Influence of different surface systems and humidity-controlled ventilation systems with heat recovery on the indoor climate in flats

Transcription

1 Influence of different surface systems and humidity-controlled ventilation systems with heat recovery on the indoor climate in flats J. Dreyer, T. Bednar & C. Hecht Department of Building Physics, Universitjl of Technology Vienna, Austria Abstract In Austria a lot of buildings suffer from rising damp and weathering, so that a high moisture content exists in the construction. Frequently problems start after the reconstruction, because new airtight windows are built-in and the high moisture content in the walls induces mould growth and other unhealthy and uncomfortable states. Investigations are done to simulate the behaviour of rooms in buildings with moisture problems to calculate the indoor climate. By the usage of prediction tools for construction behaviour and indoor climate it is possible to evaluate methods like mechanical air change with and without heat recovery, additional heating or different surface finishes. A ventilation device with humidity regulation and heat recovery can be used successfully to guarantee a comfortable and healthy indoor climate. In the paper the interaction and influences of moisture controlled ventilation with or without heat recovery on the moisture states of the indoor air are presented. In the recent past render systems, which have a low capillary moisture transport capability, have been developed for the renovation of such buildings. Investigations about the success of those measures showed different results. A sufficient exact prediction is available using a new developed program, which is able to predict the indoor climate under a given usage and the coupled heat and moisture transfer (vapour + liquid) in constructions. The simulation gives information about the moisture states inside the construction elements, the drying process, the indoor and surface temperatures and the heat loss.

2 5 82 Structural Shtdies, Repairs and Maintenance of Historical Buildings 1 Introduction For the renewal of buildings with new building materials and methods, knowledge of the construction, the current state and the destruction processes is necessary. Very often after a retrofitting measure unhealthy indoor climatic states, structural problems and technical defects appear. The reasons could be new inhabitants, change of utilisation, new heating systems or heating regimes, new materials and so on. To avoid such problems in the field of conservation, reconstruction and retrofitting of buildings the behaviour of the buildings, considering the interaction of the user, materials and climatic conditions. must be described. The thermal and hygric states inside the materials, constructions and buildings change slowly and it could last several years until dangerous states appear. Therefor a performance tool is necessary to predict the thermal, moisture and climatic states in a time period of about 10 years and more. A systematic approach for compatible renovation and retrofitting includes the measuring of the present state, the description of the real complex processes by simulation and the investigation of the long term behaviour of the construction after retrofitting. With this methods the impact of a renovation can be investigated and it is possible to predict the behaviour of the constructions and the indoor climatic states after the renovation. In this way the performance of a renewal of buildings is optimised and negative effects can be minimised. 2 Calculation of indoor air states in rooms with a high moisture load under influence of ventilation and heat recovery In some cases the construction elements of old buildings are loaded with a high moisture content because of condensation. weathering or arising moisture. Very often it is not possible to retrofit the defect, although an acceptable indoor climate is wanted. In this cases one can install a ventilation system to improve the climatic conditions. To describe the influence of moisture production by inhabitants, heating system, evaporation of wet construction elements, air change etc. to the indoor air state a modelling of the processes is done. At first a description of the heat balance of the indoor air of a building or room is necessary. The following eqn (1) demonstrates a characteristic type, which allows the calculation of the interior air temperature in dependence of the properties of the construction and climatic condition. (c. m), effective heat storage capacity of construction elements and indoor air, T temperature (int... internal, ex... external, ex,i... external temperature for element i), 4 output of the heating system per volume unit,

