Passivhaus: How to Build to This Low Energy Standard

There's building a house, then there's building a Passivhaus. It's quite an undertaking taking on a project that aims to meet Passivhaus standards, as it is a very thorough and exacting way to build, ensuring your build is airtight, well insulated and energy efficient. 

This, of course, comes at a cost, and will certainly add to your overall expenditure on your self build, but the benefits a Passivhaus can offer will potentially change how you think about living in a home forever. 

This guide examines just what it means to build to Passivhaus standards, while exploring the pros and cons, whether it's worth the extra expense and what eco house alternatives there are to this premium project. 

What are Passivhaus Buildings?

"This German-constructed standard has become the flag-bearer in the march to the best energy efficiency that can currently be achieved," explains energy-efficiency expert Tim Pullen. "Passivhaus homes need less than 30% of the energy for space heating than an equivalent house built to current Building regs standard."

Passivhaus is an entirely voluntary building performance standard that anyone setting out to build a low-energy home might be interested in. It has gained lots of attention in the UK and is based around the principle of reducing heating demand to a very low level through a fabric first design rather than relying on renewables.

The standards far exceed those of the current Building Regulations, and it becoming more popular because it lays out a calculated way of designing a low-energy home. Designers use a software package to predict the home’s future energy demand.

The Passivhaus concept started out life as a research project to find out why previous attempts at building low-energy, eco houses had failed to deliver on what they set out to achieve. The lessons learnt from this research have been distilled into a set of guidelines about how to get it right. 

The good news is that numerous studies and thousands of houses worldwide provide you with the evidence that they work.

Passivhaus only applies to new buildings. However, the Passivhaus Institute has a separate certification called EnerPHit which can be applied to retrofitting similar standards. 

How is Passivhaus Standard Achieved?

"Deciding on a Passiv approach early is key. Small changes early on, such as orientation of the house, mix of glazing and openings as well as the ‘form factor’ (ratio of internal floor area to the surface area of walls & roofs) is a big factor in how comfortably the design will meet the Passivhaus criteria," advises architect Craig Alexander from Oakwrights.

"The Passivhaus Planning Package, or PHPP, is a modelling software developed by the Passivhaus Institue and used when designing energy efficiency in homes and buildings to calculate it operational energy use and carbon emissions," adds Tara Gbolade, founder of Gbolade Design Studio. 

To design a Passivhaus using this complex spreadsheet all of the relevant construction details need to be input, including: 

• the insulation depths
• the window sizes and orientations
• the junction details.

This gives you a predicted space heating demand (expressed in kWh/m²/yr). You keep adjusting the design until your outcome meets the standard. 

"There is then more detailed design investment in the pre-construction phase, ensuring that all the details such as avoiding thermal bridges and maximises the air-tightness of the house can be achieved on site when building," concludes Craig Alexander. 

What are the Key Principles of Passivhaus Design?

Passivhaus can be built using almost any construction system. However, there are some universal features, including:

• Low heating demand: Space heating demand of less than 15kWh/m²/yr. This means a Passivhaus uses 90% less energy to heat than an average home, and 75% less than the average new build.
• High levels of insulation: Exterior walls to achieve a U-value of less than 0.15, and insulation is designed to minimise thermal bridging. 
• High-performance windows: U-values less than 0.8.
• Airtightness: Less than 0.6 air changes/hour at 50Pa (Pascal). This is 20 times more airtight than a standard build.
• Ventilation: Over 80% heat recovery from ventilation exhaust air. Mechanical ventilation with heat recovery (MVHR) provides constant fresh air and retain heat inside the house. 
• Optimised solar gain: The house retains heat from sun and occupants’ activities

How Many Passivhaus are there in the UK?

As of November 2020, there were over 1,300 dwellings across the UK which had achieved Passivhaus accreditation according to the Passivhaus Trust, and over 60,000 worldwide.

"And according to the Passivhaus Institute, there are at least that number 'in the pipeline'," says Tim Pullen. 

However, not all homes built to Passivhaus standards undergo the accreditation process. 

How Much Does it Cost to Build a Passivhaus?

"The general rule of thumb is that Passivhaus buildings cost between 5 and 10% more than a ‘conventional’ build," says architect Craig Alexander. However, alternative estimates can put this cost between 10 and 25% of the build costs, which is affected to a large extent by whether certification is wanted.

