Design for energy efficiency: airtightness

As we become increasingly conscious of our impact on the planet and our local environment, it’s essential that our built environment is constructed for minimal energy consumption. Previously we touched on Passivhaus, a higher quality of design and construction with a clearly defined standard for energy use in all building typologies. In this post we’ll explore airtightness, a critical requirement of achieving Passivhaus certification and why an airtight home has significant implications for a building’s occupants and the environment.

What is airtight construction?

In a nutshell, airtight construction means that the building envelope is draught-free. This is achieved by implementing an airtightness barrier which must be continuous and thoroughly joined across all junctions of the building envelope to form a complete loop. This continuous airtightness loop comprises a combination of interconnected materials and flexible sealed joints to ensure the building envelope has no unplanned gaps that would otherwise allow air to leak in or out of a building.

Why should we consider airtightness in our buildings?

The majority of the UK’s housing stock loses a lot of heat; approximately 20-30% of the heat generated to warm our homes leaks through the building fabric. This can be down to poor design, poor quality of building or mistakes during construction. Whatever the reason may have been in the past, we should strive to develop our built environment to be as energy efficient as possible. By building airtight, ventilation becomes an important consideration for the buildings’ occupants. Fresh air is brought into the building mechanically whilst stale air is exhausted outside. A heat exchanger warms up the incoming fresh air from the exhausted stale air without the two streams ever mixing. Mechanical ventilation is a very important consideration for airtight design and is a topic we will explore in a future post.

The benefits of airtight design

The airtightness of a building has important implications for our planet and the building’s inhabitants. An airtight building results insignificantly reduced energy demand & energy bills, reduced CO2 emissions, an increased level of comfort, and an overall healthier living environment. By achieving good levels of airtightness, you also achieve structural protection from mould growth, moisture, damp, rot, and condensation which in turn reduces maintenance required over a buildings’ lifetime.

How do we achieve airtight construction?

A clear airtight strategy is required to achieve a draught-free building envelope. An architect can help plan for airtightness during early stages of the design process by planning where all air penetrations should occur and detailing how the joints between materials are sealed. Early planning is critical to ensure careful consideration has been given for the various junctions that occur throughout a building. The airtightness of a building will be checked three times during the construction process to ensure the desired performance target is on track to be achieved. The first test is taken when the building has been made air tight. The next test is taken when all services have been installed. The final test is on completion. During these tests, any defects can be identified and resolved before construction is completed.

Project Focus

In the design of Wellington Lane, smartply sheathing was used to create the primary airtightness barrier. Airtightness has been engineered into this sheathing panel and the special surfacing provides an integrated vapour barrier with consistently high vapour resistance over the entire surface. The special coating also provides a smooth durable surface for excellent bonding of airtight tape at panel joints allowing the airtightness barrier to be continuous and joined up throughout the building. A photo taken during construction of Wellington Lane shows the airtightness barrier is located on the interior side of the wall. This allows the insulation and building envelope to breathe and wick any moisture away whilst keeping the interior spaces comfortable and airtight.

Sustainable Design - Take the Passivhaus approach.

What is Passivhaus?

The Passivhaus Standard was developed in Germany in the early 1990s and is now a leading assessment method and design methodology for energy efficient buildings. It is one of the best ways of proving that a project meets very high sustainability standards.

The standard focuses on reducing the energy consumption (heat loss) of a building by having high levels of insulation, high quality windows and doors, together with an ‘air-tight’ construction which uses mechanical ventilation and heat recovery. All of these things help to ensure that heat generated within the building stays inside for as long as possible.

Advantages of Passivhaus

Beyond energy consumption, the standard has many other advantages for the for the user of a passivhaus building. Occupants record higher levels of comfort due to the prevention of drafts and more stable internal temperatures and ventilation rates. High levels of insulation means that there is much less risk of condensation forming on internal surfaces which can lead to mould growth and respiratory problems.

