HITEX RESEARCH BULLETIN

HITEX Research Bulletins summarise research projects undertaken in conjunction with the University of Auckland. The aim is to provide an understanding of what is happening in the wall of a building for the purpose of building structurally sound and healthy homes for the future.

 

1. SUMMARY

A test rig was set up in a Physics laboratory at the University of Auckland to measure the rate of the drying out of a wetted timber wall. The microclimate within the wet timber wall was monitored while the wall dried out over a 6 week period. Results showed that the HITEX Diamond systems maintained a lower humidity than the fibre cement system. Mould and fungi were found in the fibre cement system but not in the HITEX Diamond systems. The humidity data was coupled with temperature data and plotted on a psychrometric chart. Vapour pressures were determined and these show there is a positive driving force for drying by molecular diffusion.

2. BACKGROUND

2.1. Test Rig: HITEX constructed a test rig with three test walls and this was set up in the Physics laboratory at the University of Auckland. The principal aim of the tests was to measure rate of drying out of a wet wall when exposed to an Auckland summer’s day.

2.2. The Test Walls: See HITEX Research Bulletin 301 for full details (ref 1):

Wall 1: “D150”.  HITEX Diamond cladding with a 150mm high strip of building paper at the base of the wall between the polystyrene and the timber frame.

Wall 2: “D All”. HITEX Diamond cladding with full height building paper.

Wall 3: “Fibre-Cement”. Standard 7.5mm fibre-cement board with textured painted finish outside, no cavity, full height building paper, and timber framed wall filled with fibreglass insulation batts.

2.3. Test Conditions: The cavities of the three test walls were wetted to simulate a leak. The temperatures and humidities within the wall cavities were recorded during the diurnal cycles as the walls dried out when exposed to Auckland summer conditions (ref 4).

3. TEST RESULTS

3.1. Temperature and Humidity: The temperature and humidity data for a typical 24 hour interval of the test is given in figs 1and 2. Note that the lamps were turned on for 2 hours at 12 noon to simulate an external heating event.

Fig 1: Temperature of air within wet wall cavities           Fig 2: Humidity of air within wet wall cavities

Humidity data from throughout the test was very similar to that shown in fig 2 (ref 5). There was no noticeable drop off in humidity over the 6 week duration of the drying test. The temperatures throughout the test remained quite steady in line with the Auckland summer conditions.

3.2. Mould: On opening up of the cavities at the end of the drying test, mould and fungi were found on the timber studs and gib board in the fibre cement system. No mould was found in the HITEX Diamond systems. The mould and fungi have yet to be identified.

Fig 3: Mould/fungi on timber stud in fibre cement cavity.        Fig 4: Mould/fungi on gib in fibre cement cavity.

4. DISCUSSION

4.1. Microclimate in Wall Cavity: The microclimate can be represented by plotting the data from figs 1 and 2 on a psychrometric chart as follows in fig 5. Also plotted in fig 5 are the plots for a typical Auckland summer and winter’s day (ref 6).

Fig 5: Wet wall cavity microclimate plotted on psychrometric chart

With the HITEX Diamond systems the microclimate within the wet wall cavity is similar but not identical to that of the exterior environment. This is to be expected as the holes at the bottom of the expanded polystyrene provide a direct connection of the cavity air to the exterior environment, but with sufficient restriction to not allow free air flow. This is believed to be an important part of drying a wet wall by molecular diffusion and further reinforces the findings of earlier HITEX Research Bulletins.

With the fibre cement system there is no such connection from the wet wall cavity to the exterior environment and as a result the microclimate within the wall cavity is very different from that of the exterior environment. The fibre cement system responds quickly to variations in the exterior environment no doubt due to the loss of insulation value of the wet wall as reported earlier (ref 3). This fast response time opens up the possibility of interstitial condensation occurring within the wet wall and this will require further research and analysis.

The lower humidities within the HITEX Diamond systems indicate that there is a good driving force available for further drying out the wet timber. With the fibre cement system with its higher humidity, there is less capacity in the air to evaporate water from the wood and so the drying of the wood is slowed.

4.2. Mould and Fungi Growth: The fact that mould and fungi were found show that the test has simulated what is found in leaky house investigations. The data shows that mould and fungi can become established in less than 6 weeks if the humidity is over 70%, the temperature is over 22 degrees C, there is wood fibre available and there is free water available (the wood moisture content was always over 18% (ref 4)). With the HITEX Diamond systems the humidity was less than 55% and the wood moisture content was less than 18% after about 25 days (ref 4), and under these conditions mould and fungi did not become established. Note that the timber used in the tests was untreated, ie. kiln dried. Further work will be required to further define the conditions at which mould and fungi will become established.

4.3. Water Vapour Pressure within Wall Cavity: The drying mechanism for walls is by molecular diffusion as there is no ventilation provided. This has been discussed in earlier HITEX Research Bulletins (refs 3,4). The water vapour pressures within the wall cavities can be noted directly from fig 5 above. The data from this is then shown in fig 6.

Fig 6: Water vapour pressures in wet timber wall cavities

Fig 6 shows there is a positive vapour pressure difference between the cavities and the exterior environment. This means that there is a driving force available for diffusion of water vapour from the cavity to the exterior environment. With the HITEX Diamond systems water vapour is able to “escape” from the wall cavity through the small holes at the base of the expanded polystyrene. This process is termed molecular diffusion and has been talked about in earlier HITEX Research Bulletins (ref 3). With the fibre cement system there is no path for this to occur so the moisture is effectively locked within the wall. Therefore the water vapour will try to diffuse through the gib board and the fibre cement cladding. This process is believed to take considerably longer and so the wall will stay wetted for longer.

4.4. More Work Required and Comment Invited: The researchers believe that the recording of the microclimate in wet wall cavities has not been done before. More work will be required to further understand what is happening in a wet wall cavity. In particular more work is required to identify the conditions at which mould and fungi can become established.

Comment on any part of this Research Bulletin is invited from any interested party. The temperature and humidity data from the Auckland University can be made available in spreadsheet format on request to HITEX.

5. CONCLUSIONS

1.       The microclimate in the wet wall cavity of the HITEX Diamond system has a lower humidity than that in a fibre cement system.

2.       Mould and fungi can become established in less than 6 weeks with the right microclimate.

3.       There is a positive water vapour pressure available to drive the drying of the wet wall by diffusion.

6. REFERENCES

1.       HITEX Research Bulletin 301
2.       HITEX Research Bulletin 302
3.       HITEX Research Bulletin 303
4.       HITEX Research Bulletin 304
5.       University of Auckland report.  A. Gosai and G. Fisher of NIWA: “Typical temperature profiles for Auckland”.

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Page last updated Tuesday, 08 March 2005