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

Almost every building leaks within 10 years of construction resulting in wetting of the timber framework. This moisture causes the rotting of unsuitably treated timber and provides ideal conditions for the growth of potentially toxic mould and fungi and a potential loss of thermal insulation properties. To maintain a structurally sound and healthy building it is essential that not only are leaks stopped as soon as possible, but that the wet walls are also dried out once the leak has been found and repaired. Available mechanisms for drying wet timber framework are ventilation and molecular diffusion through small holes. A test rig was designed and built to measure the rate of the drying out of a wetted wall. The test rig was set up in a Physics laboratory at the University of Auckland. Results will be available in later HITEX Research Bulletins and the University of Auckland full report.

2. BACKGROUND

2.1. Sources of Leaks and Wetting of Timber Walls

·    A report that reviewed pre-purchase reports concluded that for building less than 10 years old, 75% had a defect and 50% allowed moisture entry into the building structure through a defect (ref 1).

·    Information from the BRANZ CINZ Forum in December 2002 (ref 2) listed sources of leaks as rain leaks, wind blown rain, damp air, plumbing leaks, construction moisture, flooding from outside, moisture diffusion through internal walls from showers, laundries etc, internal leaks and floods from laundries, kitchens, bathrooms etc.

·    Conclusions from a Canadian survey (ref 3) are that with leaks the source of moisture is rain penetration, and that the rain penetrates at the edges of cladding, at gaps in weather barriers, at windows, at balconies and at parapets. It was also identified that there was a lack of knowledge in the drying out of the walls affected by leaks.

2.2. The Effects of Leaks on Timber Walls

·    Fungi breakdown moist wood resulting in decay. Extracellular enzymes released by microscopic fungi attack wood lignin and cellulose, reducing the wood’s strength (ref 4).

·    Moulds will grow on a great diversity of building materials but especially those with cellulose. All moulds are a potential health problem and some such as stachybotrys and aspergillus are particularly dangerous (ref 4).

·    Water or moisture accumulation in the walls may have an affect on the thermal insulation and maintenance of R-value.

2.3. The Need to Dry Out Timber Walls

·    To stop decay (dry and wet rot), fungi and mould, all leaks into a building wall must be eliminated as soon as possible to remove the source of the water needed for their survival and growth. Fungi is able to spread at 1.8 metres per year (ref 5) so it can spread far from the source of the initial leak and eventually through the whole building.

·    Wet timber can hold more than 40% of its weight of water that can sustain the mould and fungi, so it is essential that any wet timber needs to be dried out as soon as possible after the leak is eliminated. Very little research work has been done to date on drying of wet timber walls.

·    If the wall insulation gets wet this also needs to be dried out as its insulation value is lowered.

2.4. The Need for Research into Drying Of a Wall

·    HITEX anticipates that Building Regulations and Codes will require in the future wet walls to be able to be dried out. In early 2003 legislation was introduced requiring cavities to be incorporated into the construction of a wall. The main thrust of this initiative was to act as a second line of defence against rain penetration through the cladding. Literature surveys on the drying out of wet walls confirmed comments of a knowledge gap (ref 3) in the drying out of wet walls. Therefore HITEX set out to do basic research into the drying out of a wet wall and develop a wall cladding that enabled a wet wall to be dried out. It is reported that decay will not occur if the wood moisture content is less than 18% (ref 4), that treatment of wood with preservatives can insure against decay (ref 4), and that mould occurs within buildings where weather tightness or ventilation are inadequate (ref 4). Field measurements by HITEX personnel have shown that wet walls at the time of construction dry out only very slowly over a period of months if at all.  Whilst cavities may assist cladding water egress they will not necessarily assist other forms of leaks (ref 2).

3. DRYING MECHANISMS FOR WET WALLS

Once a leak is eliminated, the following drying mechanisms are available to dry out the wet wall:

·    Natural ventilation of the timber cavity through air moving between vents at the bottom and top of a wall. This mechanism affects the thermal insulation or “R” value of the building. The holes are also sites for external rain to possibly enter.  Wet timber and insulation are often behind the cavity.

·    Forced ventilation by mechanical means such as a fan. This mechanism would be useful in emergencies for fast drying but again would affect the “R” value.

·    Aspiration of the wall through small appropriately placed vents during daily temperature fluctuations. This is a slow drying mechanism with less than 1% of the air changing daily.

·    Molecular diffusion of the water vapour through small appropriately placed holes during daily temperature cycles. This drying mechanism is driven by water vapour pressures.

