Moisture Content in Insulated Basement Walls

Transcription

1 Moisture Content in Insulated Basement Walls Peter Blom,PhD, SINTEF Building and Infrastructure; Sverre B. Holøs M.Sc. SINTEF Building and Infrastructure; KEYWORDS: moisture, basement, field measurement, simulations SUMMARY: The paper investigates moisture content in bottom and top sills in an internally insulated basement wall. Moisture and temperatures in the wall are measured from May 2007 until January The moisture content in the sills are also simulated in the program WUFI 2D. During the measurement period, the sills take up very little moisture. Long term simulations with high initial moisture content in the construction indicate that the sills in the wall dry out. However, this is not confirmed by the measurements, partly because of low initial moisture content in the sills and a short measurement period. Further, the bottom sill does not dry during the winter season as predicted by the calculations. The results from the project can not be used to confirm the validity of Norwegian guidelines concerning insulated basement walls. 1. Introduction In Norway, areas in basements are often used as normal, heated living spaces. Basements walls therefore need good heat insulation and protection against moisture damages. Given the often limited drying possibilities of built-in moisture in basement walls, one would prefer wall constructions without organic constituents. However, basement walls with an inner, insulated wooden framework are nevertheless popular, partly of economical reasons. Such walls may experience moisture problems of different causes: Built-in moisture due to exposure to rain or snow during the building process Ground water or storm water infiltrate through cracks in walls Condensation on concrete surface due to entrance of moist room air Condensation on concrete surface due to moist air entering in cracks between floor and wall In this paper, problems with built-in moisture from the building process are focused. There are numerous examples of rot in bottom sills in basements walls due to built-in moisture, especially in several kinds of prefabricated building elements combining in-situ concrete and an internal, wooden framework. In order to reduce the risk on moisture damages, the following guidelines are now recommended in Norway: At least 1/3 of the total thermal resistance should be located outside the concrete wall The wooden framework should be separated from the concrete wall with insulation, as shown in figure 1. No vapour barrier should be used on the inside of the wall, unless more than 50 % of the wall is above grade. The aim of the study is to examine whether these guidelines are sufficient to avoid moisture damages due to built-in moisture from the building process. The study is based on measurements in a basement wall (figure 1) in a new house outside Oslo and on calculations with the program WUFI 3.2 (

2 FIG. 1: Basement wall used for thermal and moisture measurements. The house is situated in Lommedalen, south of Oslo. 2. Measurements Description of the materials in the basement wall is shown in Table 1. Water vapour diffusion resistance factor and the initial water content are used in the WUFI calculations. The wall is instrumented with thermocouples, sensors for relative humidity and conductivity moisture meter for wood moisture content. Hourly data are stored on a logger inside the house. Measurement data from the period until are used in the paper. TABLE 1: Materials in basement wall. The materials are listed as shown in figure 1, ranged from inside and out. Material Width [mm] Water vapour diffusion resistance factor (-) Initial water content Gypsum board Mineral wool Sills and studs Expanded polystyrene Concrete, w/c ratio 0, Water membrane i) Expanded polystyrene i) Membrane under bottom sill and outside concrete wall Figure 2 shows measured relative humidity inside the wall and average moisture content in bottom and top sills. The relative humidity is measured between the mineral wool and the EPS-insulation. The moisture content in the sills is corrected according to actual air temperature. The moisture content in the bottom sill is higher and more constant than the moisture in the top sill. The moisture in the top sill reacts faster to changes in the surrounding climate. The sharp drop in the moisture content in the top sill towards the end of the measuring period is caused by a cold spell in the outside temperature, which in turn lead to low indoor relative humidity

3 Moisture content bottom sill Relative humidity (%) Measured RH outside mineral wool Moisture content top sill Moisture content by weight (%) jun 13.jul 03.aug 24.aug 14.sep 05.okt 26.okt 15.nov 06.des 27.des 6.0 FIG. 2: Measured moisture content by weight (%) in bottom and top sill and measured relative humidity between mineral wool and EPS. The relative humidityis measured 0,2 m above the bottom sill. 3. Simulations of heat and moisture transport 3.1 Calculation model WUFI 2D 3.2 ( is a program for calculation of coupled heat and moisture transfer in building components. WUFI 2D 3.2 calculate transient heat and mass transfer in two dimensions. The mathematical model in the program is described by differential equations for heat and moisture transfer. The differential equations are discretised by means of an implicit finite volume method and are iteratively solved (Künzel 1995). For heat transfer, the program takes in to account thermal conduction, short wave solar radiation, long wave radiation cooling and enthalpy flows through moisture movement with phase change. For moisture transfer, the program takes into account vapour diffusion, solution diffusion, surface diffusion and capillary suction. Convective heat and moisture transport is disregarded. 3.2 Simulation results The main input data for the calculations are shown in Table 1. The climate files used in the calculations are sinus-curves fitted to the measured climate data, see Table 2. Figure 3 shows the grid used in the calculation. Figure 4 shows measured and calculated moisture content in top and bottom sill throughout the measurement period. Figure 5 shows calculated moisture content in top and bottom sill assuming an initial high moisture content in sills and studs (18 % by weight) and concrete (90 % RH). The calculated moisture content fits reasonably well to the measured moisture content in the top sill. The calculations for the bottom sill, however, do not concur very well with the rather constant moisture content in the bottom sill. The long term calculations in Figure 5 indicate that the sills dry out

