What can you learn from Passivhaus for your renovation project?

We are currently working on a Passivhaus design for a new-build house in our studio (Bradley Van Der Straeten Architects www.b-vds.co.uk). Passivhaus is seen as the top standard for designing energy efficient homes, and whilst it may not be achievable for most existing homes,  there is a lot to be learnt from the design principles and the approach to designing a Passivhaus. In this article we would like to share with you an initial guide to what Passivhaus is, what the benefits are and what lessons can be learnt for typical work to existing homes.

What is Passvihaus?

Passivhaus design is a highly energy-efficient building standard that originated in Germany in the 1990s. The goal of Passivhaus design is to create a comfortable and healthy living environment while minimizing the amount of energy needed to heat and cool the building.

Passivhaus buildings are designed to be extremely airtight and well-insulated, so that they can retain heat during the winter and stay cool during the summer. This means that they require very little energy to maintain a comfortable temperature year-round. Additionally, Passivhaus buildings are designed to take advantage of natural light and air flow, which further reduces the need for artificial lighting and heating or cooling systems.

What are the benefits of Passivhaus?

One of the key benefits of Passivhaus design is that it can significantly reduce the amount of energy needed to heat and cool a building. This not only saves money on utility bills, but it also helps to reduce the building's carbon footprint and contribute to a more sustainable future.

In addition to its energy-efficient design, Passivhaus buildings are also known for their high level of comfort and indoor air quality. Because Passivhaus buildings are so airtight, they are able to maintain a consistent temperature and prevent drafts, which can make living spaces feel more comfortable. Additionally, the use of natural light and ventilation helps to improve indoor air quality and reduce the need for artificial air fresheners or other chemical products.

What are the main principles of Passivhaus Design?

-          Insulation

A Passivhaus relies on high levels of insulation that assist in keeping heat within the property. The levels of insulation required a more than typically required for Building Regs, as a comparison a wall thickness of approximately 350mm can be enough to satisfy current Building Regulations, whereas a typical Passivhaus external wall can be up to 500mm.

Another key consideration with the external walls is making sure they have high levels of thermal mass, to prevent overheating in summer. (think about a typical loft extension, they are usually made of non-dense construction (timber / soft insulation) and can be very hot to live in in the summer). High thermal mass can be achieved using denser insulation (such as dense wood fibre) or solid walls using clay or dense concrete blocks.

-          Air Tightness

Air tightness when referred to homes literally means how much air can leak out of the house (how many holes are there). The more warm internal air that can escape from a house, the more heat needs to be generated internally to replace it. Typical construction in UK homes still relies on cavity wall construction, which generally is not very easy to make air tight. Typical old homes in the UK (Victorian/Georgian) were specifically designed to not be airtight, the external walls on these properties literally breath. For a Passivhaus project airtightness is usually achieved by wrapping the house internally with a membrane to prevent air leakage. The important thing is the layer has to be continuous, which takes very careful design and planning. A successful house that achieves certification will usually have a air leak gap equivalent in size to a pound coin. That is a seriously small margin of error!

-          Harnessing solar gains

Solar gain is also an important factor in Passivhaus design. Passivhaus buildings are designed to take advantage of natural light and heat, and solar gain is a key element in achieving this goal. By carefully designing the orientation, placement, and size of windows and doors, Passivhaus buildings can maximize solar gain, which can help to reduce the need for artificial heating and cooling systems. Typically a design will require larger windows on South facing orientations, to maximise solar gains, and smaller glazed areas on North facing facades to minimise heat loss.

Windows are usually one of the weakest thermal elements of a building façade so glazing has to be designed to also be designed to minimise heat loss. This usually requires triple glazed glass and thermally efficient frames (no super thin aluminium profiles!). Some windows have Passivhaus certification, meaning you know they will pass the standard.

-          Thermal Bridge Free Design

To achieve Passivhaus certification, buildings must be designed to be thermal bridge free, which means that they must be designed to minimize the transmission of heat through building elements. This can be achieved by carefully designing the building envelope, including the walls, floors, and roofs, to prevent heat loss and ensure that the building is as energy-efficient as possible.

In practice, the best ways to achieve thermal bridge free design is to keep things simple. As a rule of thumb if something internal (like a steel column) is covered by less than two thirds of the insulation, this could be considered a thermal bridge. With the project we are working on we have found that items such as external overhangs are challenging and have to be carefully coordinated with the structural design. Essentially a Passivhaus would be happy if it was a simple rectangular box, with the structure (steels) on the inside face of the external walls.

