11,00kWh of energy to heat the house. Assuming we fit a gas boiler, the cost will be about £600 per year.

 

Comparing this to a 200m2 Passivhaus, which has a demand of around 15kWh/m2/yr. So you’ll only need about 3,000kWh to heat it. It’s safe to assume that at least half of this figure will be provided by activity in the house and passive solar gain. This means that the heating bills will be around £90 (assuming this heat is provided by a heat pump).

 

 

Should I build to a slightly lower standard than Passivhaus?

A problem with the Passivhaus standard is that it was developed in Germany, a part of the world that has relatively long, cold winters, where there is a great difference in the temperature indoors and outdoors.

 

The UK, on the other hand, rarely sees winter temperature fall below zero (around 5 days per year) and the typical average winter temperature is 7°C.

 

You’ll want to consider if you need to build to certified Passivhaus standards or incorporate some of the principals into your design and build a home close to the standards. Of course, you’ll save on build cost but it will cost more to heat even if this is just slightly.

 

You might be able to save up to 50% of the forecasted extra cost by relaxing the glazing and airtightness specifications. Even building to more relaxed standards, the heating demands should be around 25kWh/m2/yr and only add an extra of 8% to 10% to the build cost.

 

In terms of our 200m2 house, we now have an energy demand of 5,000kWh which means it’ll cost around £250 per year to heat. Quite a big saving!