Moisture behaviour of slab-on-ground structures in operating conditions:
steady-state analysis Virpi Leivo*
Tampere University of Technology, Laboratory of Structural Engineering, P.O.Box 600, 33101 Tampere, FINLAND
Jukka Rantala
Tampere University of Technology, Laboratory of Structural Engineering, P.O.Box 600, 33101 Tampere, FINLAND
Keywords: slab-on-ground, moisture, steady-state analysis
Abstract: This paper examines the moisture behaviour of slab-on-ground structures in steady-state conditions where a capillary phenomenon is excluded and only water vapour diffusion is involved. Especially the impact of temperature of the subsoil, relative humidity of the indoor air and water vapour resistances of floor covering and thermal insulation has been studied. The subsoil or fill surface beneath a heated building is warm and moist (RH is near 100%). Therefore in many cases the water vapour diffusion flow is directed from the subsoil towards the drier indoor air, especially when the relative humidity of the indoor air is low. The critical point is usually the relative humidity on the lower surface of the floor covering. The moisture behaviour of the structure depends on the water vapour gradient across the structure and the water vapour resistances of the structural materials, especially of the floor covering. The adequate thermal insulation beneath the concrete slab ensures temperature gradient between subsoil and slab-on-ground structure and decreases water vapour gradient and diffusion flow to the indoor air. The lower water vapour gradient is a precondition for using safely water vapour tight floor coverings materials. The higher water vapour resistance of the thermal insulation also decreases the relative humidity of slab/floor covering –interface.
1. Introduction
The slab-on-ground structure is the only part of the building envelope that is partially or entirely in contact with the moist and warm subsoil surface or another constructed soil layer. Therefore the external moisture loads on slab-on-ground structures diverge considerably from the rest of the building envelope and the superstructures.
In many cases some moisture failures have occurred in slab-on-ground structures despite the fact that the capillary rise of water from subsoil is prevented. The cause of the problems is the water vapour diffusion flow from the subsoil into the structure.
This type of failure is typical in un-insulated slab-on-ground structures or after a repair work or renovation in which the floor covering in replaced. After the more careful examination it is often noticed that the water vapour permeability of the new floor covering material is very low.
Careful examinations of moisture damaged slab-on-ground structures reveal that the temperature and moisture behaviour of a ground slab is more complicated than usually presumed. The moisture and water transport mechanisms inside and through ground floor structures and adjoining soil layers are various and strongly dependent on the current temperature field. The structural moisture in massive in situ cast
concrete slabs, changes in indoor air temperature and humidity as well as the warming up of the subsoil caused by the steady heat flow from the heated building above can all change the moisture balance and behaviour of the ground floor structure.
The conditions at the coarse-grained subsoil or fill layer beneath a slab structure and the thermal and moisture behaviour of soil materials are studied widely by the authors of this paper and others [1,2,3,4,5]. The soil surface beneath a heated building is warm and moist and the water vapour content of the pore air is in many cases higher than the water vapour content of the indoor air, especially during winter. In many countries, as in Finland, it is legislated that the capillary rise from subsoil to the slab- on-ground structure must be prevented. The capillary break can be built in various ways. The coarse-grained gravel layer is the most common solution. According to previous studies [2] the gravel with a minimum grain size of d ³ 1 mm, prevents capillary rise effectively. The EPS insulation layer built with two overlapped sheets breaks also the capillary rise. On the other hand, the polyethylene sheet under a concrete slab is not recommended as it slows down significantly the drying of construction moisture from the in situ cast concrete slab.
2. Boundary conditions 2.1 Indoor air
The moisture level of the indoor air depends on the outdoor air humidity, the occupancy of the space or the building, moisture emission supplies inside the building, ventilation, hygroscopicity of the building materials, moisture flow through the building or room envelope and the amount of structural moisture evaporating during the drying period of the structures. The dependency of the indoor air humidity on the outdoor conditions is strong and the moisture emission rate inside the building increases this base level to a certain extend. The indoor air water vapour content can be estimated as follows (1):
V n
G
o
i =n + ×
n (1)
where
ni = indoor air water vapour content (g/m3) no = outdoor air water vapour content (g/m3)
G = moisture emission rate inside the space or building (g/h) n = ventilation rate (1/h)
V = volume of the space (m3).
The last term in (1) is a indoor moisture supply from the indoor air. The indoor moisture supply is typically in the range of 2 –4 g/m3 in residential buildings [6].
Figure 1 presents the average relative humidity of indoor air in Finland by two different indoor moisture supplies, 2 and 4 g/m3. Average outdoor air relative humidity is about 80 RH% (70,7 … 89,7 RH%) and water vapour content about 5,3 g/m3 (2,2 … 7,7 g/m3).
