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What is Airtightness?

Niall Crosson

By Niall Crosson

Thursday 22nd June 2017

Airtightness is primarily focusing 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.  

What is Airtightness? What is Airtightness?

What is Airtightness?

Often we are asked what does the term airtightness mean. 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 materials used. 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 against wind (Solitex Plus

2 - On the inside against the hot air penetrating through it creating air movement through the insulation (Intello Plus)

Intello airtight membrane being applied over Thermo Hemp insulation
Intello airtight membrane being applied over Thermo Hemp insulation

What steps can I follow to achieve high levels of airtightness?

Intello being stuck to timber a joist
Intello being stuck to timber a joist

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 the OSTRO Passivhaus. 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 Passivhaus would only have a leakage area equivalent to that of a credit card.

Equivalent size hole in building fabric - comparing Building Regulations to Passive House construction.

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.

Blog author

Niall Crosson

Niall Crosson

Group Technical Manager (BTech, MEngSc, MIEI, CEPHC)

Niall is the Group Technical Manager for Ecological Building Systems and was also the company’s first employee. Niall holds a first class honours degree as a Bachelor of Technology and a Masters in Eng.Sc. Niall is a certified Passivhaus Consultant and has provided technical guidance on many award winning low energy projects in Ireland and the UK, and has authored or co-authored several chapters in both the Passivhaus Handbook and the Passivhaus Designers Manual. Niall is also on the board of directors of the Irish Green Building Council and provides guidance on several national standards committees.  Along with this, Niall has contributed to several construction publications including The Journal of the RIAI.

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