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Air & Vapor, Part 2: The Importance of Air Tightness

February 6, 2012

In Part 1 of Air & Vapor we started our discussion noting the importance of air tightness and vapor control in building enclosures, as second only to keeping the rain out.

Control Layer Priorities:

  1. a rain control layer
  2. an air control layer
  3. a vapor control layer
  4. a thermal control layer

In this post we’ll discuss the importance of air tightness.

Air control at the enclosure is so important (two notches above insulation) because it has profound effects on:

  1. Indoor Air Quality (IAQ)
  2. The optimization of thermal insulation
  3. Prevention of bulk vapor penetration.

For these reasons, air control and air tightness is a cornerstone attribute of any high-performance building, including  Passive House (Passivhaus) buildings.  So let’s take a closer look at the power of air sealing and air tightness:

Indoor Air Quality (IAQ)

You can’t control the quality of the air we breathe until we control it physically.   Unfortunately this is a widely ignored fundamental truth.   Until we control the air flow through a home the IAQ is a crapshoot.    And the first step in controlling the air flow in a building is an airtight enclosure with a continuous air barrier.

Ventilation fresh air filter, full of exterior soot, pollen and other contaminants

Once an airtight enclosure is established, the air inside can be controlled predictably and efficiently.   High IAQ becomes not only possible but simple to achieve.   Fresh air can be brought in continuously with stale air exhausted continuously.    The fresh air is filtered as it enters the home, removing soot, pollen and other pollutants and supplied to living areas.   Exhaust air is taken from bathrooms and kitchens and brought through a high efficiency heat exchanger, swapping its energy to the incoming fresh air before it is expelled to the outside.

In this way – made possible with an airtight enclosure – air quality inside the home may be superior to the outdoor air quality.

Optimization of Insulation

Air leaks through the enclosure can severely compromise the effectiveness of the thermal control layer.  A study by the Fraunhofer Institute for Building Physics, in Stuttgart, showed that a leaky enclosure can decrease the insulation’s effectiveness by a factor of 4.8 – making a 500 sf leaky building require the same amount of heating as a 2,400 sf airtight building!

Diagram of insulation unprotected by air tightness (at left) and protected by air tightness (at right).

Consider your sweater on a breezy day, and the effect of putting on a windbreaker.

The insulation performance is best optimized with an air-tight layer on all sides: an air-tight layer at its interior face (such as Intello Plus) and a wind-tight/air-tight layer on the exterior face (such as Solitex Mento).

If you are going to bother to insulate, get the most out of it with with complete air sealing and airtightness.

Prevention of Bulk Vapor Penetration

BSC: diffusion wetting vs. air leak wetting.

While there is much gnashing of the teeth (and some of it rightly so) about vapor diffusion resulting in condensation and structural damage, a far bigger threat to the structure is bulk water vapor penetration via air leaks in well insulated buildings.

With this iconic diagram from the Building Science Corporation paper RR-0412: Insulations, Sheathings and Vapor Retarders, one can see that the potential water intrusion from just a small air leak can be up to 90 times greater than the water that might diffuse through gypsum board into the enclosure.

With such extreme wetting, even a vapor open construction may not be able to dry fast enough to keep the structure safe.   Here we see water damage from an air leak in a such a vapor open construction.

Water damage from air leaks in vapor open construction.

Uncontrolled air movement through OR within* the building enclosure, carrying bulk vapor, causes the greatest amount of moisture related building damage.  Keeping the enclosure airtight on the interior is an essential step in keeping it dry.

*convective loops within wall cavities can transport and deposit bulk vapor when it reaches the dew point.


Because air tightness is essential for clean indoor air, optimized insulation and minimizing of wetting damage, it should be seen as a fundamental and integrated insurance policy on the performance of the building.   The more airtight your enclosure, the better the insurance policy you have.

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  1. Vented rainscreen: what a high performance building wants | Four Seven Five - March 22, 2012

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