Passipediaconducted a detailed analysis of the build of a 149m2 Passivhaus built in Germany in 2015. It concluded that the extra cost was €14,000 and the net saving in running cost €585 per year, which includes the extra mortgage cost from the higher build cost.

In London, Elizabeth Sharp constructed her Passivhaus for £250,000 (pictured below). Richard Dudzicki (founder of RDA Architects and designer of the project) says: "Looking ahead, Elizabeth can expect to make considerable savings on running costs for the next 20-plus years [her estimated energy bills in 2018 were around £300]. The house has an energy rating of around one-tenth of what a normal house uses." 

Other studies give quite different figures, from a ‘profit’ of upwards of £100,000 over the life of the house to an extra investment of over 30% of the build cost.

Bear in mind that the extra cost is a factor of the size of the development (an estate of Passivhaus houses will cost less per house than a single home) and the quality of the house being built. A high quality house with a build cost of over £2,000/m2 will have a lower proportional extra cost than a lower quality house.

Do I Need to Certify a Passivhaus?

The Passivhaus Institute acts as a training and a certification centre. It charges around £1,500 to certify eco homes, though this is not compulsory.

The advantages of opting for a certified Passivhaus are to do with getting a measure of quality control. The design is checked over to see if it really meets Passivhaus standards, and the house is tested to make sure it follows the design. 

However, it’s not compulsory if building to 'Passivhaus standards' to complete these tests. Most of the houses built to Passivhaus standard are not certified by the Institute.

What are the Advantages of Living in a Passivhaus?

There are other key advantages that come with living in a Passivhaus build, as Passivhaus consultant Alex Clifford explains:

• The air in a Passivhaus in the UK is dryer than in a standard home due to the lower relative humidity of the pre-warmed air coming through the MVHR system. This eliminates the chance of mould growth and condensation on the inside of windows and has the added advantage of drying clothes very quickly when hung up inside.
• The air quality within a Passivhaus is cleaner and healthier, with constant fresh, filtered air pre-warmed in a heat exchanger using damp warm extract air from kitchens and bathrooms.
• Lower energy bills and lower carbon emissions due to very low heating and cooling requirements.
• Warm, snug rooms at a constant temperature without any draughts or cold spots in winter, and cooler rooms in summer. Floor space right up to full height windows can be used year round as there is no chill off the triple glazing.
• Low maintenance costs as there is no need for complicated heating or cooling with hi-tech controls.
• Peace and quiet when the windows are shut. Triple glazed windows and thick insulation are excellent at keeping the noise out.

What are the Drawbacks of Passivhauses?

Passivhaus is an idea that works better for some than others. Before building, there are certain things to know about living in a Passivhaus. You need to learn how to ‘drive’ a a home designed this way, for example, knowing:

• when to open windows to help prevent overheating
• when to boost the ventilation system (if required)
• when to change filters

You do have start thinking of your home as a machine for living in. You can’t just start knocking it about, adding new bits here and there.

This may in fact be one of the major criticisms of Passivhaus — it’s not readily adaptable. Even things like running a cable through for a satellite dish can compromise the building's fabric. So building a Passivhaus is a lot of trouble to go to if you don’t respect its integrity.

However, the big plus for Passiv­haus is that it works, and has been shown to work over a large number of buildings.

Ventilation in a Passivhaus: Can You Open Windows? 

There's a bit of a fallacy that you shouldn't open windows in a Passivhaus build, but think of it more as having better control over how your home is ventilated. 

"During the heating season, your ventilation would be predominantly from an MVHR system that has pre-warmed fresh air from outside, but in the summer this may well be mainly natural ventilation from opening windows if that is what the occupants prefer," suggest Passivhaus consultant Alex Clifford of Clifford Design. 

"The impact on heating cost from an open window in winter is minimal and it can be controlled, unlike leaks in a building fabric which are affected by the wind and weather."

Windows and doors may need to be opened in summer to reduce the risk of overheating, especially on consecutively hot days, but in winter (and year round really), it's less likely that people living in Passivhaus builds will feel the need to open windows, as the air is fresher and the temperature better controlled in the home. 

How Do You Heat a Passivhaus? 

