The drying of a cellular concrete flat roof is a classic example for the application of modern computational simulation methods. Since traditional methods considering only vapor diffusion were not sufficient to explain the drying behavior of these flat roofs, a new calculation method [1] was introduced in the early 80s which allowed for capillary transport, too, and was thus able to quantitatively reproduce experimental results obtained in the 60s [2]. Since this type of construction has recently attracted the attention of experts because of moisture problems during the drying phase in more southern climates, it will be used here as an example for investigating the hygrothermal behaviour of massive flat roofs.

Fig. 1 shows the computed drying curves for three different flat roof variants, each starting with 20 vol-% of construction moisture in the prefabricated cellular concrete elements at the beginning of the first year. A comparison of the computation with measurements obtained for the 15 cm thick roof [2] shows the quality of the calculational prediction. The initial strong drying by 10 vol-% (15 kg/m²) within six months is due to strong heating of the bituminized roof surface by solar irradiation.

Since the roof can only dry towards the indoor side, a strong moisture load on the indoor air is created which must be removed promptly. While at temperate latitudes this is routinely achieved by simply opening the windows for an appropriate period of time, the air conditioning equipment used in hot and humid climate zones often cannot cope with the additional moisture so that indoor humidity may exceed permissible levels during the drying period of the roof.

The designed U-value of the roof is only reached when its moisture content falls below the reference moisture content of cellular concrete (about 1.5 vol-%). With the 15-cm-thick roof this happens within less than two years. A 20 cm thick cellular concrete roof already takes 3.5 years to dry out, which is roughly double the time. If 6 cm of polystyrene insulation are applied on top of the latter roof, its drying time reaches approximately five years. This example demonstrates the considerable influence of solar irradiation on the moisture behavior of flat roofs. While the additional insulation raises the overall temperature level of the cellular concrete, it also strongly suppresses the surface temperature peaks in summer. Since saturation vapor pressure rises exponentially with temperature, this reduction of peak temperatures slows the diffusion-dominated drying following the initial moisture loss which is supported by capillary forces.

 

Literature

Kießl, K.: Kapillarer und dampfförmiger Feuchtetransport in mehrschichtigen Bauteilen; rechnerische Erfassung und bauphysikalische Anwendung. Diss. Universität-Gesamthochschule Essen 1983.
Künzel, H.: Untersuchungen über die Feuchtigkeitsverhältnisse in Flachdachkonstruktionen. Berichte aus der Bauforschung H. 48, Verlag Ernst & Sohn, Berlin 1966

 

Page created: 09 May 2007; last update: 17 jul 2012

From a hygrothermal point of view, an exterior insulation is in general preferable over an interior insulation. However, economical aspects or the wish to preserve historical facades may rule out exterior insulation, so that interior insulation remains the only way to reduce the energy consumption of a building and to increase comfort. The resulting risk of interstitial condensation and possible countermeasures (e.g. vapor barriers) are widely known.

It is less well known, however, that an interior insulation affects the moisture content of a facade exposed to rain and may increase the risk of frost damage. The influence of this effect on the moisture behaviour of a fair-faced solid-brick masonry wall exposed to driving rain has been investigated by computational simulation in [1] and has been verified by accompanying open-air experiments. Fig. 1 shows the moisture conditions in a 40-cm-thick masonry wall without and with interior insulation after periodic equilibrium has been reached (that is, the simulation is run for several years, always applying the same yearly weather data, until the transient moisture profiles repeat from one year to the next). The blue area shows the bandwidth of the moisture contents occuring during one year, plotted over the cross-section of the walls. The dark curve describes the yearly averages of the moisture contents for the region between the exterior surface of the wall and the point close to the interior surface where the interior rendering or the polystyrene foam insulation begins.

Despite recurring water saturation of the facade during periods of intense driving rain, the average moisture content at the exterior surface corresponds roughly to the reference moisture content of the masonry because sunshine quickly dries the surface. Due to the strong moisture-dependence of capillary transport, however, deeper below the surface the average moisture content increases quickly. In the wall without interior insulation, the moisture content reaches a maximum at a depth of a few centimeters and then decreases quite smoothly until it reaches the hygroscopic dry level on the indoor side. Beyond a depth of about 20 cm, the moisture content shows no variation over the whole year, that is, the transient outdoor climate only affects the exterior half of the masonry.

Applying an interior insulation changes the moisture conditions in the masonry drastically. Below the exterior surface, the moisture content now increases even stronger, without ever decreasing towards the indoor side. There are two reasons for this: while the driving rain load has not changed, the diffusion resistance of the polystyrene insulation slab impedes the drying through the indoor surface, and in addition the average temperature level of the masonry is reduced by the insulation, so that drying towards the exterior side is slowed down, too. In comparison with the uninsulated case the total moisture content in the insulated wall is noticeably increased. The risk of frost damage is increased accordingly and can only be reduced by improved rain protection such as hydrophobic facade treatments [1].

Literature

Künzel, H.M. und Kießl, K.: Feuchte- und Wärmeschutz von Sichtmauerwerk mit und ohne Fassadenhydrophobierung. Mauerwerksbau aktuell 98 (1998) S. D48-D57.

 

Page created: 09 May 2007; last update: 17 Jul 2012

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