Measurement of moisture conditions in cellulose insulated walls and roofs with or without PE-vapour barriers in 32 occupied Danish dwellings

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1 Measurement of moisture conditions in cellulose insulated walls and roofs with or without PE-vapour barriers in 32 occupied Danish dwellings Georg Christensen & Tommy Bunch-Nielsen Building and Environment Technology LtD Staktoften Vedbæk Denmark ABSTRACT: In a Danish housing estate moisture sensors have been installed in 32 dwellings part in walls and part in roofs. The walls and roofs were insulated with cellulose insulation and were either with or without PE-vapour barriers. In the paper is described the results from moisture measurements during two winters. Also the indoor climate has been recorded in the dwellings. The result was that under Danish conditions neither the moisture conditions in walls and roofs nor the indoor climate depended on whether a PE-vapour barrier was used or not. 1 INTRODUCTION 1.1 Background When using the so called alternative insulation materials originating from e.g. cellulose or flax it is often a strong desire to construct walls and roofs without using the traditional PE-vapour barrier. A reason for this is that it is often claimed that omitting the PE-vapour barrier will improve the indoor climate. In order to investigate the effect of omitting the PE-vapour barrier the moisture conditions have been recorded with built in moisture sensors in 16 walls and 16 roofs during a two year period in normal occupied dwellings. The indoor climate was recorded in the same period. The walls were traditional wood stud walls and the roofs were traditional ventilated roofs with wooden roof trusses, a metal roof covering and a horizontal ceiling. Roughly half of the studied walls and roofs were constructed without and the other half with a vapour barrier in order to compare moisture conditions in the two situations. 1.2 Moisture sensors The moisture sensors consist of small dowels with built in electrodes and thermocouples. The diameter is 10 mm and the length is 25 mm. The sensors are shown in fig 1. A sensor is calibrated so that the measurement of an electrical resistance adjusted for the temperature gives the moisture content in a piece of fir placed in the same position. This means that when a dowel is placed in the insulation material it does not in principle indicate the moisture content in this material. However previous experience has shown that for cellulose insulation materials the deviation is quite small. Figure 1. Moisture sensor consisting of wood dowel, electrodes and thermocouple.

2 The outputs from the sensor are in LogMohm and millivolt and are recorded on an instrument shown in figure 2. Measurements were carried out with rather long intervals one or two months - since moisture contents vary very slowly. the studs and in the insulation material just behind the wind barrier. A section of the wall is shown in figure x 150 mm wood profiles 22 x 50 mm lath 9 mm gypsum board 220 mm Cellulose insulation 13 mm plywood 13 mm gypsum board Moisture sensor Photo has been sent okt. 25, 2002 Figure 2. Instrument for measuring electrical resistance and temperature. 1.3 Meters for measurement of indoor climate The indoor climate (temperature and relative humidity) was measured using small Dickson data loggers programmed on a PC and the results were processed by a computer programme. The data loggers have a size approximately as a package of cigarettes and are powered by means of batteries. They were during the two winters placed in the dwellings on e.g. a shelf away from any heat or moisture producing appliances. 1.4 Walls The walls were ordinary wood stud walls with 220 mm of insulation in the form of cellulose fibres (granulated newspaper). Boric salts are added to improve the resistance against rot and decay and to improve the fire properties. On the interior side there was a 13 mm plywood and as the inmost layer a 13 mm gypsum board. This combination of the boards was chosen partly in order to stabilize the walls and partly in order to give fire protection to the combustible insulation material. On the exterior of the walls there was a wind barrier consisting of a 9 mm gypsum board and a traditional ventilated wood facade. All moisture sensors were placed between Figure 3. Section of wall with moisture sensor behind wind barrier. The ratio between the moisture diffusion resistances of the interior layers (13 mm gypsum board, 12 mm plywood and 220 mm celluloseinsulation) and exterior wind barrier is: Zint Zex 0,6 + 4,0 + 1,0 = = 14 0,4 Where Z is in GPa m 2 s/kg This ratio is more than 5-10 which is normally accepted as a safe ratio to avoid in terstitial condensation. Measurements were carried out in a two storey row housing estate and both the ground floor and first floor walls were monitored. In some parts (app. 10 m 2 ) of the gable walls a vapour barrier in the form of a 0.15 mm PE-foil was placed and the rest of the walls were without a vapour barrier. 1.5 Roofs The roof construction consisted of traditional wood trusses with a span of 8 m. The roof had a slope of 25 degrees and was covered with steel panels with an anti-condensation layer on the bottom side.

