Why build an airtight house? – Leakage problems

The external envelope of a building should be as airtight as possible and this doesn’t only apply to Passive Houses.

• building damage caused by water vapour that is transported in air draughts can only be prevented by airtightness of the envelope (see illustration). Building damage is mostly due to the absence of airtightness in the roof area.

• Draughty living spaces and cold air pockets are no longer acceptable for inhabitants: a truly airtight method of construction leads to greater comfort.

• Ventilation systems with supply air and exhaust air spaces only function if the building envelope is sufficiently airtight.

• Airtightness results in better sound protection

Therefore, in accordance with construction technology regulations, a good level of airtightness is generally required today, and rightly so. This applies even more for a comfortable Passive House.

Due to insufficiently airtight building components, warm and moist air can flow from the inside towards the outside and thus lead to condensation on colder parts of the construction. Most building damage occurs in this way. This can be remedied by careful airtight construction.

Regarding energy saving construction, airtightness isn’t a pastime, it is vital for the prevention of moisture penetration in building components. As the illustration shows, air flowing out from leaks transports substantial amounts of moisture into the construction. As the air cools on the way out, the water in the air condenses and remains inside the construction.

Airtightness should not be confused with thermal insulation. Both are important for the building envelope, but they must be implemented independently of each other:

• A well-insulating building component doesn’t have to be airtight: e.g. one can easily “blow through” a mat of coconut fibre, cellulose filling or mineral wool insulation. These materials are very good insulators, but they are not airtight. The only insulation material that can also be used as an airtight layer is foam glass.

• Conversely, an airtight building component doesn’t have to be insulating, e.g. an aluminium sheet is absolutely airtight, but has practically no insulating effect.

Airtightness is an important requirement for energy-saving construction, but it is not the most important one (as sometimes misrepresented by popular publications - good insulation is the most essential requirement).

Airtightness should not be confused with diffusion impermeability either: for example, oiled paper is airtight but diffusion permeable. Ordinary interior plaster (gypsum plaster, lime plaster, render, or fibre reinforced loam render) is also sufficiently airtight while being diffusion permeable at the same time.

For a quality assured Passive House (certificat of the PHI) an airtightness with a very low remaining leakage is required:
The n₅₀ value has to be not higher than 0.6 h^-1. This is a quite demanding requirement, but experience has shown

• after some learning gained with first built projects or a Passive House course, these very good values can be achieved regularly. (To see these figures as “very tough” is just a beginners view; we have seen that again and again: after some experience, well designed projects of an experienced team will have n50-values of some 0,3 to 0,5 h^-1.)

• the costs for achieving these values are not higher than those to achieve less demanding ones. (To be better than say 1.5 h^-1, you'll need an airtightness concept. But: If you do an airtightness concept, it doesn't make a big difference to go for say 0.5 than for 1.5 h^-1)

• these values guarantee a good performance of the building: moisture problems are avoided this way

• the energy conservation by these measures is significant. The influence on peak load is even higher (because of the changing weather conditions). Therefor, building really airtight significantly reduces the risks of missing performance. This is one of the reasons, why there is no “performance gap” with passive houses - but there seem to be such problems within other ambitious low energy projects. Yes, it's difficult to fully track air infiltration losses and to exactly calculate their influence on the performance. But: The best and easiest (and cheapest) way to cope with this is to build the envelope tight - and avoid all these problems.

• there is an additional benefit for better comfort by avoiding drafts. Persons seem to be especially sensitive to cold drafts (see ISO 7730) - and there are two important causes for such drafts: first, an untight envelope; and second, cold surfaces.

It's a bad idea to think, that you'll gain anything from alleviating the airtightness requirements. The only result will be, that yo'll get higher risks for bad performance. On the other side, it's not much an effort to learn how to deign and to build really airtight.

General Principles

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