31 Jul 2014

The Airtightness Test

We set out to achieve a low energy house with the principles of "super-insulate, build tight and ventilate right".  This means that the building needs to be pretty airtight. This ensures that the air that you heat can't just escape through gaps in the building, it is directed through the MVHR to heat/cool the incoming air - minimising additional energy input.  The other thing to worry about when air leaks is that the internal air will be warm and moist, when this moves through a gap in the fabric it can cause interstitial condensation - damp and mold!  No good!

To conduct the test the building is sealed and a calibrated fan is mounted in a doorway.  It sucks air out of the building to depressurise it.  Then they can test the pressure differences between outside and inside and watch what happens as they test at different pressures.  A number of different data points are collected and graphed, creating a final averaged figure.  We have tested twice now and will have one final test when the building is finished.  So the first two tests have been used as aides to discover where in the building the airtightness needs to be improved.  As the inside is depressurised it pulls in air from outside (actually air from outside pushes in and tries to fill the space) and you feel with the back of your hand where air is moving through the building envelope. 

Searching for leaks

Two figures are calculated in the test; air permeability and air changes per hour.  The two figures are very similar and to be honest i'm yet to really understand the difference! (Dimitri says "One is flux and the other is just a measurement that builders and architects can understand"!) However it is the Air Changes per hour that we are really interested in.  Air permeability is measured in cubic metes per hour per square metre of the dwelling envelope area when the building is subjected to a differential pressure of 50 Pascals (m3/(h.m2)@50Pa). Air changes per hour is the number of times all of the air in the house is replaced in an hour at the same pressure difference as above(without MVHR obviously).

Building Regulations state a maximum figure of 10 air changes per hour, although 3 is the minimum standard for AECB (Association Environmentally Conscious Buildings) Silver Standard for environmental building.  Passive Houses have to achieve 0.6. Getting a score of 10 down to 5 is relatively easy, however from 1 to 0.6 is very challenging...

The first airtightness test was conducted once the windows and roof had been installed.  Where the concrete blockwork meets windows and doors and the roof, we have used a lot of very expensive airtightness tapes. We've been told these tapes will last for 50yrs, but to be honest no one has had them in use for that long so nobody really knows as lab conditions are never quite the same thing as reality.

Paul Jennings, the airtightness guru, arrived with all his gear and got to work. We achieved a result of 4 in the first test, we were all disappointed! Leaks were searched for around junctions of the walls and windows and roof and sealed with more tape.  This got the figure down to 3.6.  Not the start we were hoping for.  

Unusually for a passive-style house, we have used traditional concrete blockwork.  Blockwork can vary a lot in air permeability and the more environmentally friendly blocks composed of fuel ash can make the blocks even more porous and very leaky.  So the blockwork could not be relied upon to give us the airtightness.  The redeeming feature of blockwork is that it is a great thermal store, absorbing heat and very slowly releasing it.  It will take a lot of energy to change the temperature of the mass.  At this point in construction we had used blackjack, a paintable bitumen, as a seal when perforating the blocks, so we guess that this had not been as effective as we had hoped either.  

Once our internal render had been completed we conducted our 2nd test.  This time the first score was 0.9 - hooray!  We were under 1, which was the figure we were aiming for when we first began this journey.  A huge accomplishment.  We always said that the aim was to do the best we could without throwing huge amounts of money at it. The costs can increase exponentially as you get down to the final 0.1s.  Just goes to show what a difference a layer of render can do.

But Andrew and Charlie were really hoping they could make it to 0.6. It's boys- they see a target and they want to hit it, goal driven! With more rushing around with smoke pens and backs of hands against junctions they found some more air movement and used an airtightness sealant and mastic to fill the voids.  Apparently this stuff is the business and doesn't fully harden and leak at a later date.  A lot of our issues were around the electrical switch boxes and the wire perforations through the render.

There was a lot of nervous tension as we waited for the final series of tests to be performed.

We managed to get a result of 0.72!  I think that is amazingly good! The photo below shows a result if 0.62, however each result has to be plotted on a graph and it is the average which determines the final score.

I am so delighted.  0.6 would be great but to be honest the difference of 0.12 is nothing that we will notice, our house is going to be super low energy and that is what we wanted.  It remains to be seen whether we'll improve the score by the 3rd test.  Andrew is planning to use some of the mastic inside the switch boxes in an attempt to improve the score.  We've also completed the plaster layer now which may have an impact.  However it has been known for scores to increase in the final test, as during the final finish more perforations are made in the airtight skin when you hang shelves and so on.

(I have to confess that I failed to write down the scores at the time, believing that they would be indelibly etched into my memory, however that has turned out not to be so!  Paul may correct me if I've got any of these numbers wrong.)

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