3 V interior volume of the building, 4 area of a construction element i, U, heat transmission coefficient of a construction element i, n air exchange rate. rl efficiency of the heat recovery system ( q =O means no heat recovery), c,,,.p,,, specific heat capacity per volume unit of air. Furthermore it is necessary to calculate the surface temperature of the elements including thermal bridges, to identify condensation problems. For the determination of the indoor humidity under influence of air change, climatic condition and construction qualities one must consider the moisture balance of the indoor air, that means the evaporation of wet construction element, the condensation at thermal bridges, the moisture load respectively unload by air change and so on, given in eqn (2). P R ~~0 P.i partial vapour pressure (int..indoor air, sj... saturation pressure at surface of element j, e..external specific gas coefficient for water vapour surface coefficient for water vapour transfer 3 Investigation of Indoor Climate with a High Moisture Load under Influence of Ventilation and Heat Recovery To investigate the influence of several measures, the interior humidity is calculated considering the thermal insulation level and several critical points at the surface of buildings elements. The critical points are positions at thermal bridges and construction elements in contact with the earth ground. The moisture content of the walls and the floor is high. For several conditions, like heating, air change, humidity regulation and heat recovery the behaviour of the indoor states are simulated for a whole year. An example of this simulation shows Figure l. In this figure one can see that during the cold period of the year the interior humidity is normal. In times with higher outdoor temperature the humidity increases and condensation happens. The humidity exchange with outdoor air can be positive and negative in dependence of the climatic states. The graph "amount of removed moisture" describes the drying process over the whole year which works in spite of some moisture load processes. By the help of this simulation the influence of technical measures like humidity control and ventilation with heat recovery are investigated. The humidity control works in the following way. Two pairs of moisture and temperature sensors measure the temperature and the humidity inside and outside. If the inside moisture content is

4 584 Structural Studies, Repairs and Maintenance of Historical Brtildings higher the air change is turned on using a fan, in the opposite case the device will be switched off. This ventilation system guarantees that no moisture load happens by the mechanical air change. temperatur ["C], relative humidity [%l, amount of removed moisture [kg] time [h] Figure 1 : Simulation of the interior temperature and humidity versus time in the period of one year for a room with a low moisture load by evaporation of wet walls and floor (reduced moisture transport through plasters and coatings). In the case of ventilation with heat recovery one can avoid that during cold time periods of a year unheated rooms cool down and the heat loss of heated rooms increases. Naturally the influence of this measures depends on the qualities of the construction elements, especially the heat insulation level, the air change per hour and the climatic conditions. In spite of this fact a generalisation is possible. In table 1 some results are summarised. Table 1 : Influence of measures on condensation, cooling and evaporation Ventilation Humidity control humidity control and heat recovery l influence on \ I I I I

5 In the next figure 2 some simulation results of the evaporation In dependence of the efficiency of the heat recovery are presented. The different amount of evaporation in the case of an unheated and a heated room are presented. The unheated room is for example a cellar used only for storage. The heated room is a room for storage of sensitive goods, where a minimal temperature level is necessary. The power of the heating system is low only to guaranty a lowest temperature level in the room. Clearly it can be seen, that a heating of the rooms increases the amount of water, which evaporates. This is the effect of higher temperatures by heating. This result is not unexpected. Through simulations kvith real climatic conditions the quantity of the effect can be calculated. Figure 2 shows, how the lower temperature level and the dehumidification effect increases by the efficiency of the heat recovery. In case of an retrofitting measure of buildings with high moisture contents inside the construction a control of the drying process is necessary. amount of removed water [kgtyear] amount of removed water [kglyear] wlthout heat lnsu~a~on without heat tnsulabon l T l minimum and maximum temperature pc] l I ith heat insulation ut heat insulation l effictency of heat recovery [-l Air change 0,5 h", humidity control minimum and maximum temperature rc] Max. 1 L -- n wllh heat lnsulabon Mm. A l l FZ/~~th~~t heat msuiation ~ l efficiency of heat recovery [-] Air change 0,5 h-', humidity control heating 5 W/m3 Figure 2 : Evaporation and moisture exhaust of rooms with wet buildings constructions (high moisture load) under the influence of air change, heating and heat recovery. The efficiency of the heat recovery system influences the minimum temperature of the indoor air and increases the amount of evaporation. A good insulation level supports this process.