2.2. Subsoil
According to the literature and previous studies [1,2,3,4,5], the subsoil surface in contact with the slab structure of a heated building is relatively warm throughout the year, especially underneath the centre part of the slab. The temperature of the subsoil under slab-on-ground which has no thermal insulation is near indoor temperature.
Some cases the temperature of the subsoil can be even higher than indoor temperature if there is un-insulated heat pipes under the slab. The temperature level of the subsoil under insulated slab depends on thermal resistances of slab, subsoil and footing structure and temperature of indoor and outdoor air. At the same time surveys suggest that the relative humidity at the pore structure of the fill and drainage layers is close to RH = 100 % [1,2], though the capillary rise was prevented. This indicates that the water vapour content at the subsoil surface beneath the slab structure is in many cases higher than the vapour content of the indoor air, especially during the cold winter months. Thus, in normal conditions the direction of the water vapour diffusion flow occurs from the warm and moist subsoil surface towards the relatively dry indoor air.
2.3. Construction moisture
One of the most important moisture sources of slab-on-ground structures after the construction is the construction moisture of the in situ cast concrete slabs. Ordinary structural concretes contain a large amount of mixing water, and therefore the drying period of a slab is relatively long compared to the modern building schedules. An adequately long drying period of the concrete is especially important in structures where the drying process is possible in only one direction, such as composite slabs and slab-on-ground structures. Drying period to the covering criteria is essential compared to building schedules but the drying of the concrete slab towards steady- state takes still a long time, even years. Drying of construction moisture in concrete is a very complex issue, which is depended on properties of concrete and drying conditions of surroundings.
2.4. Critical moisture levels
Many building materials have a critical moisture level. If the moisture level in material exceeds the critical level, there is a risk of damage. In slab-on-ground structures the typical critical part is the interface between floor covering and concrete slab. The critical moisture levels according to [7,8]:
· Wood and wood based materials: RHcrit = 80%
· Organic contaminants (such as sawdust other building residues left on a concrete slab or in a crawling space): RHcrit = 80%
· Vinyl floor coverings with a backing which may provide nutrients for
mycological growth: RHcrit = 80%
· Bonded floor coverings, which do not tolerate degradation of floor adhesive by alkali in the concrete:
o Long-term moisture state (moisture remains for more than six months) For layered products: RHcrit = 90%
For homogeneous vinyl materials: RHcrit = 85%
o Short-term moisture states (moisture can dry out in a few months):
For layered products: RHcrit = 95%
For homogeneous vinyl materials: RHcrit = 90%
· Cork tiles:
o Without vinyl layer on the underside: RHcrit = 80%
o With a vinyl layer on the underside: RHcrit = 85%
· Screeds vapour barriers, filler, ceramic tiles. These shall withstand being laid in a moist alkaline environment for a long time. RHcrit = 100%.
However, if a vapour barrier is to be bonded to the concrete, the RH in the surface layer must be no higher than 90% before the adhesive has cured.
In general the relative humidity of RH = 75 % is considered as a limit value after which some fungus growth is possible.
3. Material parameters
The average values of thermal and moisture parameters of the structural materials used in the calculations are shown in Table 1. The applied parameters are detected from the literature [9,10].
4. Steady-state analysis
Thus the thermal and moisture behaviour of the slab-on-ground structure is depended on many varying variables (indoor and outdoor temperature and moisture) for many purposes the steady-state analysis is practicable. The steady-state analysis is adequate accurate in design phase when comparing different structures and in planning repairs when examining impacts of the different repair solutions.
The moisture behaviour of slab-on-ground structures is studied theoretically under steady-state conditions. In the analysis two different ground floor structures and varying combinations of thermal and moisture parameters of the structural materials are used under changing surrounding conditions. Both structures studied in this paper are very common in Finnish construction and moisture problems related in floor coverings of these structures are widely reported. The basic assumption of the moisture design of slab-on-ground structures is that the capillary rise from subsoil to the structural layers (insulation and slab itself) is prevented, thus the temperature depended water vapour content in the subsoil is the dimensioning factor. Therefore the only assumed moisture migration mechanism in all the calculations is water vapour diffusion.
The boundary values for the calculations are the indoor air temperature Ti and relative humidity RHi, and the corresponding values for the subsoil or fill surface (Ts, RHs). In all comparisons of this paper one of the variables is the temperature of the subsoil varying between Ts = +12…+24°C. The relative humidity at the contact surface between the slab and fill is assumed to be RHs = 100 %. In the performed calculations the relative humidity of the indoor air vary between RHi = 25 % … 50 %. The minimum value corresponds to the conditions in winter and the latter the average humidity level during summer. In both cases the indoor air temperature is assumed to be Ti = +19°C.
