Part 6- How to build an airtight structure

22 June 2016 Brent Ackerman, Delta Project Management
An airtight building is essential when creating a Passivhaus home. Here we eplain why attention to detail and careful planning are required to achieve this.

Build Tight

The Passivhaus standard requires very low air leakage rates which must be demonstrated by means of an airtightness test, carried out using a ‘blower door’.

The air change rate must be less than or equal to 0.60ac/h (air changes per hour), under test conditions, to meet Passivhaus levels.

It is worth noting that in practice an airtightness level of 0.60ac/h is roughly equivalent to having a hole in the envelope area of the building less than the size of a five-pence piece for every 5m2 of building envelope.

In comparison, a building that merely achieves the limiting figure for airtightness to comply with the current version of Part L of the Building Regulations will have an equivalent hole the size of a 20-pence piece for every 1m2 of envelope.

How is airtightness measured?

Airtest on Potton Passivhaus show house

In typical fashion, the methodology for testing airtightness for Passivhaus buildings differs from that used in UK Building Regulations — although they do both use a ‘blower door’ to carry out the actual physical test.

The Passivhaus standard measures the air changes per hour at a pressure of 50 pascals or @50Pa (the n50 measurement).

In other words, this is the number of times the volume of air within the building is changed in an hour under test conditions; it’s a volumetric measure.

The standard is a measure of air infiltration, and hence the heating energy cost of the building. It considers the volume of air that needs to be heated and excludes internal walls and floors.

The UK standard measures air permeability in m3/hr/m2@50Pa (the q50 measurement) — the air leakage per m2 of building envelope. The UK standard defines the building envelope as everything within the air barrier line.

In practice, this could be anywhere within the building envelope and is a measure of building envelope airtightness.

The current backstop airtightness figure for UK Building Regulations is 10m³/hr/m²@50Pa and reportedly the average figure of all UK homes (in other words, our housing stock) is around 12m3/hr/m2@50Pa.

While clearly technically incorrect, it is often convenient to consider the air change rate and air permeability for dwellings as roughly similar and, in this case, it will help me with the next explanation.

Building a Passivhaus Show Home

A complete step by step guide, we take you through the story of building our Passivhaus show home. From the design process, constructing the superstructure, achieving airtightness right through to the finishing touches.

Download the guide here.

Building a Passivhaus Download

Airtightness Case Study: Potton’s Passivhaus Showhome

Image_Show Home_Elsworth_6
Our show home is the first Passivhaus that we have constructed and we are learning all the time.

However, we have constructed a very large number of homes that can reasonably be considered as low energy and have needed to be particularly airtight.

For a great number of these we have had the responsibility of delivering the airtight envelope of the home at an early stage of the construction.

We know from our experience that whenever the airtightness target is less than 3m3/hr/m2@50Pa for an early stage test (ie as soon as the airtight envelope is constructed) it’s of great benefit. I would go one step further and argue that in any dwelling with a target of less than 1.5m3/hr/m2@50Pa, an early stage test is essential.

Clearly then, when working to the Passivhaus standard, where the target is 0.6ac/h, an early stage test is an absolute must.

We know from our experience that, using the Kingspan TEK® Building System, and by carefully sealing and taping external wall junctions we can readily – I hesitate to use the word easily – achieve airtightness figures of between 1.5-3.0m3/hr/m2@50Pa.

We know also that figures of 1.0-1.5m3/hr/m2@50Pa require considerable care and engagement from the whole construction team.

We have some experience of properties where the requirement is 1.0m3/hr/m2@50Pa and know that this is hard to achieve, but equally far from impossible.

Test Results

So, what about our show home? Our first test yielded a result of 0.76ac/h — irritatingly short of our Passivhaus requirement of 0.60ac/h, but perhaps better than one might reasonably expect straight out of the box.

We spent a short period of time with smoke sticks and ‘the skin on the backs of our hands’ test finding some of the more obvious leakage points, including some schoolboy errors, and addressing them.

We then carried out a second test which gave us a result of precisely 0.60ac/h.

Finally, we carried out a smoke test, which involved filling the entire house with ‘smoke’ from a machine – similar to the type of thing that one might expect to find in a nightclub – and then gently pressurising the house using the ‘blower door’.

This test, while not perfect, can help reveal leaks in the building envelope that had previously remained undetected as the smoke finds its way out of the structure.

The smoke test was helpful and identified a number of leakage points, mostly higher up, where some attention was required.

By the time you read this we will have conducted our first test in public and we will know how much we have managed to improve upon our target of 0.60ac/h.

Ventilating and Heating a Passivhaus

There is always a temptation when designing and constructing a very high-performance property such as a Passivhaus, to fill it full of the latest high-tech, complex technologies and this is particularly so when looking at the systems that heat and service our homes.

From day one we were determined to resist this temptation and to construct a home that is both energy efficient (concentrating efforts on the building envelope), but also very simple to live in.

I think there is a real danger that the population at large receive a message that Passivhaus buildings are complicated to live in — this should simply not be the case.

Potton Elsworth Passivhaus Show House

The Elsworth is open to the public

Come and expereience the Elsworth show house for yourself. Built using the Kingspan TEK Building System, this is the UK's first show home built to Passivhaus standard.

Take me to the Elsworth Show House


Potton Head Office

Eltisley Road

Great Gransden


SG19 3AR

441767 676400

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