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

The EMC (equilibrium moisture content) of wood is dependant on the RH (relative humidity) of the air it is in contact with and this Research Bulletin provides data on the EMC for the normal range of RH. Moulds are able to grow above a RH of 65% which corresponds to a wood EMC of 14%, and moulds that result in rot of wood grow above a RH of 80% which corresponds to a wood EMC of 18%. Information on climatic conditions around New Zealand show these figures are exceeded for greater parts of the year in many locations. To avoid the growth of moulds, timber frame walls must be chemically treated or engineered to achieve a microclimate within the wall that avoids these trigger points. The HITEX Diamond system with its insulation exterior to the wall does this and has achieved an EMC of 14%. Other claddings will be different and will require research. Systems using a vented cavity constructed with timber battens will require special engineering and research.

2. INTRODUCTION

2.1. Background: Much is known about the EMC of Radiata Pine in relation to manufacturing and processing, and particularly for kiln drying. It appears that relatively little research has been done in relation to EMC and timber durability when used in timber frame walls. The EMC of the wood in timber frame walls changes continually in response to changes in atmospheric conditions and applied moisture such as leaks.  Whilst wood EMC changes have predictable effects on shrinkage and swelling of more notable concern is the effect changes have on timber durability.

2.2. Wood EMC and Relative Humidity: Wood is a hydroscopic material and has an EMC when in contact with air at any time. The EMC for woods at a range of temperature and RH have been researched and the following figs 1 & 2 are representative of data available in the public domain (refs 2,3,4). The data shows that the EMC is strongly dependent on the RH, and that the effect of temperature for ambient applications is small.

Fig 1: EMC’s of pinus radiata at different RH and temperature.

There is a reported difference between the EMC’s as to whether the wood is in the state of drying (desorption) or wetting (absorption) as is shown above in the dashed lines in fig 1. It was not certain from the data whether this was because the wood was very slow in coming to equilibrium, and if the wood had been left longer the absorption and desorption lines would have coincided in the centre. For the purposes of all HITEX Research Bulletins the centre line in fig 1 is the data used for the EMC of wood.

Fig 2: EMC’s of hardwood at different RH and temperature.

The data in fig 2 shows that there is little difference between the EMC of pinus radiata and hardwood. 

2.3. Wood Moisture Content and Mould Growth: Mould specialist Dr. Nick Waipara of Landcare Research provided information on the RH (relative humidity) ranges at which moulds grow. This information was presented in an earlier HITEX Research Bulletin (ref 1). The three RH and mould ranges are:

	“Some Moulds”: RH 65-80%: Moulds that are typically found on surfaces and can affect e.g. paints.
	“Common Moulds”: RH 80-95%: Moulds that can cause decay of wood.
	“Toxic Moulds”: RH 95-100%: Moulds that are toxic and cause health effects.

These RH ranges of moulds are considered the lower limits the moulds can continue growing. It would be expected that to begin growth the RH would be higher initially. Fig 3 overlays the EMC curve for wood with the plots of the ranges at which moulds grow as presented in an earlier HITEX Research Bulletin (ref 1).

Fig 3: EMC Curve for Wood overlaid with ranges at which moulds can grow.

It is evident from fig 3 that wood needs to be maintained at an EMC of less than 18% to avoid continuing decay or rotting. Further this means that the air the wood is in equilibrium with must have a relative humidity of less than 80%. Further it is evident that wood needs to be maintained at an EMC of less than 14% to avoid any formation of mould, and this means that the air the wood is in equilibrium with must have a relative humidity less than 65%. The BIA (Building Industry Association) has advised HITEX that no mould formation is acceptable (ref 5) and this will form part of the new E2/AS1 requirements. Therefore it is appropriate to look at when the EMC exceeds 18% and 14%, and the air RH exceeds both 80% and 65%.

3. WOOD EMC PREDICTIONS

3.1. EMC Predictions Around New Zealand: EMC predictions for Radiata Pine in various localities around New Zealand have been published (ref 6) and the figures are given below in table 1. The data is for wood when exposed to ambient air conditions but sheltered from direct rainfall.