The varying parameters are the water vapour permeability of the thermal insulation and water vapour resistance of the floor covering material.
In steady-state studies the principal assumption is that both the heat and moisture flow are stationary and the temperature and moisture distribution over the structural cross- section can be determined by equations (2) and (3), respectively:
d T q T2 - 1
=l (2)
where q is the heat flow through a material layer with a thickness of d and thermal conductivityl. Term(T2 – T1) presents the thermal gradient over the layer andl/d =R is thermal resistance.
g =dvn2d-n1 (3)
whereg is the moisture flow through a material layer with a thickness ofd and water vapour permeabilitydv. Term(n1 -n2) presents the water vapour content gradient over the layer anddv/d =Zv is water vapour resistance.
The temperature and moisture content in different layer of the structure can be determined by equations (4) and (5), respectively.
(
i e)
T j j
j T T
R T R
T = -1+ -1 - (4)
whereTj is the temperature in layer j, Rj thermal resistance (=lj/d) of the layer j and RT total thermal resistance between Ti and Te. Term (Ti – Te) is the thermal gradient over the whole structure.
(
i e)
T j j
j v v
Z v Z
v = -1+ -1 - (5)
where vj is the moisture content in layer j, Zj water vapour resistance (=dv/d) of the layer j and ZT total water vapour resistance between Ti and Te. Term (Ti – Te) is the moisture gradient over the whole structure.
The relative humidity(jj) in the layerj can then be determined by equation (6):
( )
j sj
j v T
= v
j (6)
wherevs(Tj) is the humidity by volume at saturation at temperatureTj.
The dependency between humidity by volume at saturation, ns, and temperature,T, is assumed to be linear in calculations, although the dependency is actually non-linear.
The following expression (eq. 7) is used [11]:
) 15 , 273 (
4 , 461
100) (
+
= +
T b T a v
n
s (7)
0[Tá +30 ºC a = 288,68 Pa b = 1,098 n = 8,02 -20[ Tá 0 ºC a = 4,689 Pa b = 1,486 n = 12,3
The similar steady-state calculation method used for determining the possible dew point at the structure is a so-called Dew-Point method.
The two examined structural cross-sections are presented in Fig. 2. Both of them consist of in situ cast concrete slab built on top of a coarse-grained fill layer and subsoil. The latter structure also includes an underneath thermal insulation layer of expanded polystyrene (EPS). The material parameters of structural materials are presented in Table 1.
The varying parameter in calculations is the relative humidity of the indoor air (Fig. 3 and 4).
In all comparisons one of the variables was the water vapour resistance of the floor covering material. The vapour resistance of Zv = 370 × 103 (s/m) denotes a typical linoleum sheet and the higher value Zv = 1333 ´ 103(s/m) equals the resistance of a typical plastic floor covering used in public spaces.
Finally the effect of the water vapour permeability of the thermal insulation on the moisture level of the slab structure is considered.
The relative humidity of the lower surface of the floor covering is usually critical for the behaviour of the structure. Therefore the results of the steady-state comparison calculations are given as the humidity values at the slab/floor covering interface.
4.1 Analysis of results
Fig. 3 and 4 present the relative humidity at the slab/floor covering interface in varying surrounding conditions. The slab type A does not have an thermal insulation layer beneath the structure and therefore the overall moisture level at the interface is significantly higher than with the type B structure with ahi = 50 mm EPS-insulation.
Especially with the low water vapour permeable floor coverings the relative humidity at the interface is hazardously high even if the subsoil temperatures remain low (Fig.
3). The higher average indoor humidity increases the humidity at the interface. In Fig.
4 the curves end after the condensation occurs somewhere in the studied slab cross- section. Usually this happens somewhere in the insulation layer. After the first condensation the steady-state vapour diffusion examination is no longer relevant.
Effect of water vapour resistance of the thermal insulation on the relative humidity of the surface structure is presented in Fig. 5. Two different types of insulations are used:
an expanded polystyrene insulation (EPS) and mineral wool insulation (Table 1). The applied water vapour resistance of the materials is presented in Table 1. The water vapour resistance of the polystyrene layer is approximately 70 times higher than the resistance of the mineral wool layer (Table 1). The higher the water vapour resistance of the thermal insulation the lower is the relative humidity of slab/floor covering – interface. The effect of the water vapour resistance is larger in high subsoil temperature.
