This is an excellent question. Too often in my experience, developers chased lower and lower air tightness targets, believing that the lower the score, the better. This is not strictly true. In ATSPACE, our motto is ‘Delivering Beyond Compliance', and nowhere is that better put into practice than in our air tightness testing. We don’t just chase that low score blindly, but we advise you on what an appropriate score for your build should be. True, the lower your build scores, the less heat loss you will suffer, and the more energy efficient your property will be, but a property needs to breathe. The emphasis should be that the lower the score, the better, as long as you have an appropriate ventilation system to match.
Perhaps it is wrong to say a build is too airtight, but more accurately, it should be said that the build doesn’t have appropriate ventilation. Without sufficient ventilation, the air within your property will be trapped, along with all the moisture in it. This coupled with all the heat you’ve retained will lead to great patches of condensation, which in turn will lead to mould and bacteria. A property that doesn’t have enough airflow will end up suffocating in stale air, not to mention the odour caused by large patches of mould.
Originally, a build would only need to achieve an airtightness score of under 10 AP50 (m3/h/m2). What this means is that the air permeability (AP) at a given pressure of 50 Pa should not exceed 10 meters cubed per hour per square meter of the build's external area. Nowadays, though, it is not common for developers to aim for a target of 5. Other developers aim even further below that with a target of 3. The difference between those that aim below 5 and those that aim below 3 is that any development with a target below 3 will have mechanical ventilation fitted as standard. In fact, mechanical ventilation and heat recovery systems do not function properly unless the build is that airtight.
It is all about finding the right balance for your project. There are projects out there building what’s known as passive houses whose airtightness requirement is 0.6. It is as airtight as you can build a property. However, they have the ventilation systems to support this and ensure the correct circulation of air.
If you'd like more information, don't hesitate to call!