The answer to the question is 'because that's what the regulations allow and as many people regard the building regulations as a target standard that's what most people do.'
Current building regulations stipulate a minimum air leakage rate to be no more than 10m3/hr/m2, in the example below you can see that by comparison to Passive standard this equates to leaving an open hole in the building fabric equivalent in size to a typical a cash machine, whereas the Passive standard this hole size would be reduced to the size of a credit card.
Obviously, people do not leave a single gaping hole but the equivalent size will be distributed over the whole building which means that the energy efficiency is stripped away by the movement of air through the building fabric especially the insulation. This is why airtightness matters and sits at the core of improved energy efficiency.
What is Airtightness?
Often we are asked what the term airtightness means. Airtightness primarily focuses on the elimination of all unintended gaps and cracks on the external envelope of the building. Airtightness is an essential part of creating a healthy, comfortable, energy-efficient living environment. In contrast, air leakage is where leaks occur due to gaps and cracks that should not be there in the first place. This can account for up to 50% of all heat losses through the external envelope of a building. There are many factors which can cause air leakage such as poor build design, poor workmanship, or indeed the inappropriate use of materials. It is important to remember that an airtight building does not mean it is hermetically sealed, rather it means that the air leakage has been reduced to a minimum.
What role does ventilation play in airtightness?
Ventilation is crucial in all buildings, not just airtight ones. It is key to construct buildings which are both airtight and gap-free and then introduce a designed and controlled ventilation system which ensures that adequate fresh air is supplied to meet the needs of the occupants.
Can I not just add more Insulation?
Insulation requires high levels of airtightness to perform. This can be explained by the "woolly jumper" effect. Imagine going hill walking and you only wear a single layer then the wind blows through the woolly jumper quite easily. However, if you apply a light windshield over the single layer it has a dramatic impact as it reduces air movement through the jumper and consequently, the woolly jumper insulates much better
Therefore, for insulation in a building to perform it needs to be protected against air movement on both sides
1 - on the outside protecting against wind by using a windtight external membrane
2 - On the inside protecting against the hot air penetrating through it creating air movement through the insulation by using an airtight membrane
Short Video - Intelligent Airtightness Explained
What are the benefits of airtightness?
- Reduced heating costs
- Improved health - substances which can provoke allergies can be carried into a building via air leakage - air coming from outside in or from within the building fabric itself
- Improved building durability - Airtightness protects the building fabric against damage due to moisture-laden air leaking into the building envelope and condensing
- Reduced callbacks - Airtightness focuses on build quality and quality workmanship
- Improved comfort levels - Airtightness is a key component in reducing overheating in summer and insulating better in winter
- Improved Acoustics - Air is a very effective medium for transporting sound. Higher levels of airtightness means more effective reduction of sound transfer
What steps can I follow to achieve high levels of airtightness?
- Design for airtightness - ensure the architect designs the building with key airtightness details in mind. Keep it simple with the details
- Build for airtightness - Now that it is designed correctly, ensure all personnel who interact with the airtightness layer are trained and install products correctly. Workmanship can be validated with a WINCON test.
- Test for airtightness - We can only understand how something is performing by attaching a metric to it, airtightness is no different. Blower door test should be carried out to measure the airtightness.
Airtightness - The Facts
On average we spend up to 90% of our time indoors - it makes sense to make this environment as stable and comfortable as possible, free from any draught and cold spots.
Based on the envelope area of a 1,900 square foot certified Passive House If built just to building regulations (a leakage rate 10m3/hr/m2) the equivalent size hole in the building once everything has been sealed up would be approximately 440 x 440mm. Whereas what was achieved on this Passivhaus was a leakage area that is 10 times smaller at just 44 x 44mm
To put this leakage area into perspective, if a building was built to the backstop allowable leakage rate for building regulations, a hole in the wall the size of a typical ATM machine would still be an allowable leakage area whereas for an extremely airtight would only have a leakage area equivalent to that of a credit card.
This article is an abridged version of an original article published by By Niall Crosson, Senior Engineer, MEng Sc, BTECH, MIEI, CEPHC in June 2017