5. Discussion
The primary function of the slab-on-ground insulation is to reduce the heat flow through the slab structure and the overall energy consumption of the building. Yet, the slab insulation has another important task by reducing the water vapour diffusion flow potential from the moist subsoil by reducing temperature of the subsoil. Under typical conditions of slab-on-ground structures, the water vapour content of the pore air in warm subsoil is higher than the vapour content of the indoor air, assuming that the humidity of the pore air is close to RH = 100 % at all times. Therefore the slab-on- ground insulation must be placed mainly or entirely underneath the slab. This ensures a higher slab temperature and lower moisture content of the structural layers above the insulation. This is evident also in the comparisons performed for the un-insulated and insulated ground slabs (Fig. 3 and 4). The relative humidity at the slab/covering – interface is significantly lower with the insulated ground slabs due to the high water vapour permeability of the underneath EPS –insulation and thermal gradient between the slab and the moist subsoil.
The effect of the water vapour permeability of thermal insulation on the relative humidity of the surface structure is compared in Fig. 5. The expanded polystyrene and mineral wool insulations both have the same thermal resistance, but the water vapour resistance of the polystyrene layer is approximately 70 –times higher than the resistance of the mineral wool layer (Table 1). The effect is evident in Fig. 4 as the humidity at the slab/covering –interface is significantly lower with the EPS- insulation.
The warmer the subsoil, the higher the vapour diffusion potential and the diffusion flow through the slab structure. According to Swedish research [12] the required temperature difference between subsoil and indoor air should be at least 2 … 3°C.
Compared with the values performed in Fig. 4 the requirement seems to be at correct level.
The importance of the floor covering material for the overall moisture behaviour of the structure is significant. The relative humidity at the slab/covering –interface increases as the vapour diffusion potential, i.e. the partial vapour content difference over the structure increases. Surface structures and floor coverings can be divided into two categories according to their water vapour resistance: permeable coverings and impermeable coverings. Permeable coatings include materials with an average water vapour resistance ofZv = 400 ´ 103 s/m (Zp ~ 50 ´ 109 (m2 s Pa)/kg). Impermeable coverings are materials with an average water vapour resistance ofZv = 1100 … 1400
´ 103 s/m (Zp < 180´ 109 (m2 s Pa)/kg ).
The impermeable covering material, especially with an un-insulated floor structure, increases the relative humidity of the slab significantly. In steady-state calculations the humidity at the slab/coating –interface is over RH = 80 % even if the subsoil temperature remains below Ts = +13°C in winter (RHi = 25 %) (Fig. 3) Use of the insulation improves the overall conditions of the slab, as the RH = 80 % level exceeds only after the subsoil temperature rises over Ts = +18°C in summer (RHi = 50 %) (Fig. 4) Therefore the impermeable floor coverings can be safely used only on adequately thermally insulated slab-on-ground structures.
The permeable covering material allows the use of un-insulated slab structure, especially if the flooring material itself and the possible adhesive have a high critical moisture level. The relative humidity at the slab/coating –interface remains below RH
= 80 % at all times, even if the subsoil temperature rises relatively high, Ts = +24°C (Fig. 3. Therefore the only permeable floor coverings should be used on un-insulated slab-on-ground structures. This should be also remembered in repairs and renovations of the slab structures.
6. Conclusions
The basic assumption of the moisture design of slab-on-ground structures should always be that the capillary rise from subsoil to the structural layers (insulation and slab itself) is prevented, thus the water vapour concentration in the subsoil is the dimensioning factor. The assumption is that the pore air relative humidity of the subsoil is RH = 100 %.
The subsoil or fill surface beneath a heated building is warm and moist. Therefore in many cases the water vapour diffusion flow is directed from the subsoil towards the drier indoor air, especially when the relative humidity of the indoor air is low. The moisture behaviour of the structure depends on the water vapour gradient across the structure and the water vapour resistances of the structural materials, especially of the floor covering. The importance of the water vapour resistance properties of the floor covering material for the overall moisture behaviour of the structure is significant.
The slab-on-ground thermal insulation must be placed mainly or entirely underneath the slab. This ensures a higher slab temperature and lower moisture content of the structural layers above the insulation. The adequate thermal insulation ensures temperature gradient between subsoil and slab-on-ground structure and decreases water vapour gradient and diffusion flow to the indoor air. The lower water vapour gradient is a precondition for safely using water vapour tight floor coverings materials.
7. References
[1] Leivo V., Rantala J. Thermal and Moisture conditions of Coarse-grained Fill Layer Under a Slab-on-Ground Structures in Cold Climate. Journal of Thermal Envelope and Building Science, 2004, Vol.28(1):45-60.