Table 1: General Equilibrium Moisture Contents (%) Around New Zealand

*Based on the monthly mean of daily relative humidity at 9am and monthly mean of the daily values
for (Maximum Temperature+Minimum Temperature) /2. Source: Orman, 1955

The figures in table 1 shows that there are no locations around New Zealand that have a predicted EMC of less than 14% in July. There are many locations around New Zealand that have a predicted EMC in excess of 18% for considerable parts of the year.

3.2. Detailed Climatic Conditions in Auckland: NIWA has supplied temperature and RH data for a typical Auckland summer and winter days (ref 7). Similar plots can be done for other centres around New Zealand.

Fig 4: Temperature and Relative Humidity Data for Typical Auckland Summer Day (ref 7).

The data in fig 4 shows that on a typical Auckland summer day, a RH of 80% is exceeded for 4 hours a day and a RH of 65% is exceeded for 15 hours per day. The data in fig 5 shows that on a typical Auckland winter day, a RH of 80% is exceeded for practically the entire day. From fig 3 it is predicted that in Auckland in winter, the EMC of wood exposed to ambient air conditions will exceed 20% and so moulds that cause rot are likely to or will grow.  It will be a similar situation for many of the New Zealand centres shown in table 1.

4. CONSEQUENCES

The consequences of the high relative humidity in typical ambient conditions around New Zealand is that wood exposed to these ambient conditions but protected from direct rainfall will have a high EMC and moulds are likely to or will grow. When the RH is between 65% and 80% and the EMC is between 14% and 18%, “some moulds” will grow that affect such things as paint application. When the RH exceeds 80% and the EMC exceeds

Fig 5: Temperature and Relative Humidity Data for Typical Auckland Winter Day (ref 7).

18%, “common moulds” will grow and these will rot the wood. Both of these are predictable but undesirable outcomes. It is predicted that any wall connected to the outside conditions would have an EMC higher than 14% in summer and 18% in winter. During construction of a building is an example of when this will occur. For timber frame walls there are the following options to avoid the formation of moulds in order to make the wood durable.

4.1. Chemically treat the timber so that moulds cannot grow. Appropriate treatment levels can be obtained from Forest Research Institute (ref 6). This has the downside of increasing the use of potentially toxic chemicals, for example especially if CCA treatment is used.

4.2. Engineer the timber frame wall for low RH and EMC: The timber frame wall can be engineered so that the microclimate within the wall gives a RH of less than the trigger points of 80% and 65%. This requires detailed engineering, research and monitoring. External claddings are used to protect the timber frame wall from exposure to wind and rain and so claddings are an essential part of the engineering of the microclimate.  The engineering will be different for each type of cladding system.

4.3. Combination of both the above: The appropriate combination of a lesser chemical treatment combined with suitable engineering of the timber frame wall.

5. DISCUSSION

5.1. HITEX Diamond System: The HITEX Diamond system is engineered to achieve a low RH and wood EMC in the timber frame wall. The HITEX Diamond system places the thermal insulation exterior to the timber frame wall, and so keeps the timber frame wall and wood at a higher temperature and lower humidity than the prevailing outside conditions. This also causes the temperature and RH within the timber frame wall to remain relatively stable compared to changes in the outside ambient conditions. These effects were observed during the HITEX Drying Project (ref 8,9). This HITEX research has found that the wood in the timber frame wall clad with the HITEX Diamond system typically has an EMC of 14% or lower and a RH or 65% or lower. These figures are below all the trigger points for mould growth identified above.

5.2. Other Wall Claddings: Other systems of cladding will achieve different microclimates within the timber frame wall and so the RH and EMC values within the wall will be different to that achieved by the HITEX Diamond system. Houses with timber walls are required to have insulation to be thermally efficient.  The type and placement of insulation affects the engineering of the wall. In particular cladding systems that have little or no insulation value, and use fibreglass batts within the timber frame wall for their insulation, are expected to have a microclimate within the timber frame wall of a lower temperature, higher humidity, higher wood EMC, and larger changes of temperature and humidity as ambient conditions change. Therefore it is expected that moulds will be much more likely to grow within these systems unless the appropriate chemical treatment of the wood and other building materials is done. There appears to have been little research done by other wall cladding suppliers on the microclimates within timber frame walls using their cladding.

