In sandwich wall constructions the outer leaf provides the weathering protection. Even if there is no air gap between the outer leaf and the core insulation, capillary infiltration of rain water is limited to the outer leaf if hydrophobic insulation materials are used and the whole construction is windproof [1]. Thus, if the wall is properly executed, the construction layers behind the facework are durably protected from precipitation water.

In summer, however, moisture may be transported into these deeper layers by so-called reverse diffusion (also known as summer condensation). If the outer leaf is heated by sunshine, the rain water it contains will be carried across the core insulation by vapor diffusion and condense in the cooler inner leaf. If the inner leaf is masonry, the condensation moisture is harmlessly absorbed as capillary water in its pore spaces and given off in cooler periods. However, if the inner leaf contains materials which are susceptible to moisture damage, such as wood, summer condensation may create problems.

The effect of summer condensation occurring in the wall construction shown in Fig. 1 has been investigated by computational simulation. The inner leaf is a simple wooden post-and-beam structure. The outer leaf is clinker facework with a relatively low water absorption coefficient (A-value) of 1,0 kg/m²h^½. The 12-cm-thick cavity between the facework and the OSB board covering the inner leaf is completely filled with hydrophobic mineral wool insulation. The investigation considers a typical cross-section of the west-facing wall during the fifth year of exposure to weather and focuses on the moisture content of the OSB board. The indoor climate used for the calculations is shown in Fig. 2, a cold and a warm year measured in Holzkirchen (HRY – Hygrothermal Reference Years) were used for the outdoor climate conditions.

Fig. 3 shows the spread of moisture conditions that occur in the OSB board, depending on the outdoor climate. In contrast to what is usually expected, the OSB board dries out during winter, starts to continuously accumulate moisture around May, reaches a maximum in autumn and then starts to dry again. As described above, the moisture accumulation during summer is due to reverse vapor diffusion. In a cold year, this does not create a critical situation for the wall construction investigated here. In a warm year, however, the moisture content in the OSB board exceeds 20 mass-%. Since relatively high temperatures occur at the same time, long-term damage caused by microorganisms can not be excluded.

This example is a striking demonstration of the fact that under certain circumstances warm weather can hold a higher risk than colder weather.

Literature

Künzel, H.: Zweischalige Außenwände mit Kerndämmung und Klinkerverblendung. wksb 37 (1996), S. 15-19.

 

Page created: 08 May 2007; last update: 17 Jul 2012

External Thermal Insulating Composite Systems (ETICS) provide both thermal and weathering protection for exterior walls. Therefore they are not only applied to new buildings but also successfully used for the rehabilitation of old buildings. In the latter case they also provide durable corrosion protection for the reinforcements in prefabricated slab constructions [1]. In all these cases, quick drying-out of the underlying construction is desirable in order to stop corrosion or moisture-induced heat loss. In the following, the drying-out of construction moisture in a 24 cm thick lime silica brick wall with an 80 mm thick ETICS containing mineral wool (MW) or polystyrene foam (EPS) insulation will be investigated by calculation and experiment, and conclusions based on further calculations will be presented.

For details on wall construction, material properties and how the outdoor tests and WUFI calculations were performed, please refer to [2]. Fig. 1 shows a comparison of the computed and measured moisture distributions for various points in time after completion of the test house (the measurements were done on drill cores). For both the wall with EPS insulation and the wall with MW insulation good agreement between computed and measured results is achieved.

The shapes of the moisture profiles show that in the wall with EPS insulation most of the moisture is drying toward the interior, while the mineral wool also allows noticeable drying of the masonry toward the exterior. It takes the wall with mineral wool insulation about a year and a half to reach hygroscopic equilibrium, and about twice that time for the EPS insulation. During the drying phase, the heat losses caused by increased thermal transmission and by the increased ventilation necessary for removing the construction moisture are not negligible. In the case of the lime silica brick masonry with its low insulation quality the moisture level during the first year increases the U-value by about 5%. With masonry of aerated honeycomb bricks under the ETICS, the U-value would be increased by about 25%, as shown by computations in [3].

Since the designed U-value is reached only at the end of the drying period, quickly drying wall constructions help to save heating energy. Constructions which are vapor-permeable on both sides and thus can dry equally to the interior and the exterior side are particularly favorable. On the other hand, in walls with vapor-retarding covering on both sides, e.g. with an EPS insulation on the outside and tiles on the inside, it may take the construction moisture more than five years to dry out completely.

Literature

Cziesielski, E.: Energiegerechte Sanierung von Korrosionsschäden bei Stahlbetongebäuden. Bauphysik 13 (1991), H.5, S.138-143.
Künzel, H.M.: Austrocknung von Wandkonstruktionen mit Wärmedämm-Verbundsystemen. Bauphysik 20 (1998), H.1, S.18-23.
Holm, A. und Künzel, H.M.: Trocknung von Mauerwerk mit Wärmedämmverbundsystemen und Einfluß auf den Wärmedurchgang. Tagungsband 10. Bauklimatisches Symposium, Dresden 1999, S.549-558

 

Page created: 07 May 2007; last update: 17 Jul 2012

Under Central European climate conditions a perimeter insulation usually presents no hygric problems. However, this does not apply to insulation exposed to ground water. In this case only closed-cell insulation materials such as foam glass or extruded polystyrene rigid foam (XPS) are admissible.

Since XPS – as opposed to foam glass – is not completely impermeable to water vapor, care must be taken to prevent water from ever seeping behind the insulation slabs. This can only be ensured by installing the insulation with durable full-face bonding between cellar wall and insulation slabs. Unfortunately, practical experience shows that this is not always done carefully enough and that moisture can then penetrate into areas with no or with incomplete bonding. From the warm side of the insulation slab, vapor diffusion will then transport moisture into the cooler parts of the insulation material. This leads to moisture accumulation in the insulation slabs around these defects and thus degrades the thermal insulation of the cellar.

In [1] the moisture behavior of an XPS perimeter insulation applied to walls of a heated cellar has been simulated, assuming that ground water has penetrated behind the insulation slabs. Under the ground temperature conditions shown in Fig. 1 diffusion of the ground water that has seeped behind the warm side of the insulation leads to continuous moisture accumulation in the insulation slabs (Fig. 2).

As opposed to measurements on sporadically taken samples, the simulation allows to extrapolate the moisture conditions over a longer period of time. The long-term moisture accumulation is much more pronounced in a perimeter insulation with only 50 mm thickness than in one with 80 mm thickness. The smaller temperature gradient in the thicker insulation layer gives rise to a smaller vapor pressure gradient on the warm side, which leads to less accumulated moisture because of the reduced vapor diffusion flow. In both cases, in the area around the defect the U-value of the insulated cellar wall increases markedly over the course of 30 years (by about 70% in the 80-mm-insulation and by about 140% in the 50-mm-insulation). Such a degradation of the insulation quality is not acceptable. It is therefore important to exercise great care when applying XPS perimeter insulation slabs exposed to groundwater.

Literature

Künzel, H.M.: Feuchteaufnahme von Perimeterdämmplatten aus extrudiertem Polystyrol-Hartschaum im Grundwasserbereich bei nicht vollflächiger Verklebung. IBP-Bericht FtB-38/1995.

 

Page created: 27 Apr 2007; last update: 17 Jul 2012