paper igloo architecture and design

0.1796ach @ 50Pa!

March 13, 2015

After a few false starts due to the wind, today, on Friday the 13th, we finally had our intermediate airtightness test... and the result was a fantastic 0.1796 ach!!

We had planned to have this a few days ago, however when testing buildings that are as airtight as those built to passivhaus standards, any external winds can play havoc with the fan, so we had to postpone until a still enough day. 

We organised our airtightness test through Heat Recovery Scotland (who designed and are installing our ventilation system), and they in turn engaged Stuart King Architecture & Design to carry out the testing for us.

It is important to have an intermediate airtightness test when all (or as many as possible) of the service penetrations are made through the envelope, but not yet covered, so that if any leakage is detected it can more easily be remedied.

The requirements for a passivhaus differ to those for the Technical Standards: for passivhaus the n50 value is a measure of how many times the entire volume of (heated) air within the building is exchanged within 1 hour, so it is a volumetric measurement, and stated as air changes per hour (ach) @ 50Pa. The Technical Standards require the Q50 value to be taken (m3/m2.hr), which is the volume of air leakage per square metre of building envelope per hour, and as such is irrespective of a building's volume.

Also the methodology is different between the two tests: passivhaus requires that the test is undertaken during both pressurisation and de-pressurisation, whereas the Technical Standards only require one or the other (and it is usually pressurisation). The 50Pa stated is a 50Pa difference between the air pressure within the building and atmospheric pressure, which is why the external wind speed, amongst other things, plays a part. (Here is a link to Stuart King's website, where they explain the general airtightness testing procedures used.)

There is no direct relationship between the n50 value used in passivhaus certification, and the Q50 air permeability value used in the Technical Standards calculations, so a conversion factor cannot be applied to translate one figure into the other. For a specific building obviously both values can be calculated as all of the information regarding surface area and volume are known, and so the two can be related in some sense.

(In a typical detached dwelling for a 0.6ach rate the Q50 value might be around 1m3/m2.hr. It is typically quoted online that a passivhaus building is approximately 10x better in terms of air leakage than a building built to Building Regulations standards - as we understand it this stems from this typical relationship of 0.6ach to 1m3/m2.hr for a detached dwelling, and the fact that the Building Regulations in England have a backstop permeability rating of 10m3/m2.hr.)

The limiting standard for a passivhaus building is that the resultant air leakage at 50 Pascals pressure must be no greater than 0.6 air changes per hour (0.6 ac/h @50 Pa). 

In the Scottish Technical Standards the backstop value for permeability (or infiltration) for domestic properties is 10m3/m2.hr, however the rate used in the TER in a SAP calculation in Scotland is 7m3/m2.hr - the value used in the DER should be related to the specific ventilation strategy for the dwelling and as such varies from project to project. 

So, after all of that, here are some photos of the test process!


Dale lays out the adjustable frame and attaches it to the cover that will surround the fan...



The frame and cover fitted into our front door opening... We wanted to use the front door as the patio doors have fewer seals at the base so we wanted to test these properly.


Dale taking some preliminary readings to set the base-line for the tests...


Our building is relatively small (compared to say a commercial building for instance), and is so airtight that only one of the removable covers was required to be open to get the correct air pressure and fan speed relationship! (Each of the black circular discs can be removed as required to increase the airflow, as well as the various rings around the fan, enabling one fan to test a variety of building volumes.)


The ceiling membrane during the de-pressurisation tests - you can see the very slight bulge between the joists as the membrane is sucked inwards as air pressure within the building is decreased. This was the test that made us most nervous as our windows open inwards, so during this test the seals are pulled away from, rather than pushed towards, the window frames...


Dale switched the fan around for the pressurisation tests...


In this photo during the pressurisation tests the membrane is doing the opposite to before, and you can just see it becoming concave between the joists...


A final photo showing the same effect at the rooflight edges.

Finally, our intermediate test results were as follows:

De-pressurisation - 0.19 ach @ 50Pa; permeability 0.26m3/m2.hr
Pressurisation - 0.17 ach @ 50Pa; permeability 0.23m3/m2.hr

This gives us our final averaged results of 0.1796 (or 0.18) ach @ 50Pa, and an air permeability rating of 0.25m3/m2.hr, so we were delighted as both are better than the respective ratings we need to achieve for our passivhaus certification and our Building Warrant!