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Draughty house? What an airtightness test can tell you

Posted by Gabby Mallett on 17 April 2015 at 12:30 pm

What does an airtightness test involve?

I recently had an air tightness test done on my home. The goal was to get a good idea of where my house may be leaking air - especially when the heating is on.

The conventional blower door test was used.  This requires a large fan to be placed in the front door with a flexible panel which covers the rest of the opening.  A young man called Adam came to do this for me.

Adam needed to calculate the size of the house and the volume of air in the house to enable him to calculate the air leakage.

Firstly, he measured the house.  He drew a plan and began measuring the size of the rooms and height of the ceiling. 

My house isn’t that easy to measure.  Half of the garage has been converted into a study. The kitchen has been extended and the new bit has a higher ceiling than the old.  Does the porch form part of the envelope of the house?  It is unheated and not original, but it was here when we moved in so we don’t know anything about its insulation... 

Adam then set up the temporary blower 'door' within the front door frame (see photo) and proceeded to set the fan to blow air out of the house.  As the test was going we could already go around to windows and doors and feel the cold air being sucked in, to replace that being blown out by the fan.  All I needed to do was put my hand up next to the windows and doors. 

The amount of air being sucked in was really amazing.  Those new French doors were pretty poor.  I could certainly see, or more accurately feel, where we need some additional or better draught excluders.

The house very quickly began to feel pretty cold, but that’s to be expected as it was only 3 degrees outside.  It might have been a perfect day for doing some thermal imaging too?

Then Adam taped over all the air outlets in the house (air bricks, extractor fans etc).  He even taped up the cooker extractor fan because we were interested in the quality of the fabric of the house, rather than how much air can move through the air bricks. 

In my case the process was completed four times, with the house pressurised and depressurised each time with vents and extractor fans unsealed and then sealed.  This enabled us to measure how much is due to planned ventilation and how much is genuine unplanned air leakage (and therefore unplanned potential heat loss).

And here are the results, they are around about what one would expect to find.  I got 8.35m3/m2/hr @50 Pa (unsealed) or 7.9m3/m2/hr @50 Pa (sealed).*  The house was built in 1994, so that’s not too bad. 

For reference the worst permissible standard for new build under building regulations (ADL1A) is 10m3/m2/hr @50 Pa. Most new builds today are in the range 4-7. According to BRE research the most energy efficient homes (Passivhaus) are now in the range 0.2 – 1.0, but during the first half of the 20th century a mean air permeability figure between 14 - 16 was more typical.
What have I learned?  Well I clearly need to do something about the draught proofing on the back doors and something about one of the windows at the front, but other than that there’s not much else I can do.  I have already added cavity wall insulation (which would have made a huge difference) and topped up what insulation there was in the loft. I also had a wood burning stove put in to an open chimney (again this would have made a measureable difference). 

I know you are wondering if this would be a good idea for you.  It’s not a cheap thing to do, these tests cost several hundred pounds as a one-off each and are usually only used in new build properties or following major renovations.

If you just want to see simple air leakage it may be easier to wait for a windy day and then use a smoke pencil or even a joss stick.  Take it around all windows, doors, cat flaps etc and see how much the smoke moves.  It really is amazing how much draft comes through an ill-fitting window.  Then you need to improve your draught proofing, or possibly even replace windows and doors, but that’s for another blog.

* Don’t be fooled by the 50 Pascals (Pa) part.  Atmospheric pressure is actually around 101,325 Pascals.  This 50 Pascals reading is the difference created between the outside pressure and that inside the house (that is what the fan is calibrated to do).


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jessicacolesComment left on: 8 October 2020 at 6:22 am

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