6 5 86 Structural Studies, Repairs and Muintenance of Historical Buildings 4 Modelling of the drying process after retrofitting Traditional performance evaluation methods are not applicable to simulate drying processes. By using the standard calculation methods no information is available about the relative humidity in and outside the construction, about evaporation and condensation. about rain load and rismg damp and about built-in moisture during the renovation time. To calculate the moisture content of construction by weathering, condensation, defects etc. and to get a better understanding of the moisture behaviour of the constructions, one can use simulation tools for the heat and moisture transfer processes in constructions. The models presented in [I] are capable of calculating the moisture transfer in the vapour and the liquid phase and in this way the behaviour of materials under the influence of several effects can be investigated with them. Those models incorporate several different assumptions. It is well known, that the driving force for liquid moisture transfer are capillary forces. To calculate the diffusive transport, the mass transport caused by the motion of gas molecules, one has to account for two different driving forces. One driving force is the number density and the other one is the temperature which determines the kinetic energy of the molecules. By considering a temperature dependent collision cross-section one can get the following equation for the mass flux in water vapour under partial pressure and temperature gradients. Usually the second term is of minor importance and therefore neglected. *PO vapour difhsion coefficient of water in air [kglm s Pal P partial vapour pressure [Pal T Temperature [K] By using eqn 3 and the suction pressure as the driving force for capillary water transport the following differential equation describes the moisture transport W moisture content [kg/m3], p,,, suction pressure [Pal gas constant for water [J/ kg K], p, vapour concentration [kg/m3] p,(t) vapour saturation pressure [Pal p liquid water density [kg/m3] w(p,,,)storage function [kg/m3] K(p,,,) liquid water conductivity [kg~m s Pal p diffusion resistance [-] DM(w) moisture transfer function [m2!s]

7 The heat transport by heat conduction and latent heat transport is described by P partial vapour pressure [Pal C(w) heat capacity [ ~ i m ~ ~ ] h(w) thermal conductivity [W/mK] 5 Investigation about the indoor humidity after renovation and the influence of surface systems To demonstrate the described method different renovation strategies for a flat were investigated. At the ground floor the walls are assumed to suffer from raising damp. To evaluate the retrofitting measure, under the assumption, that a working moisture barrier was build. the effect of an inside plaster system or an inside lining board was taken into consideration. The table 2 gives some information about the investigated render systems Table 2. List of investigated inside plasters! systems. name I Inside renovation system KM dry I Lime plaster KM 1 Lime plaster KZM Lime cement plaster SP Hydrophobic renovation render SanSys Renovation lining board based on calciumsilicate Initial moisture content of masonrv 80% r.h. 100 kg/m3 100 kglm3 100 kg/m3 l00 kg/m3 Construction, geometry utilisation ventilation Table 3. Parameters describing the investigated flat. 60 cm full brickwork new windows household with 2 persons minimum ach Opended windows tilted windows n=lih, for 20 minutes heating Temperature Q H.,~ < l0 kw, Q H.a,=O if T>2Z C 1 controlled heating 1. flat area: 70 m2,room height: 3 m, moisture evaporation area: 250 m2 glazed area 9 m2,uw=1.73w/m2k moisture production 4.8 kg/d mean heat production 570 W nz0.2 l/h (tight windows) N=10 1k;two times per day lasting 10 minutes only if room temperature >22S C l device - / or Q 000 (~~oc-t,)w 1