5.3. Battened Cavities. For a building wall with a vented cavity, the cavity will tend to take on the temperature and humidity of the outside environment due to its direct connection to this outside environment. The more the cavity is vented the more this will occur. If the cavity is constructed with timber battens, the EMC of the timber battens will be higher than the EMC of the wood in the timber wall frame. There will then be a driving force available to drive moisture from the timber battens through the building paper and into the wood timber frame. The moisture will also be driven into other building materials such as fibreglass insulation batts and gib board. This was identified in computer simulations done by the MEWS Task 8 Project (ref 10). The timber battens themselves with their high EMC will require a high degree of chemical treatment so that mould does not grow on them. For cladding systems that have a significant EMC of their own such as fibre cement board, then the back of the cladding will also absorb moisture that enters the vented cavity. This moisture may also transfer to the timber battens and through to the timber wall frame. These issues need to be taken into account when engineering a vented cavity constructed with timber battens.

5.4. Leaks in Buildings: Any leak into a timber frame wall will add additional moisture that will increase the RH and the wood EMC. Leaks are a foreseeable event based on the history of homes leaking. Once a leak has been identified, it needs to be fixed before the RH and wood EMC can be returned to their engineered values. No consideration is given in this bulletin for moisture accumulations by leaks.

5.5. Releasing Moisture Accumulated during Construction: A building is typically open to the atmosphere during construction and hence the wood in the timber frames will have a relatively high EMC at this time. Current building practice is to place the interior lining when the moisture content of the wood is below 20%, a figure that is above the trigger points for mould growth. Therefore the timber frame wall needs to be able to release this excess moisture and this can best be done through the cladding. The HITEX Diamond system is able to do this (ref 9).

5.6. Activities in Building.  Building occupant activities range from showers, cooking and breathing which give off vapour increasing the RH within the home. This moisture must either escape through ventilation devices or diffuse out of the structure.  The greater the number of occupants and their activity, the greater the amount of moisture released and the greater the impact on the timber frame wall. Higher moisture loads arise in specific rooms such as bathrooms and laundries. More understanding is needed on the effects of this.

5.7. Building Details: Colours of the building, its orientation to the sun, hours of sunlight, rainfall subjected to, wind exposure, materials used, design of the building and temperatures within the building and outside the building are all predicted to affect the timber frame wall RH and EMC values. These variables affect the conditions of buildings and it is for this reason more must be understood to ensure timber EMC remains within prescribed levels.  With E2/AS1 calling for timber to remain below 18% moisture content sound engineering methods are needed to achieve this figure, as the predictions are that these EMC values are conservative and will be exceeded.

5.8. Climatic Variations: The data given in earlier sections is average figures and a typical winter day. It is predictable that considerable variations will occur within each region. It is expected that colder and wetter climatic variations such as storms will cause the wood EMC to be higher than predicted.

6. CONCLUSIONS

	Detailed engineering of a timber frame wall is required to ensure it is durable. This requires the appropriate combination of treated timber, placement of insulation and type of cladding materials to achieve a suitable microclimate within the timber frame wall. This requires research and development by the cladding supplier.
	Research on the Hitex Diamond Cavity system shows a microclimate within the timber frame wall can be achieved that gives a wood EMC below that required for the formation of moulds. However provide 50 year durability and to handle events such as leaks, a degree of timber treatment is required.
	As the industry places more reliance on cavities this increases the connection of the outside air to the timber framing.  Additional engineering is required for cavities to keep the timber EMC within safe limits.
	It is envisaged increasing the level of chemical treatment of the wood will provide durability to the structure where EMC levels exceed those for mould formation.  Each engineered structure may have a different EMC and hence a different treatment requirement.

7. REFERENCES

1.  HITEX Research Bulletin 307.
2.  “Properties and Uses of NZ Radiata Pine” by J.A. Kininmonth and L.J. Whitehouse.
3.  Supplied by BRANZ from public domain reference material.
4.  Northern Hardware Initiative of North America Website.
5.  Personal communication between I. Holyoake of HITEX and C. Benge of BIA, March 2003.
6.  Forest Research Institute Bulletin – Treatment Levels for Radiata Pine.
7.  A. Gosai and G. Fisher of NIWA: “Typical temperature profiles for Auckland”.
8.  Hitex Research Bulletin 305
9.  Hitex Research Bulletin 304
10. Hitex Research Bulletin 311

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Page last updated Tuesday, 08 March 2005