As rain

• As water spilled in bathrooms or kitchens.

• As airborne moisture from inside or outside.

• As moisture from installations that leak, such as drains, water supply or heating systems.

The last point does not need to be discussed, since it depends on good execution and technology alone. Humidity requires special consideration in the case of highly insulated buildings. A wall with 250 mm of insulation will take 70% longer to dry out than a wall with 150 mm insulation (Folleras, 2007).

Moisture within building materials

During construction, a new house may contain about 10 000 litres of water within its building materials. Drying time for these is strongly dependent upon the structure of the material. There is an unnamed relative material factor, s. The drying capacity of a material increases when this factor value falls. Lime mortar has an s-factor of 0.25, brick 0.28, timber 0.9, lightweight concrete 1.4 and cement mortar 2.5.

To avoid moisture, materials should be carefully kept in dry storage at the building site. Ideally one may cover the whole building, so-called Weather Protection Systems (WPS); other systems enable one to complete the roof covering well before the other parts.

Concrete, earth and timber structures should be allowed to dry out before they are used with less moisture-diffusive coverings or surface treatments. Many defects, for example, arise from insufficiently dried concrete being covered by PVC or other impermeable layers so that the humidity is effectively trapped and cannot dry out.

Ground moisture

A building site should be dry and well-drained. However, foundations are exposed to dampness throughout their lifetime. This can be reduced by adding something that breaks the capillary action of water rising from the ground: for example, a layer of broken stone (hard core), expanded glass or clay aggregate, or else heavy grade plastic sheeting. It is common for structures to have a damp-proof course between the foundation and the rest of the structure; usually a layer of bitumen felt placed along the base of the wall a short distance above ground level, but below the level of the interior floor. High quality slate, slabs of granite, etc. may also be used. It is nonetheless difficult to prevent a certain amount of moisture entering the fabric of the building. This will increase as the damp-proofing course ages. There have been a series of damp problems associated with organic floor coverings such as timber, vinyl sheeting, etc., that were laid directly on concrete in slab-on-grade constructions, even where there is a plastic membrane in between. As insurance against such problems, concrete slabs on the ground should preferably have mineral floor coverings such as slate or ceramic tiles.

Rain

External cladding and roof coverings is discussed in greater detail in Chapter 15. There is also a need for special components to protect exposed parts of the building; in particular the critical points where chimneys, pipes, ventilation ducts, roof lights and so on penetrate the building fabric. Roof gutters and downpipes are also critical; these are often made of heavy plastics or metal sheeting, either prepared on site or prefabricated. All these points are subject to exceptional weathering. The fact that they need inspection and replacement more often than most other external components is often overlooked. It has been documented that buildings with rooflights and other roof protuberances have a considerably higher risk of water damage

Roof

Pitched roof Flat roof

Pitched with doimers

Bathroom

Timber wall and floor Mineral wall and floor

Small risk Large risk

14.5

Risk of moisture damage in different constructions. Results from an investigation of 16 000 Danish houses. Source: Valbjorn, 2001.

(Valbjorn et al., 2001) (Figure 14.5). The same applies to flat roofs, especially in rainy climates. This is primarily due to incorrect design choices, no matter what materials are used.

Water spills

In bathrooms and kitchens there is often a lot of water spilled. Water penetration is prevented by waterproof membranes. These are most often of bitumen or plastics. Their lifetime is coinsiderably shorter than that of the building, and they are difficult to inspect and often concealed behind other layers. It has been shown that damage is less common with the use of pure mineral constructions for bathroom floors; and even less common when mineral materials are also used in walls (Valbjorn etal., 2001) (Figure 14.5).

Air moisture

Air moisture is almost entirely produced inside the building by people, animals and plants, or from cooking and using bathrooms. The building

14.6

Interior wooden and rendered surfaces with high moisture buffer capacity. Gaia Lista, 2007.

14.6

Interior wooden and rendered surfaces with high moisture buffer capacity. Gaia Lista, 2007.

Roof

Pitched roof Flat roof

Pitched with doimers

Bathroom

Timber wall and floor Mineral wall and floor

Small risk Large risk

itself gets damaged when air moisture penetrates the external walls and condenses there.

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