-          Mechanical Heat Recovery and Ventilation (MVHR)

Mechanical heat ventilation recovery (MVHR) is a type of ventilation system that is designed to improve the indoor air quality of a home.

MVHR systems work by drawing in fresh air from outside the home and filtering it to remove pollutants and contaminants. The fresh air is then supplied to the home, while the stale air inside the home is exhausted to the outside. The heat exchanger transfers heat from the stale air to the fresh air (ie it utilises heat generated in the home), which helps to improve the energy efficiency of the system.

The benefits of MVHR systems include improved indoor air quality, reduced energy bills, and improved comfort. For Passivhaus design, it removes the need for ventilating the home by opening windows, therefore making it very energy efficient. The unit can usually sit in the loft or in a purpose designed cupboard and then requires duct pipes linking to most rooms. This is something that needs to be carefully designed and planned alongside an MVHR supplier / installer

Can I obtain Passivhaus certification for work to existing property?

It is very difficult to achieve the results required for certification on an existing building. To achieve certification for your completed house, the project has to be signed off by a qualified certifier and has to meet all of the stringent criteria. One of the key principles of Passivhaus is considering the design of the building from the start and optimising the orientation and window sizes for solar gain. This is difficult to control with an existing building. The insulation and air tightness standards are so high, that achieving them can be prohibitive for an existing building.

As an alternative, the Passivhaus trust has a the EnerPHit Standard that is designed specifically for refurbishing existing homes. (we know, they need to work on a catchier name). Energy efficiency measures need to be appropriate and flexible when working with an existing house, and this standard addresses that and provides more scope for some compromise. The principles of air tightness, insulation, heat recovery are all the same but can be adapted to an existing situation.

Interestingly we have had a client on The Two Architects platform working with an EnerPHit designer to refurbish their existing Victorian Terrace property (we helped with the design and layout of the spaces) and they shared the full design report with us. The property had high levels of insulation and had an MVHR unit installed to recover heat from humid areas of the house and supply the warm air extracted from them to other parts of the house. It is worth noting that EnerPHit is still a very high standard and challenging to achieve. However, applying the principles without necessarily achieving the certification is still a good approach.

What principles can be applied to my home?

Some or all of them. With our residential refurbishment and extension projects to existing buildings we strongly advocate what we call a fabric first approach.

A fabric first approach to renovating and refurbishing a house to make it more energy efficient involves prioritizing the insulation, sealing, and weatherproofing of the building envelope (the physical structure that separates the interior and exterior environments of a building) as the first step in the process. This means that before focusing on upgrading or replacing heating and cooling systems, appliances, or lighting, attention is paid to ensuring that the house is well-insulated and sealed to prevent heat loss and drafts.

Some specific steps that might be taken as part of a fabric first approach to renovating a house include:

1 Adding insulation to the walls, ceiling, and floors to reduce heat loss through the building envelope.

2 Sealing gaps and cracks in the walls, floor, and ceiling to prevent drafts.

3 Weatherstripping doors and windows to reduce air leakage.

4 Installing energy-efficient windows to reduce heat loss through the windows.

By taking a fabric first approach to renovating and refurbishing a house, it is possible to significantly reduce the energy needed to heat and cool the building, which can lead to significant cost savings and a reduction in the building's carbon footprint. It also means your house can be ready to suitably support the future installation of an Air Source Heat Pump (ASHP), and possibly an MVHR system. The following two steps will also help;

5 Upgrade your heating distribution system (pipes, radiators, under floor heating) to be Air Source Heat Pump (ASHP) ready. For example if you currently have radiators, they are probably sized to suit the high heat output of a gas boiler (70c water) and will need to be larger for the lower heat output of an ASHP (typically 55c).

6 Plan space for a future MVHR system installation, including duct routes.

Conclusion

In conclusion, Passivhaus design is a highly energy-efficient and sustainable way of designing homes. By focusing on airtightness, insulation, natural light and ventilation, Passivhaus buildings are able to provide a comfortable and healthy living environment while minimizing their energy consumption and carbon footprint. The standards compared to current Building Regulations are very high, and prohibitive for most householders. However, following the principles and doing the best you can to improve the thermal performance of your property and potentially the heat distribution in your home can help to get you as close as possible to those very high standards.

If you were to take home one piece of advice from this article, remember that the air tightness of the fabric of your home is probably as important, if not more important than the thermal performance of your building envelope. This is one of the main things we have learnt with the Passivhaus calculations we have been doing on our studio project. This is something that most people overlook. Put simply, it is no good surrounding your house with loads of insulation if there is going to be gaps in that insulation letting in cool air that will undermine it.