Airtightness testing - the man with the fan..!

Early last week we took up an offer to have the house tested for airtightness by the Leeds Met. Uni's Centre for the Built Environment. Unlike in England, such test are not (yet) a requirement of building reg's in Scotland, but given that we are aiming to achieve the AECB's Silver standard I was keen to ensure we were at the very least close to our design target!So how do we measure airtightness? Until Friday, this was a mystery - then it all fell into place; method as follows:1. Close all trickle vents, windows and doors, fill all drain/water traps.2. Open main door of house and fit blower fan sealed within frame.3. Switch on fan and measure air flow rate into house (in m.cu. per hr) when air pressure in house is a steady 50 Pascals higher than outside.4. Record this flow rate and divide by surface area of house. Hey presto, this give the air permeability figure in m3 per m2 per hr - or m/hr.The results? Pretty good for a first test appeared to be the concensus. Under the current building reg's in England, the number has to be less than 10; ours was 4.8. Put another way, if we add up all the gaps in our building we have a hole the size of a dinner plate between us and fully airtight.However, the AECB's Silver target level is 3.0 for a house using a Mechanical Extract Ventilation system, or 1.5 for MVHR (our option); in other words we need to try and reduce our dinner plate to a saucer!So where do we look? Thermal imaging and smoke tests under pressure showed airflow through the service void, all of which are connected to the 1st floor joist void and loft space. This leads me to suspect we have some leakage at the soil vent pipe penetrations and possibly the airtightness membrane joints within the loft. Another significant 'problem' is clearly the seals around the large sliding patio doors, of which we have three sets, and which were actually letting water in last week during a driving rain storm. Our tester reckoned that sorting the doors alone could bring our figure down a futher 1 to 2 m/hr, getting us much closer to our target. Other issues included leaks between plasterboard and window frames.As a postscript to this, today I noticed a 1.5 inch waste pipe in the kitchen open to the elements with a gale blowing out of it into the house which we had omitted to seal; so that's an egg cup off our dinner plate!