8 588 Structural Studies, Repairs and Maintenance of Historical Buildings Figure 3 shows the result of the calculation of the drying process of the wall after a successful fitting of an moisture barrier against rising damp and by consideration of rain load and evaporation at the inside and outside surfaces. Table 3 shows the using conditions of the flat and the influence of the inhabitants. The drying process lasts over a time period of about ten years. During the years with an increased moisture content inside the walls, an increased humidity load for the indoor environment exists. This increased humidity load can induce a grow of mould at critical areas of the surfaces of the outside walls. Therefor a calculation of the relative humidity must be carried out distinguish between the different inside surface renovation systems. N. E cn Y C t Q) C c 0 0 Q) L 3.- fjy 0 c time from start of drying [years] Figure 3. Moisture content of the whole wall over 10 years. The influence of the different surface systems is only minor. As a reference a wall with an initial moisture content equalling 80% relative humidity is added to the figure. Figure 4 and 5 show the calculated relative humidity at the surface and for the inside air over a period of ten years. It can be seen, that the built-in moisture due to the defective moisture barrier causes a strong increase of the relative humidity. This problem is essential for about three years. The relative humidity at the surface is about 15 percentage higher than in the indoor air. The risk for mould growth does exist if the relative humidity is higher than 80 percentage. The figures show that such a risk exists in the first three years for the surface of constructions and two for objects in the room. The lowest surface humidity during the first year can be achieved with the renovation render system which has a hydrophobic character. From the second year on, the renovation lining board has the lowest relative humidity at the surface and inside the room, because of its low thermal conductivity. The surfaces of the construction stay a little bit warmer than for the other render systems and therefore relative humidity is lower at the surfaces.

9 Figure 4. Surface relative humidity over ten years As the difference in the behaviour for the render systems is small they are combined to the grey area. The black line is the behaviour of the flat with the renovation lining board ' 9 time from start of drying [years] Figure 5. The grey area belongs to lime, lime cement and a hydrophobic renovation render and the black line is the renovation lining board The results of the investigation show that during the first year the risk for mould growth is very high. After the first year the heat insulating lining board results in the lowest relative humidity and therefor the risk for mould growth is lower than in the other cases.

10 590 Structurai Studies, Repairs and Maintenance of Historical Buildings 6 Discussion For the renewal and renovation of old buildings it is very important to pay attention to the possibility of critical states of the indoor climate. To find the best building solution the calculation of the humidity in the rooms by considering built-in moisture and all moisture transfer processes is necessary. For the case of high moisture contents inside the walls due to damages or bad moisture barriers one must use technical possibilities to decrease the inside humidity. Ventilation with a moisture control is a good way to take away the built-in moisture. The usage of a heat recovery in the ventilation is essential for heated rooms. For an acceptable retrofitting moisture barriers and a good weather protection are important. The simulation of the drying process shows that it is relative long and the indoor air reaches critical humidity levels during the first years. Several inside renovation systems like lime render, cement-lime-render, renovation render or renovation lining board have an influence on the relative humidity at the surface. Systems with a high capillary moisture transfer, like lime render and cement-lime-render produce a quicker drying and a higher surface humidity. The high moisture transfer to the indoor environment from those surfaces increases the danger of mould growth because of the higher indoor air relative humidity. Systems like renovation render and renovation lining board have a little lower possibility to evaporate the moisture and produce lower surface humidity. This effect helps to avoid mould growth because of the lower indoor relative humidity. During the first years the relative humidity behind the lining board stays above 80%. As the ph factor is very high and the fixing of the boards has to prevent air circulation behind the board no mould growth occurs behind the board. An advantage of the usage of an insulating lining board is the enhancement of the surface temperature. Generally it is necessary to take care of the indoor air quality during the first years after renovation measures because of the high moisture evaporation from the walls. If it is not possible to ensure a high air change rate to get acceptable indoor relative humidity, drying of air can not be avoided. References [l] Kiinzel, H.M. (Hrsg.), Praktische Beurteilung des Feuchteverhaltens von Bauteilen durch moderne Rechenverfahren, WTA Schriftenreihe, Heft 18, Aedificatio Verlag, 1999 [2] Adan, O.C.G, 1994, On the fungal defacement of interior finishes, Thesis, University of Technology Eindhoven, Netherlands