Written by Jon Davies
Technical Sales Support and Education Manager, Pro Clima New Zealand

In the winter, thermally insulated buildings separate the warm indoor air with its higher absolute humidity, from the cold outdoor air with its lower absolute humidity.

In the summer, these buildings separate the warm outdoor air with its higher absolute humidity, from the indoor air with its lower absolute humidity.

This is accompanied by the risk of condensation (water condensing):

• On the external construction components in the winter

• On the internal construction components in the summer

Whether water actually condenses depends on the amount of moisture that penetrates the construction and the diffusion resistance of the various layers of materials in the construction. In the winter, the temperature within the building envelope decreases towards the outside. Depending on the absolute humidity there is the risk that a temperature may be reached at which the air is no longer able to hold the moisture that it originally contained (the saturation temperature). It is therefore worthwhile to predictably regulate the diffusion flow.

Vapour diffusion flow

If the vapour diffusion flow is limited (for example, by an intelligent airtightness membrane) and if the construction is open to diffusion on the outside, a certain amount of moisture that penetrates the construction is able to escape again without forming condensation. If, on the other hand, there is a diffusion-inhibiting (eg. plastic sheet) or even completely impermeable layer (eg. aluminium foil) on the outside, the vapour is prevented from passing through and there is a risk of condensation. There is no condensation if less moisture enters on the warm side than can escape on the cold side. There are two ways of influencing the moisture balance: on the warmer side, by limiting the diffusion flow, and on the colder side by increasing the permeability for diffusion.

Vapour diffusion flow in the summer

While diffusion takes place in winter from the inside toward the outside (warm to cool), it also takes place from the outside to the inside in the summer (also warm toward cool). The vapour partial pressure (combination of the temperature and the relative humidity) is higher outdoors than indoors.

If you have the perfect materials for winter conditions only, (eg. a vapour retarder on the inside face and a layer completely open to diffusion on the outside), you can end up with a lot of moisture penetrating from the outside in the summer. The vapour retarder on the inside can then become a moisture trap on which the moisture from outside can condense. In other words, in the summer there needs to be a vapour permeable layer on the inside, instead of a vapour retarder. One solution to this challenge is to use a humidity-variable vapour control layer such as INTELLO which operates predictably in both summer and winter to prevent moisture accumulation within the structure.

In the next post we will look at both material selection and how to predict where moisture accumulation may occur while a design is still on the drawing board.

If you'd like to know more about pro clima please visit the pro clima website. Our team can organise a CPD points presentation to help you confidently predict success in designing dry and durable buildings.