·    Wind pressure fluctuations causing the cavity to breathe. Of importance for wind affected buildings such as multistoried, but not as useful in sheltered locations.

·    Condensation within the wall collected and drained. Any condensation within a wall is not desirable as this directly feeds the fungi and mould.

·    Diffusion/permeation of water vapour through the building paper and external cladding and/or through the internal gib board. This mechanism takes weeks to months and has doubtful merits in drying out a wet wall.

Conclusion: Available mechanisms for drying wet timber framework are ventilation and molecular diffusion through small holes. Calculations by The University of Auckland confirmed molecular diffusion through small holes was a promising drying mechanism for HITEX that warranted testing.

4. TEST RIG TO INVESTIGATE DRYING OF WET TIMBER WALL

4.1. 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. While the principal aim of the tests was the drying out of a wet wall, other tests including temperature profiles through walls and measurement of the thermal insulation properties also considered important were able to be done.

4.2. HITEX Diamond System: HITEX developed a new style of expanded polystyrene cladding (patent pending) that has an integral cavity sculptured into the inside surface. This “diamond” pattern provides a cavity that is not blocked off by studs or nogs. This cavity provides a connection for all air cavities in a building wall, thus giving a wet wall a greater chance to be dried out (see photo opposite).

4.3. The 3 Test Walls: The “external walls” were located on the inside of the test rig to which the outside atmospheric conditions were ducted by fans. The “interior” walls were clad with acrylic painted gib board and faced the conditions inside the laboratory. The timber walls were constructed with kiln dried (untreated) 90/45 pine. Details of first three test walls tested are listed below.

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 between the polystyrene and the timber frame.

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

Further Walls constructed of other cladding systems will be described in ensuring reports.

4.4. Test Conditions: The purpose of the tests was to measure the rate of drying out of a wet timber wall during normal everyday conditions. NIWA provided data (ref 7) for typical summer and winter conditions. An attempt was made to simulate these for the typical summer’s day by ducting in air from outside the laboratory to the interior of the test rig. A bank of lights was installed to simulate a period of sun in the middle of the day. Thus a diurnal temperature cycle was achieved.

4.5. Holes for Drying By Molecular Diffusion: The diamond pattern sculptured on the inside surface of the polystyrene is open at the base of the wall resulting in many small holes connecting the cavity air with the outside atmosphere. It is through these holes that drying of the wet timber wall by molecular diffusion is to be tested.

Photos from left to right: (1) Temperature sensor, (2) Humidity sensor, (3) Probes for measuring moisture content of timber, (5) Bank of lights inside test rig.

4.6. Instrumentation: To understand the drying mechanisms during the test, temperature and humidity instrumentation was placed inside the timber cavities. The temperature and humidity of the “external” and “internal” air was also recorded. The moisture content of timber was measured during the drying phase of the work. For details on the instruments and set up, refer to the full University of Auckland report when completed.

4.7. Results: These will be detailed in the full University of Auckland report when completed. Interim results will be highlighted in upcoming HITEX Research Bulletins.

5. DISCUSSION

Before the tests were devised extensive literature surveys found that very little work had been done in drying out of a wet timber wall. Similarly research looking at the actual temperatures and humidities within a wall were sought but again almost nothing was found. Many references and texts mentioned the possibility of interstitial condensation advising designers to avoid this for good building design (ref 5,6).

The whole test procedure and test rig was done within a limited budget. The researchers were left wondering why such basic research had not been conducted before. It appears from reports from Canada (ref 3) that some work had gone into developing computer models to simulate this, but that no one had worked on an actual test rig.

REFERENCES

1. Report by UNITEC for BIA, December 2000 “Auckland House Cladding Survey”.
2. Address by BRANZ at Claddings Institute of NZ Forum, December 2002.
3. Address by D. Hazleden “The Vancouver Experience” at The Science of Building Weathertightness seminar, Auckland, March 2002.
4. Address by M. Hedley and R. Wakeling of Forest Research at The Science of Building Weathertightness seminar, Auckland, March 2002.
5. T.A. Oxley and E.G.Gobert, “Dampness in Buildings”. 2nd Edition published 1994.
6. P. Burberry “Environment and Services - Mitchells Building Construction” Chapter 2.
7. A. Gosai and G. Fisher of NIWA: “Typical temperature profiles for Auckland”.

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Page last updated Tuesday, 08 March 2005