[2] Leivo V., Rantala J. Moisture behaviour of slab-on-ground structures.
Tampere Universty of Technology. Department of Civil Engineering. Laboratory of structral engineering. Research report 122, 2003. 100 p. + 12 annexp.
[3] De Vries D. A., Afgan N. H. Heat and mass transfer in biosphere. John Wiley
& Sons, 1975.
[4] Kersten M. S. Thermal properties of soils. Engineering Experimental Station.
University of Minnesota, Bull. No. 28, 1949.
[5] Sundberg J. Thermal Prperties of Soils and Rocks. Publ. A 57, dissertation.
Department of geology, Chalmers University of Technology and University of Göteborg, 1988. 310 p.
[6] Hagentoft C-E. Introduction to Building Physics. Lund. Studentlitteratur.
2001. 422 p.
[7] HusAMA 83. (General Materials and Workmanship Specifications for Buildings). Swedish Building Centre, Solna. (in Swedish).
[8] Hedenblad G. Drying of conctruction water in concrete, drying times and moisture measurement. Stockholm; Byggforskningrådet, 1997. 54 p.
[9] Hedenblad G. Fuktsäkerhet I Byggnader: materialdata för Fukttransportberäkningar. Stockholm; Byggforskningrådet, 1996. 55 p. (in Swedish) [10] Hedenblad G. Moisture Permeability of Mature Concrete, Cement Mortar and Cement Paste. Dissertation. Division of Building Materials. Lund University of Technology, 1993. 250 p.
[11] DIN 4108-5. Thermal insulation and energy economy in buildings - Part 4.
DIN Deutsches Institut für Normung.
[12] Harderup L-E. Concrete slab on the ground and moisture control. Verification of some methods to improve the moisture conditions in the foundation. Lund Institute of Technology, 1991. Doctoral dissertation. 174 p.
Table 1 Moisture and thermal parameters of the structural materials.
Material Thermal
conductivity l W/m°C
Water vapour permeability
dv
´ 10-6 m2/s
Water vapour resistance Zv=d/dv
´ 103 s/m
Concrete 1.5 0.27 -
Pexpanded
polystyrene (EPS)
0.037 0.16 -
Mineral wool 0.041 11.5 -
Construction paper - - 14.8
Fig. 1 Average relative humidity of indoor air in Finnish climate.
Average indoor air relative humidity
0 10 20 30 40 50 60 70 80 90
1 2 3 4 5 6 7 8 9 10 11 12
Month
Relative humidity, RH%
Moisture supply 4 g/m3 Moisture supply 2 g/m3
Fig. 2 Structure of the examined slab-on-grounds.
1 floor covering
2 concrete slab 80 mm
3 expanded polystyrene EPS 50 mm reinforced building paper
4 capillary break layer 1 floor covering
2 concrete slab 80 mm
reinforced building paper 3 capillary break layer
1 2
3 4 1 2 3
Slab B Slab A
Fig. 3 Effect of relative humidity of the indoor air and the water vapour resistance of the floor covering on the relative humidity of the slab/coating interface. Slab A
Fig. 4 Effect of relative humidity of the indoor air and the water vapour resistance of the floor covering on the relative humidity of the slab/coating interface. Slab B
Relative humidity RH (%) at the lower surface of the floor covering
12 13 14 15 16 17 18 19 20 21 22 23 24 Subsoil temperature (oC)
100 90 80 70 60 50 40 30 20 10 0
Effect of indoor air RHi (%) Slab A
Zv= 1333*103 s/m
Zv= 370*103 s/m
RHi =25 % RHi =25 % RHi =50 %
RHi =50 %
Typical subsoil temperature area in Finnish climate
Relative humidity RH (%) at the lower surface of the floor covering
Subsoil temperature (oC)
Effect of indoor air RHi (%) Slab B
Zv= 1333*103 s/m
Zv= 370*103 s/m RHi =25 % RHi =25 % RHi =50 %
RHi =50 % 100
90 80 70 60 50 40 30 20 10 0
12 13 14 15 16 17 18 19 20 21 22 23 24 Typical subsoil temperature area
in Finnish climate
Fig. 5 Effect of water vapour resistance of the thermal insulation on the relative humidity at the slab/covering –interface.
Zv= 1333 *103 s/m
Relative humidity RH (%) at the lower surface of the floor covering
12 13 14 15 16 17 18 19 20 21 22 23 24 Subsoil temperature (oC)
100 90 80 70 60 50 40 30 20
Effect of water vapour resistance of the thermal insulation, Slab B
Zv= 370 *103 s/m
10 0
Polystyrene Mineral wool
Mineral wool Polystyrene