3 The ceiling consisted of two layers of 13 mm gypsum board nailed to laths fixed to the foot of the wood trusses. In the houses with a PE-vapour barrier this was placed between the two layers of gypsum board. The insulation had a thickness of 300 mm and the same material was used as for the walls. Figure 4 shows a section of the ceiling. 300 mm 200 mm Moisture sensors in the wood Cellulose insulation Wood truss foot Laths Two layers of 13 mm gypsum board 1.6 Indoor climate Indoor climate was measured in all dwellings during the two winter periods. The reason for making these measurements was partly to explain possible extraordinary results from the moisture measurements in the walls and roofs and partly to study any differences in the indoor climate depending on the presence of PE-vapour barriers. Figure 4. Section of ceiling with the position of two moisture sensors. In part of the dwellings a 0.15 mm PE-foil is placed between the gypsum boards. Figure 5b. Moisture content in walls facing west 1 and 2 first floor, 3 and 4 ground floor.

4 2 RESULTS 2.1 Walls Typical examples on moisture measurements in façade walls without a vapour barrier but facing different directions are shown in figure 5. Measurements are caried out in ground floor walls as well as first floor walls. In both stories walls are facing east as well as west. In both cases moisture levels in the end of the winter period are around 20% but only for short periods and only when the temperature is low. Everything then seems to indicate that the moisture conditions in both cases are quite satisfactory. Figure 6 shows results from measurements in gable walls where part of the wall is without a vapour barrier and the other part with a vapour barrier. The gable walls are facing north and south. In both cases the moisture conditions are satisfactory since the moisture content is only around 20% at its maximum. This is the case in the wall facing north and only during a short period in the end of the winter. In none of the cases it can be demonstrated that a PE-vapour barrier will give better moisture conditions in the wall. Figure 6. Moisture content in gable walls with and without PE-vapour barriers. In top north and bottom south facing wall.

5 2.2 Roofs Moisture sensors were placed in four different positions in the roof trusses. Three were drilled into the wood in places where the wood truss was exposed to the ventilation air. One was drilled into the bottom of the roof truss foot and completely covered by the insulation material. Figure 7 (top) shows measurements from a roof without a vapour barrier in the ceiling and figure 8 (bottom) the similar with a PE-vapour barrier. No difference can be seen. It should be noted that the very low moisture content shown in the figure is the result from the sensor placed in the bottom of the roof truss. (3 and 7). Figure 7. Moisture content in wood truss roof construction. Top without and bottom with PE-vapour barrier.

6 2.3 Indoor climate During the first winter the average water content in the dwellings was 7.2 g/m 3 with a standard deviation of 1.16 g/m 3. In the second winter the similar numbers were 7.5 g/m 3 and 1.07 g/m 3. No distinction is here made between dwellings with and without vapour barriers since the measurement showed very similar results. The indoor climate has in all cases been in what in Denmark is named room climate class 2. According to the Danish definitions this means a water content in the room air between 5 and 10 g/m 3. The maximum value measured in the dwellings was 9.5 g/m 3 which means that all dwellings had room climates within room climate class 2. Under Danish winter conditions it is common to assume that the outdoor air has a water content of app. 4 g/m 3 which means that in average water vapour is added in an amount of about 3.5 g/m 3 in the dwellings. It should be noted that in four of the dwellings PE-vapour barriers were covering the total area of the walls as well as the roofs. In four dwellings PE-vapour barriers were used in part of the walls. The rest of the dwellings were without vapour barriers at all. Comparison between the recorded temperatures and humidities between the three groups showed no significant differences. It should also be mentioned that calculations have shown that water vapour diffusion through walls and roofs will only account for up to 10% of the moisture removed from the dwelling, the rest is removed by ventilation. For this reason it is not surprising that no difference was found between the three situations. 3 CONCLUSIONS Measurements of the moisture content in traditional wood stud walls and traditional ventilated wood truss roofs both insulated with cellulose fibre insulation have shown, that it is not possible to find any significant difference (if any) between the moisture content in the walls and roofs with or without the traditional PE-vapour barrier on the warm side of the insulation material. Regarding the indoor climate it was not here possible to detect any difference between the dwellings with and without PE-vapour barriers. It should be mentioned that these conclusions are based on field measurements under Danish climate conditions and further, that in the design of the walls and roofs great emphasis had been placed on securing air tightness in the constructions in order to avoid moisture transport by convection. 4 ACKNOWLEDGEMENT This field study has been supported by the Danish Ministry of Energy and Environment. 5 REFERENCES Christensen, Georg & Bunch-Nielsen, Tommy, 2002, Munksøgaard. Measurement of moisture content in roofs and walls in a housing estate encompassing 100 dwellings. (In Danish). Building and Environment Technology LtD, June Building envelopes with cellulose and flax insulation, examples. (In Danish). Building and Environment Technology LtD & Danish Fire Technical Institute, June Thermal and Moisture Technical Investigations of Alternative Insulation Materials. (In Danish) K.K. Hansen et. al. Report R Danish Technical University, Dec. 1999

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