Part 6: How to Build an Airtight House

22 June 2016 Brent Ackerman, Delta Project Management

How to Build Airtight

Close-up image of a SIP under construction
Airtight building regulations are essential when creating a Passivhaus home. Below, Brent Ackerman, from Delta Project Management, explains why attention to detail and careful planning is required to achieve a smooth and compliant build.

The Passivhaus standard requires very low air leakage rates, which must be demonstrated with the help of a “blower door” — a machine used to measure the airtightness of buildings. To meet Passivhaus levels, the air change rate should be less than or equal to 0.60ac/h (air changes per hour) under test conditions.

It’s 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 of 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 I of the Building Regulations will have an equivalent hole the size of a 20-pence piece for every 1m2 of building envelope.

How Is the Airtightness of a Building Measured?

Airtest on Potton Passivhaus show house
In typical fashion, the technique for testing airtightness on Passivhaus buildings differs from that used in UK Building Regulations, although they do both use a “blower door” to carry out the 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 volumetric measure is the number of times the volume of air within the building is changed in an hour under test conditions. The standard is a measure of air infiltration, 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 (how much can pass through) in m3/hr/m2@50Pa (the q50 measurement) — the air leakage per m2 of a 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 the average figure of all UK homes is reportedly around 12m3/hr/m2@50Pa. While 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

Airtight Building Solutions Case Study: Potton’s Passivhaus Showhome

Image_Show Home_Elsworth_6
The first Passivhaus building that we constructed was our Elsworth Show Home. However, we have also constructed a large number of homes that needed to be especially airtight in order to be considered “low energy”.

For a great number of these homes, we have had the responsibility of delivering the airtight envelope of the home at an early stage of the construction. We know from experience that whenever the airtightness target is less than 3m3/hr/m2@50Pa for an early stage test (i.e. 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.

Therefore, when working to the Passivhaus standard where the target is 0.6ac/h, an early stage test is an absolute must. From experience, we know that by using the Kingspan TEK® Building System and 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 also know 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 working on properties where the requirement is 1.0m3/hr/m2@50Pa, so we know that this is hard to achieve, but equally, it is far from impossible.

Test Results

So what about our Elsworth Show Home? Our first test yielded a result of 0.76ac/h, which is irritatingly short of our Passivhaus requirement of 0.60ac/h. However, this is perhaps better than what you would 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, which included making 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 what you would find in a nightclub) and then gently pressurising the house using the “blower door”. While not perfect, this test 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.

We have since conducted our first test in public. The result was 0.50ach@50 Pa — a pleasing figure given the complex geometry of our new show home (the more complex the design, the more challenging achieving airtightness is).

Ventilating and Heating a Passivhaus

There is always a temptation when designing and constructing a high-performance property (such as a Passivhaus) to fill it with the latest and most complex technologies, which is especially true when looking at the systems that heat and service our homes.

From day one, we were determined to resist this temptation and construct a home that is both energy efficient (concentrating efforts on the building envelope) and simple to live in. I think there is a real danger that the general population thinks that Passivhaus buildings are complicated to live in, which is definitely not the case.

Read Part 7: Getting the Details Of Your Passivhaus Design Right

Potton Elsworth Passivhaus Show House

The Elsworth is open to the public

We have five self-build homes for you to browse, but the Elsworth is the UK's first permanent show home built to Passivhaus standard, created using our Kingspan TEK Building System.

Come and experience the Elsworth show house for yourself.

Editor’s note: This blog post was updated in June 2019 for accuracy.

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