Hygrothermal Performance of Ventilated Cold Roofs an Experimental Study

Transcription

1 Hygrothermal Performance of Ventilated Cold Roofs an Experimental Study Andreas Holm ABSTRACT Kristin Lengsfeld Attics with an insulation on the ceiling plane inherently are more prone to moisture damage than conventional attic construction. This paper will describe a series of realistic field tests on different types of ventilated roofs in order to evaluate their hygrothermal performance. During the testing period wind washing, thermal performance and relative humidity in the cold roof, on the interior surface of the underlay, moisture of the rafters and amount of condensation were monitored. The room under the insulated ceiling was air-conditioned. The amount of migrated warm air into the cold area of the roof was well defined and controlled. The influence of different underlays and ventilation strategies was studied. Investigations show that there are only very slight differences of the individual roof constructions especially in the climatic conditions to be reached in the attics. Yet, at times, the differences between the constructions and ventilation strategies are principally only visually discernable. INTRODUCTION Attics on the insulated ceiling plane are common in West and North-European countries and in North America. Humid indoor air from subjacent heated rooms may reach the attic via a roof access, usually a makeshift staircase with an access cover, or via unfinished air-tight floor breakthroughs, light fixtures, bath fans or partition walls. In the process it is frequently observed that the result is the formation of condensation water or frost on the underside of the sheathing. The hygrothermal conditions in three unfinished attics with diffusion open underlays with and without additional ventilation are measured by means of hygrothermal measurements at the outdoor testing site of the Fraunhofer Institute of Building Physics (IBP) in Holzkirchen, Germany. DESCRIPTION OF THE INVESTIGATIONS Within the period from December 2003to March 2004, hygrothermal measurements were carried out at three unfinished attics (ventilated roofs) at the outdoor testing site of the Fraunhofer Institute of Building Physics in Holzkirchen. The following constructional options with a North- South-oriented steep-pitched roof of approx. 24 are compared: field 1: diffusion-open underlays (membrane A), laid over the ridge field 2: diffusion-open underlays (membrane A) with approx. 2 cm thick ventilation slots at the ridge (fig. 2) field 3: diffusion-open underlays (membrane B) with approx. 2 cm thick ventilations slots at the ridge (fig. 2) Figure 1 shows the schematic structure of a ventilated roof. The pitched roofs (inclination 24 ) had an area of approx. 2.5 x 4.0 m and were orientated to the north and the south. During the testing period wind washing, thermal performance and relative humidity in the cold roof, on the

2 interior surface of the underlay, moisture of the rafters and amount of dew water were monitored. The room under the roof ceiling was air-conditioned during the measurements between 20 and 22 degrees Celsius and % RH. The amount of migrated warm air into the cold area of the roof was well defined and controlled. A ventilator is installed in the floor to the attics allowing defined flows of humid air from the subjacent room to be transported to the above attics at certain time slices. An underlay is laid directly on the rafter. Figure 2 shows the installation of the membranes at the ridge in field 2 and 3. The 2 cm thick air gap for ventilation is clearly visible. According to the client s manufacturer s notes, the underlays have metric permeances between 925 and 617 ng/pa*s²*m. The concrete roof tiles are laid directly on the battens, i.e. without counter battens. An approx. 3 mm thick gap is beneath the underlay at the eaves providing for a continuous ventilation of the attic (figure 3). Foil closed at the ridge Foil open at the ridge (2 cm) Foil open at the ridge (2 cm) Field 1 Foil A Field 2 Foil A Field 3 Foil B natural Ventilation insulation air-conditioned air-conditioned Humidifier Heater Humidifier Heater Figure 1: Schematic structure of the test construction for measurements at the attics.

3 Figure 2: Construction of the ridge details in the fields 2 and 3 during re-construction of the ventilated roof. There is an approx. 2 cm thick gap in the membrane at the ridge. Figure 3: View of the 3 mm thick ventilation slot at the eaves for continuous ventilation. Figure 4 shows the exterior climatic conditions (temperature, relative humidity, normal rain, global radiation and wind velocity), registered during the measuring period by the Institute s meteorological station. As the hygrothermal behaviour of the ventilated roof is dependent on the

4 respective ratio of snow coverage, this ratio is shown in figure 5 for the northern as well as for the southern side of the test roof. Figure 4: Diagrams of the exterior climatic conditions, registered during the period of measurements. Above : outdoor air temperature and relative humidity as a means value per day. Centre: normal rain and global radiation. Below: wind velocity.

5 Figure 5: Diagram of snow coverage ratio on the test roof during the period of measurements. Figure 6: Diagram of the characteristics of room temperature and relative humidity in airconditioned rooms.