How to get Certified?

In order to be certified as a Passivhaus, a building must meet all of the requirements established by the Passivhaus Trust. This involves modelling the proposed design in a software package (known as PPHP) to ensure that it meets the necessary standards for heat loss. Further documentation, including construction drawings and photos during construction are then issued to a registered Certifier for completion.

Project Focus

Wellington Lane in was an eco house project in Bristol which adopted Passivhaus principles. I led the project from concept to completion, gaining planning consent and producing the necessary technical drawings for the project. I remained in close contact with the contractors Earthwise Construction throughout the construction process in order to monitor air-tightness, thermal bridges and respond to technical queries as they arose.

In this case the client chose not to pursue the full certification for the project, but the principles and methods were adopted in full. 4 years later, the clients remain very happy with how the design has reduced their energy costs and keep the house warm and cosy throughout the year.

Material Choices - Which insulation is right for you?

It is the kind of question which doesn’t normally get asked until planning permission is out of the way and the client is undertaking the detailed design. But it is important to consider the different insulation options early on in the project, as it could have a big impact upon the wall thicknesses, appearance, as well as the environmental targets for the scheme.

What are the options?

There are a range of products on the market which broadly fit into three categories: Oil based polymers, mineral and natural. Oil based products are normally rigid boards made from polystyrene (EPS or XPS) or polyurethane from suppliers such as Celotex and Kingspan. Mineral products include rock and glass wool. These two catagories have dominated the construction market for many years but are not necessarily the most effective. Recently there has been a resurgence in the use of natural materials available including wood-wool, woodfibre, sheepswool, cork and hemp.

Insulation requirements

In England, the current building regulations give the performance requirements for insulation in buildings. The unit of measurement given is called the U-value (W/m2K) which is the amount of heat lost through a meter squared area of the walls, floors etc, per 1 degree temperature difference between inside and outside. It sounds complicated! But remember the lower the U-value, the better the performance of the building element. Current regulations state that the maximum U-value for a wall should be 0.28 for an extension and 0.16 for new builds.

Clients, especially self-builders may decide to go for insulation levels which are better than the governments minimum targets, so as to improve the energy efficiency of the building. This will provide long term savings in terms of energy costs, bring better comfort to the space. In my Wellington Lane project we adopted a Passivhaus approach and achieved a U-value of 0.10 for the walls, with a build-up of 300mm mineral wool between timber I-joists clad with 100mm woodfibre external wall insulation. This made the walls very thick but the house super toasty! I will look closer at this project and the materials used in a future post.

Passivhaus project. showing walls filled with insulation before they are overclad with wood fibre insulation

Passivhaus project. showing walls overclad with wood fibre insulation, before being rendered with lime

Which insulation performs best?

Rigid synthetic board products such as Celotex are, as a rule of thumb, twice as good as mineral wool and natural materials such as sheep’s wool and wood-fibre. But remember that is not the full story. The performance can be bumped up with more insulation so it is worth considering the overall costs of the product, installation and other factors which are important to you.

Which is cheapest?

It used to be that rigid board was the most economical option even though it has always been the most expensive, because much less of the material is required to hit the required U-values. But over the last few years the price has gone up considerably, and now there is much more of a level playing field compared with the eco products which are far better for the environment.

Which is most environmentally friendly?

Whilst synthetic rigid-boards are an effective insulator they come with great environmental cost to the planet. They are manufactured from petrochemicals which cause resource depletion and pollution risks from oil and plastics production, and have a high embodied energy. Natural products such as wood-fibre can lock carbon carbon inside the material reducing the impact on global warming and generally have a lower embodied energy.

Other positives for natural insulation

If you are undertaking renovation work to historic buildings it is important to use a breathable material, such as wood-fibre insulation, which will not lock moisture within the structural fabric of the building. Natural products such as sheep’s wool have proven to reduce problems with asthma, and help to create a healthy internal environment.