Airtightness: why is it so important?

With this article, we address one of the most important topics in the field of energy efficiency: the airtightness of the thermal envelope.

Whether the building is a new construction or a renovation, its airtightness plays an important role for comfort, energy efficiency and durability of the structure.

AIRTIGHTNESS AND TRANSPIRABILITY

In the first place, we need to draw a line between airtightness and transpirability: they are two very different concepts, although they are used often (and mistakenly) as synonyms.

Transpirability is the ability of the thermal envelope to be “open” to water vapor only.

Airtightness is the perviousness of a structure of the building envelope to air, through holes, gaps, cracks – you name it. These represent defects, whether they are caused by poor design or faulty construction. The more holes in the structure, the more air is going to leak in and out of it.

 

AIRTIGHTNESS OF THE THERMAL ENVELOPE

When you talk about airtightness of a building, you refer to its thermal envelope: unheated rooms don’t play a role in it.

The airtightness can be measured on site, with a blower door test according to EN 13829. During the test, all exterior openings (doors, windows) are closed, and the building is pressurized to find air leaks. This test allows to evaluate the overall airtightness of the whole envelope.

With the test, you can measure the volume of air extracted by the testing appliance (see photo): by dividing that value by the net ventilated volume of the envelope, you obtain the airtightness value of the building. The higher the volume of air extracted, the more air leaks are present.

As an example, airtightness values of a building envelope can be:

Masonry house from the 70s: 5,0 1/h;

Timber house from the 70s: 11,0 1/h;

Current house: 2,5 1/h;

Passive House: 0,6 1/h.

A complete airtight building would achieve 0,0 1/h: this is basically impossible to obtain regardless from the type of structure you choose (masonry, wood etc.).
COMFORT

A non-airtight envelope allows for air leaks. In winter, this phenomenon has two negative effect.

In the first place, in the proximity of the air leak, you can feel the cold air moving, which is per se a cause of discomfort.

In the second place, these leaks allow for more air stratification inside the building, with a relevant temperature difference between the floor and the ceiling: this is perceived by the human body as discomfort, even if the room temperature may be higher than 20°C (68°F).
ENERGY

Airtightness equals absence of air leaks, so it is quite straightforward how:

airtight = efficient

The “energy weight” of the air leaks on the overall energy demand for heating depends on the efficiency of the building: we’re going to dedicate a specific article on this matter.

As an example, for the two Passive Houses in Cavriago, the building envelopes are designed to allow a maximum “leakage” of 0,6 1/h. In this case, the energy demand for heating due to air leaks is 11% of the total for each building. If the very same building, with the same insulation level (walls, roof etc.) had a less airtight envelope of about 1,5 1/h, the “energy weight” of the air leaks would count for more than 20% of its total heating demand: you’d even have to change its heating system!

DURABILITY

So far, we’ve described airtightness as cold air leaking into the thermal envelope: this is correct, but incomplete.

In winter, when the indoor environment is warmer than the outside, external cold air is denser and heavier than the internal one. For this reason, cold air tends to leak into the building from the bottom, pushing out the warm air from the top. If this warm air manages to escape through holes and craks of the thermal envelope, it is going to release water condensation along its path.

(*) source: Fraunhofer Institut

You can easily understand the damage that such an amount of water can cause to a building. The insulative properties of a wet material drop dramatically: condensation undermines the energy performance of the building.

In just a few years, however, the condensation can permanently damage the structures of the building. It is the case of wooden structures (in all the wide variety they come in): if these assemblies are not airtight, they are most likely doomed to rot.

Masonry or concrete structures are less easily damaged by the condensation, because they are born “wet”, meaning that their construction process involves the use of water, so they are less sensitive to it.

AIRTIGHTNESS AND MOLD

If innovation in comfort and efficiency pushes architects to design more performing buildings, at the same time, building users need to learn how to use these buildings properly.

You cannot use an airtight building the same way you’d use a leaky one, otherwise you’re going to face a very likely mold and condensation problem.

Airtightness is not to blame: the user is!

In case the building is not provided with a mechanical ventilation system, the user is responsible to regularly exchange the indoor air, to provide the rooms with fresh air, and to get rid of water vapor created by breathing, cooking, washing etc.

This topic is extremely important for the indoor air quality of a building, and we invite you to read our article on mechanical ventilation.

DESIGN AND CONSTRUCTION

Airtightness needs to be designed: whoever is in charge of the thermal envelope, needs to address this feature for all the structures of the thermal envelope (roof, wall etc.), and for their junctions as well.

You cannot hope to address airtightness by guessing solutions when the building is under construction.

Design aside, airtightness is probably one of the most difficult goals to be achieved during the construction. A sigle crack can cause very large damage. This is why a very detailed design cannot replace a good site supervision, carried out by a competent professional.

To guarantee the airtightness of the envelope, it’s good business practice to run a preliminary blower door test before the final finishes are in place, in case you need to seal some unexpected leaks.

AIRTIGHTNESS AND RENOVATION / RETROFIT

Airtightness is very important in any project, including building renovations and energy retrofits.

The level or airtightness you can achieve in this kind of projects depends on many variables – the type of structures, the depth of the renovation and so on. Boundary conditions in existing buidlings increase the complexity of the problem, but nothing is impossible.

Compared to the airtightness levels listed above, in a renovation you can dramatically improve the existing conditions, and cut air leaks by a considerable amount. From a very leaky thermal envelope (5,0 1/h), it is usually quite feasible and cost effective to achieve an acceptable tightness level (1,0-1,5 1/h), with a reduction of leaks around 80%.

NOTE (1): THE ROOF IN THE PHOTO

The roof in the photo is provided with an aluminum vapor barrier on the inside (warm side), and a plywood panel with a bituminous membrane on the outside. We could have a whole conversation on  vapor barriers versus structure transpirability, but that would be off-topic. From the point of view of building physics, the structure was designed properly.

The cause of the beams rotting is not a widespread condensation problem – otherwise all beams would have suffered from the same issue. You can see how part of the structure remained untouched by the problem (bottom left). The phenomenon is caused by localized lack of airtightness, which caused some of the beams to rot.