4 Table 2: Description of sinus climate files used in the calculations Climate Temperature/ amplitude Day of maximum Relative humidity (%) Day of maximum Indoor air 20/0-60/ aug Soil, m 11.5/3 3. aug Soil, 0-1 m 10/10 3. aug Outside air 10/20 3. jul 70/ aug FIG. 3: Grid used in the WUFI2D 3.2 simulations. The right figure is an enlargement of the corner of the wall

5 Bottom sill, calculated Top sill, calculated Moisture content by weight (%) Top sill, measured Bottom sill, measured apr 17.jun 06.aug 25.sep 14.nov 03.jan 22.feb FIG. 4: Measured and calculated moisture content by weight in top and bottom sill Moisture content by weight (%) Top sill Bottom sill nov.90 jun.91 des.91 jul.92 jan.93 aug.93 mar.94 sep.94 FIG. 5: Calculated development moisture content in sills, assuming a high initial moisture content in sills, studs and concrete - 5 -

6 4. Discussion The sills in the wall were quite dry when the building process was completed and the measurements started. During the measurement period, the sills take up very little moisture. The moisture content in the top sill seems to follow the changes in the surrounding atmosphere, while the moisture content in the bottom sill show small variations. The calculated moisture content fits reasonably well to the measured moisture content in the top sill (figure 4). The fit between measured and calculated values are poorer for the bottom sill. One possible explanation for this could be air leakages to the ground. Moist air from the ground may prevent the seasonal drying of the bottom sill during wintertime. Long term simulations with high initial moisture content in the construction indicate that the sills in the wall dry out. This is not confirmed by the measurements, partly because of low initial moisture content and a short measurement period. The wall construction does not fully comply with the guidelines for insulation of basement walls in Norway. Only about ¼ of the thermal resistance in the wall is located outside the concrete, it should have been ⅓. Secondly, the external insulation should have covered the concrete wall also above grade. Without the external insulation in this area, there is a condensation risk on the inner surface of the concrete. This type of condensation is not confirmed in this project, but both simple condensate calculations and general experience from practice indicate that no external insulation in this area is a problem. In general, it is a challenge for the building industry in Norway to prioritise external insulation of basement walls. 5. Conclusion Field measurements in an internally insulated basement wall show that the sills take up very little moisture during the first year after the building period. The calculated moisture content fits reasonably well to the measured moisture content in the top sill. The fit between measured and calculated values are poorer for the bottom sill, possibly because of air leakages to the ground. Long term simulations with high initial moisture content in the construction indicate that the sills in the wall dry out, with seasonal variations. However, this is not confirmed by the measurements, partly because of low initial moisture content in the sills and a short measurement period. Further, the bottom sill does not dry during the winter season as predicted by the calculations. Consequently, the results from the project can not be used to confirm the validity of the Norwegian guidelines concerning insulated basement walls. 6. Acknowledgements This paper has been written within the ongoing SINTEF strategic institute projects Climate 2000 Weather Protection in the Construction Process. The authors gratefully acknowledge the Research Council of Norway. 7. References Künzel, H.M. Simultaneous Heat and Moisture Transport in Building Components: One and two-dimensional calculations using simple parameters. Fraunhofer-Informationszentrum Raum und Bau. IRB Verlag, Stuttgart, Oustad, M., Gustavsen, A. Uvsløkk, S. Calculation of Moisture and Heat Transfer in Compact Roofs and Comparison with Experimental data. Proceedings of the 7 th Symposium on Building Physics in the Nordic Countries. Reykjavik, Swinton, M.C., Maref, W., Bomberg, M.T., Kumaran, M.K., Normandin, N..In situ performance evaluation of spray polyurethane foam in the exterior insulation basement system (EIBS)Building and Environment, Volume 41, Issue 12, December 2006, Pages