"The definition of Passivhaus is 'a building for which thermal comfort can be achieved solely by post-heating or post-cooling of the fresh air mass', which means very little heating is required, for example just warming the incoming air, rather than requiring a full traditional heating system," explains Oakwrights' Craig Alexander.

"This is because the building is designed a) to lose very little heat and b) make the most of gains from the sun, and internal occupation, such as heat generated from cooking, washing and day-to-day activities. 

"In practice, in the UK, most domestic Passivhaus projects incorporate some heating elements such as radiators (for example you would likely still want heated towel radiators in bathrooms).

"As a reference, I was involved in the design of a 200m2 detached Passivhaus which had a heating demand equivalent to a single 2kw electric fan heater, which in a conventional house (certainly an example of an older UK housing stock) would struggle to keep a single room warm!"

How Much Will a Passivhaus Cost to Heat?

The Passivhaus standard seeks to reduce the space heating requirements to such a low level that you no longer require a conventional heating system.

The level is set at 15kWh/m2/yr, and it’s the key Passivhaus target. This means that if you build a 160m² house to Passivhaus standards, you would need just 2,400kWh of energy throughout the year. That’s about a tenth of what a typical British home would use, and about a third of what many so-called eco-homes would consume.

But it’s not no heating at all — that would be expressed as 0kWh/m2/yr.

There isn’t a set method for providing space heating for a Passivhaus. The most popular seems to be adding a small heating element to the ventilation system, turning it into a warm-air heating system. These units are generally rated at no more than 3kW, and they only kick in when the outside temperatures are close to zero. 

You also have to provide for domestic hot water, so some form of boiler is useful.

As another example, a 200m2 house built to Building Regulations standard will need 11,000kWh of energy for space heating. Assuming the house has a gas boiler, that will cost around £600 per year. 

Building Regulations require consideration also be given to overheating, so the cooling demand could be the same as for a Passivhaus. It could also use the same LED lights and high-efficiency appliances.

The 200m2 Passivhaus, with a demand of only 15kWh/m2/yr, will only need 3,000kWh for space heating and it is safe to assume that at least half of that will be provided by activity in the house and passive solar gain. This means that the heating bills could be around £90 (assuming this heat is provided by a heat pump).

Is Building to Passivhaus Standards Worth it in the UK? 

The idea of thermally efficient, fabric-first construction is gaining in popularity, due in no small part to the influence of the Passivhaus standard.

It can be argued that the rarity of weather extremes in the UK does not warrant all that investment; and the ‘Passivhaus standard’ items like windows and doors are priced at a level attractive only to the manufacturer. Plus, there are savings to be made in tweaking the standard to suit the UK climate. This is where fabric first starts.

The purist may argue that driving the energy demand of the house to the lowest level is good for the homeowner and for the planet. But this rather steps over the tight focus Passivhaus has on space heating.

The pragmatist might argue that ours is a warm, wet maritime climate, that renewable energy is now cheap, and that not troubling the power station more than we have to is also good for both.

You’ll need to decide whether to build a certified Passivhaus home, or perhaps save on the build cost and build close to Passivhaus level.

Backing off on glazing and airtightness can make a big difference, with the potential to save up to 50% of the investment. With thought and care, it is possible to achieve a space heating demand of 25kWh/m2/yr for an extra cost of 8% to 10%.

The Low Energy Alternatives to Building to Passivhaus Standards

With any low energy build it is important to set a target of what  'low energy' or 'green' actually is as this gives a target for architects, builders and client to measure against. 

"This may be to achieve an EPC 'A' rating or a % improvement over building regs," explains Passivhaus consultant Alex Clifford. 

"The AECB has introduced a new standard that uses Passivhaus principles and design modelling but has less stringent targets than Passivhaus that some clients or contractors may have thought too onerous. It does not require third-party certification and instead can be certified by a Passive House consultant reducing the cost as compared to Passivhaus."

A zero-carbon house has much less exacting standards than the Passivhaus standard, typically 46kWh/m²/yr for detached houses. That’s still a demanding target, but not nearly as difficult to meet as the Passivhaus standard of 15kWh/m²/yr. The ‘zero carbon’ bit is then made up of a mixture of on-site renewables and carbon offsetting.

In contrast, while renewables can be added a Passivhaus home, it’s all about saving energy, not generating it.