The waiting is over, the experts from Leeds Metropolitan Univeristy have tested Denby Dale for airtightness and this is what they found…

We have just had the results of the initial blower door airtightness test on the Denby Dale Passivhaus and I feel like you do at school when the exams are over.

Dr David Johnston and Dominic Miles-Shenton from Leeds Metropolitan Universitys buildings and sustainability group kindly came and undertook the test for us last Thursday.

As we’ve mentioned before, the airtightness barrier in cavity wall construction is the wet plaster, so we needed to test the build at this stage before we got any further to check whether we needed to do any remedial work.

The airtightness of the building was measured using the fan pressurisation technique. During the test, all of the openings in the house were sealed off, including the mechanical ventilation and heat recovery ducting, and any services to the outside which were still awaiting the second-fix.

How does the blower door test work?

The technique involves sealing a portable variable speed fan into an external doorway, using an adjustable door frame and panel. The fan is then used to pressurise and depressurise the building. The airflow rate that is required to maintain a number of particular pressure differences across the building envelope is measured and recorded.

Blower door airtightness test

The leakier the building, the greater the air flow required to maintain a given pressure differential.

It was quite nerve-wracking, with the building team, client and I following every stage with trepidation.

We knew it was going well when the team kept changing the aperture of the fan to smaller and smaller sizes – starting off with a largeish B size aperture (used for a normal house) and going down to a tiny D size, which they’d rarely used before.

The results

Then the testers started smiling and it became obvious that we’d achieved something. Their findings were as follows:

• UK measurement method - 0.38 ach @50Pa or 0.41m3/(hm2) @ 50Pa, (Building Regulations require a maximum allowable backstop air permeability of 10.0m3/(h.m2)@ 50Pa). The calculations were undertaken in accordance with ATTMA Technical Standard 1 and are based upon an internal envelope area of 277.1m2 and an internal volume of 303.45m3.

• Euronorm and Passivhaus method - 0.41 ach @ 50Pa (Passivhaus requirements are 0.6 ach @ 50Pa). For Passivhaus certification (and Euronorm methodology), the volume of all internal partitions, stairs, floor void etc are discounted in the airtightness calculations, resulting in a lower overall internal volume (in this case internal volume calculated as 277m3).

The blower door test was followed by a smoke test, where the Leeds Met team went around vulnerable points such as window openings, ceilings, anywhere where there’s a break or a change and where you might get leakage.

I hope that the results will also help blow a myth that traditional cavity wall construction can’t be made airtight for Passivhaus needs


We found three weak points, which are, luckily, very easy to rectify.

A bit of Pro Clima Tescon Profile airtightness tape had come loose, due to condensation from the plastering, at the bottom of one of the windows. It occurred in a spot where there is a groove for the window board running along the bottom rail of the window, where it had been difficult for us to get continuity of taping.

We can rectify this by replacing the tape and supplementing with some Orcon adhesive, as an extra ‘belt and braces’ measure.

Two external doors also showed some air leakage – where the seals weren’t fully engaging. This has already been remedied by adjusting the hinges to pull the leaves in harder against the seals – but we won’t know how much better that’s going to make the performance until we get the next test.

We are completely delighted with the results – it is a real achievement for the whole team particularly for the building team on site who were taken out for a meal the next day by clients Geoff and Kate Tunstall to congratulate them all.

Passivhaus can be reached with cavity wall construction

I hope that the results will also help blow a myth that traditional cavity wall construction can’t be made airtight for Passivhaus needs. The results also prove that British builders have the skills to achieve the necessary levels of airtightness and that you don’t have to use expensive materials or methods to build a Passivhaus.

Good simple robust design, knowledge and care in application are all that are needed to achieve Passivhaus levels. I hope we have proven that you can still build in the local vernacular and reach the performance levels that you need for 2050s goals.

This isn’t to say that everyone’s got to build Passivhaus buildings using cavity wall construction, but that it does offer a method of having thermal mass within the building - offering an added bonus for performance because it gives you the ‘cave effect’ of temperature stability.

It also offers the advantage of familiarity in that it is a construction technique that British builders are used to with materials that are easily accessible and familiar. Timber frame might be easier to get airtight but you tend to lose the advantages of ‘thermal mass’ (See also: Why we chose cavity walls).

I’ve already had a lot of nice feedback through Twitter including from Dr Wolfgang Feist – originator of the Passivhaus methodology and founder of the Passivhaus Institut – who has been supportive of the project from the outset. He has always had faith that cavity wall construction would work with the Passivhaus methodology and we are delighted to have proved him right.

Next stages

Leeds Metropolitan University are kindly going to do a further interim test once we’ve done our final apertures. There are three further penetrations needed through the wall – for the boiler flue, and the pressure relief from the boiler and mains pressure hot water – although we’re very confident that we can cope with that, without reducing airtightness significantly.

We also still have to close the cavities externally around window and door reveals helping to restrict further air movement which should help us get an even better final airtightness result.

We’re currently making final preparations for a training day and site visit for building professionals on the Denby Dale Passivhaus here in Huddersfield in early February. A few places are still available, so do contact me if you’re interested in having a closer look the project.