6 Figure 7: Diagram of the quantity of air injected by ventilators into the test fields daily. The indoor climate to be reached in the attics and in the subjacent room is continuously and simultaneously registered to the weather data by means of a datalogger in stand-alone operation. Figure 6 shows the measured indoor air temperature or relative humidity of the indoor air for the subjacent air-conditioned room. From the beginning of January in 2004, the ventilating performance was a continuous 135 m³ per day (figure 7). This amount is approximately equal to the quantity of air possibly flowing through a leaky attic staircase per day. The volume of the air flow is made up of the joint permeability, loading group A, pressure difference of 10 Pa and a length of the joint of 1,5 m. These values are regulated in the DIN [1] The moisture load in the subjacent room is adjusted to 50 % relative humidity until the end of January, and increased to 60 % in the following period until the end of March in 2004 to simulate the building humidity contents of a new building. In order to analyze the influence of the ventilation at the ridge with the same underlay, field 1 and 2 are re-constructed in the beginning of March. The membrane at the ridge in field 2 is closed and the membrane in field 1 is installed with a 2 cm thick gap as in field 2 before. The moisture content of the wooden rafter surface at a distance of approx. 2 cm from the underlays was repeatedly measured during a week by means of an electrical hygrometer. Beginning in mid-january, daily pictures (always in the morning) were taken and documented of the condensation water forming at the underlays and the condensation water forming at the underlays of the respective rafter field of the roof area was measured in addition by wiping and subsequent weighing. RESULTS OF THE INVESTIGATIONS Figure 8 show the cycles of the indoor temperatures (top) and relative humidity (bottom) in the test rooms for the period from January 7 until March 19, A comparison with the respective exterior climatic conditions shows that the hygrothermal conditions in the ventilated roofs match the exterior climatic conditions. The resulting dew point temperature in the three test rooms as well as the exterior dew point temperature is shown in figure 9. Figure 10 shows the measured values of the moisture content of the wooden rafter surfaces at a distance of 2 cm from the underlays for the northern and southern roof respectively. The results show that for the northern

7 as well as for the southern side, the moisture content of the wooden rafters in field 1, i.e. in the ventilated roof with underlays on the ridge, is slightly higher compared to the other two options. The evaluation of the condensation formation within three days is to be seen in figures 11 and 12. A higher condensation in field 1 or after re-construction in field 2 is clearly visible. Figure 8: Diagram of the temperature characteristics in the three test attic spaces during the period from January 7 to March 19, 2004 in comparison to the outdoor air temperature.

8 Diagram of the characteristics of the relative room humidity in the three test attic spaces during the period from January 7 to march 19, 2004 in comparison to the outer relative humidity.

9 Figure 9: Diagram of the characteristics of dew point temperature in the three test attic spaces during the period from January 7 to March 19, 2004 in comparison to the exterior dew point.

10 Figure 10: Diagrams of the characteristics of the moisture measured in the wooden rafters during the period from January 7 to March 19, above: measurement results of the humidity in the wood at the northern roof side. below: measurement results of the humidity in the wood at the southern roof side.

11 southern side northern side Field 1 Foil A First Spalt zu Field 2 Foil A First Spalt 2 cm offen Field 3 Foil B First Spalt 2 cm offen Figure 11: Photographical evaluation of the condensation water formed on the lower surface of the underlays in the fields 1 to 3 Top: On the southern side of the roof in the morning of January 7, Bottom: On the northern side of the roof in the morning of January 9, th Canadian Conference on Building Science and Technology

12 Field 1 Foil A First Spalt 2 cm offen Field 2 Foil A First Spalt zu Figure 12: Photographical evaluation of the condensation water formed on the lower surface of the underlays in the fields 1 and 2 on the northern side of the roof in the morning of March 8, Condensation water for a rafter field per orientation forming at the underlays is shown in figure 13. It is obvious that the quantity of condensation water measured after wiping and weighing shows different results depending on the orientation of the roof. The area of the measured condensation is about 0,06 m².these effects depend on the snow covering on the roof. During the period from 26 th February to 1st March the northern side of the roofs is completely snow covered and on the southern side there is mostly no snow or maximal half snow covered. The snow cover on the north side makes an impact as an additional insulation layer on the roof. Through this effect the surface temperature on the underlay of the north side is higher than on the southern side and therefore on the south side a lot of condensation water appears through a lower temperature as the dew point temperature in the attics. The differences between field 1 and 2 depends on the accuracy of the measurements of the condensation by wiping and additional unknown of air leakages of the constructions. On the north side there is one day with a lot of condensation water on the underlays despite snow covering. On the 26 th January it s more condensation water on the northern side. The cause for this effect depends on the outdoor boundary conditions. A few days before (23rd January) the southern side isn t snow covered and the northern side is completely snow covered. and during the weekend the weather was sunny and warmer. The sunshine on the south side of the roof makes an impact on the temperature layering in the attics and the outcome of this is a moisture gradient between the north and south side. The following of these effects are condensation water on the underlays only on the northern side.

13 Figure 13: Diagram of the quantity of condensation water measured at the lower surface of the underlays in the fields 1 and 2 for the northern and southern side respectively. CONCLUSIONS Investigations showed that there are only very slight differences of the individual roof constructions especially in the climatic conditions to be reached in the attics. Yet, at times, the differences are principally only visually discernable. These visual differences and the quantity of condensation water measured by wiping the underlays show with regard to the prevailing testing conditions that the boundary conditions and the air leakages of the constructions have the biggest

14 influences in view of problems with condensation water on the underlays. For a more detailed and exact overview of the influences of different ventilation strategies and membranes additional measurements are necessary. Because for such constructions it is very important to know how the infiltration rate by air leakages and the air change rate of the complete constructions is. From the point of view to mitigate the condensation on the membranes of the attics one measure is to reduce air leakages from the insulated ceiling to the attics. Another way could be to increase the air change rate of the attics by ventilation but this measure means more heat energy for the rooms below the insulated ceiling. The results of these measurements show that attics with insulation on the ceiling plane are more prone to moisture damages as conventional attic construction because independent of the air gap at the ridge condensation occurs on the surface of the underlays and this is the basis for mould growth. REFERENCES [1] DIN 18055: Fenster Fugendurchlässigkeit, Schlagregendichtheit und mechanische Beanspruchung, Anforderungen und Prüfung. Beuth Verlag. (Oct. 1981)