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 wall section. It was found that HITEX Diamond wall sections held less water and dried out faster than a fibre cement wall section in the event of a leak. The sole plates became the wettest and were the last part of the wall to dry out or begin drying out. It is concluded that:

	HITEX Diamond systems dried out a wet wall section, with parts of the wall drying out to less than 18% moisture in 7 days. No timber in the fibre cement system reached the 18% during the test.
	Molecular diffusion combined with the Diamond interconnecting cavity provides the drying mechanism.
	After 35 days testing, mould and fungi had become established in the fibre cement wall section, but were not found in the HITEX Diamond wall sections.

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 section when exposed to an Auckland summer’s day.

2.2. Test Walls: See HITEX Research Bulletin 301 for full details:

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 7.5 mm 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: Sections of the three test walls were wetted to simulate a large leak. The sections of the walls wetted were the lower wall cavities bounded by the sole plate, two studs at 600 mm centres and the nog at 750 mm above the sole plate. Wet towels were used to wet the walls and the fiberglass insulation batts were soaked. The moisture contents of the wood and gib board were measured as the wall dried out when exposed to Auckland summer conditions. The target dryness was 18% moisture content, below which it is reported that decay of wood does not occur (ref 3).

3. TEST RESULTS

3.1. Moisture Content of Nog: Data recorded (ref 2) is shown in fig 1.

Fig 1: Moisture Content of Nog During Drying Out.

3.2. Moisture Content of Stud Top and Bottom: Data recorded (ref 2) is shown in fig 2.

Fig 2: Moisture Content Stud Top and Bottom During Drying Out

3.3. Moisture Content of Sole Plate: Data recorded (ref 2) is shown in fig 3.

Fig 3: Moisture Content of Sole Plate During Drying Out

3.4. Moisture Content of Plasterboard. Data recorded (ref 2) is shown in fig 4.

Fig 4: Moisture Content of Plasterboard During Drying Out

The data presented in fig 1, fig 2, fig 3 and fig 4 show that the HITEX Diamond wall sections dry out faster than the fibre cement system wall systems. Parts of the wall in the HITEX Diamond sections reached the target 18% moisture content in as little as 7 days. The timber in the fibre cement wall section never reached 18% at any location at any time during the test.

4. DISCUSSION

4.1. Wetting of the Wall Sections: The researchers found it more difficult to wet the HITEX Diamond wall sections than the fibre cement wall section. The fibreglass insulation batts were found to be a very water absorbent material that literally soaked up and held the water. This water was then able to transfer to the timber sole plate, studs and nogs more easily. It was found that the timber soaked up the water only slowly, and even with the use of totally saturated towels, the researchers found it hard to get the moisture content of the timber in the HITEX Diamond wall sections much over 25%. This has its benefits in the event of a leak in a house wall as it suggests that wall sections in a HITEX Diamond system will not absorb or hold as much water as wall sections in a fibre cement section that is filled with batts. Thus there will be less water to feed mould and fungi and less water to dry out once the leak is found and stopped. The quantity of water used to wet the wall sections was not measured.

4.2. Inconsistencies of Wetting Out: The researchers again found it difficult to get uniform wetting out of the timber in all the wall sections. This is probably due to the irregular nature of timber with its knots being harder and less absorbent than clean sections.

4.3. Timber Moisture Content Measurement Limit 40%: The moisture content meter was not capable of measuring moisture contents in timber over 40%. All data presented with a value of 40% means that the actual value was 40% or higher.

4.4. HITEX Diamond Walls Dry Out Faster: The data shows the HITEX Diamond walls dry out faster than the fibre cement wall. The drying mechanism is through small holes at the base of the HITEX DIAMOND cladding through which the moisture escapes by molecular diffusion. Drying by ventilation is not feasible as there are no holes in the top of the wall section to allow air movement. Drying by aspiration of the cavity directly to the outside atmosphere due to temperature changes is happening but is thought to account for only a small amount of the drying. The fibre cement system design seals off each air cavity and moisture cannot therefore escape, but remains trapped within the wall. The HITEX “D150” dries out faster than the D All, and this is thought to be due to the inclusion of the full layer of building paper which slows but does not stop the movement of moisture within the wall.

4.5. Sole Plates Take Longer to Dry Out: Fig 2 shows that the sole plates take much longer to dry out than the studs and nogs. Therefore it seems that the wall section is drying out from the top down. This is typical of what is seen in a house with a leak with the worst affected timber usually being the sole plate. The studs and nogs above the floor level are in a position with the Hitex Diamond walls as they have no batts to get air all around them so they are able to let their moisture go more readily. The sole plates sit on the base of the wall and only the top surface is exposed so drying out can only really occur from the top surface. Also when the walls were wetted any surplus water ran down to the sole plate and accumulated there so the sole plates started off much wetter than the studs and nogs. It is also possible that air is circulating within the wall cavity with the heavier more humid air sinking. Another mechanism that may be occurring is interstitial condensation within the wall and this drains by gravity to the bottom of the wall.

4.6. Moisture Content Taken At Centre of Timber:  It was part of the parameters of the test to dry out the timber and therefore moisture content probes were installed permanently into the central ninth of the timber as these would be the measure of the total dryness.  When the test was concluded it was noted that although the moisture contents in the central ninth were in some instances above 18% the surface moisture content where evaporation was taking place with the Hitex Diamond Cavity walls were below 18%.  This leads the researchers to believe the process of drying the centre of timber takes time especially if the timber was fully saturated.  This will go some way to explaining why the bottom plate takes longer to dry as it relies on just one drying surface.  The effect of the wetter centre of the wood is unclear as to the formation of mould.

4.7. Linear or Exponential Drying Curve:  The data shows a fig 1, fig 2 and fig 3 for the Hitex Diamond walls an almost linear drying rate rather than the predicted exponential curve.  It is thought that this is probably due to evaporation of the timber is more of a constant with the Hitex Diamond walls as the temperature and humidity of the cavity remained relatively constant and supported drying conditions.  This may prove useful in shortening test programs in the future.

4.8. Gib Board Becomes Wet: The data shows the gib board also takes up moisture. On inspection the gib board was soft and spongy, especially where it was in direct contact with the timber and fibreglass insulation. The moisture content was measured with the same instrument as was used for the timber wall, and it is not certain that the data recorded is totally accurate. Nevertheless the data shows moisture take up and drying out.

4.9. Mould and Fungi Growth: At the end of the 6 week test the wall sections were opened and inspected. There was no sign of mould or fungi in the HITEX Diamond systems. However in the fibre cement wall mould and fungi were found growing. There was a greenish hairy mould on the timber studs, and a black fungi on the inside of the gib board. This mould and fungi and the microclimate that existed within the wall cavities will be the topics of later HITEX Research Bulletins.

Fig 5 (Far Left): Black mould on inside of gib in fibre cement system at end of test.
Fig 6 (Left): Greenish hairy mould on wall stud in fibre cement system at end of test.

5. CONCLUSIONS

1.       The HITEX Diamond expanded polystyrene systems dry out faster than the fibre cement system and parts of the walls with the HITEX Systems reached the target dryness of 18% in as little as 7 days.

2.       Molecular diffusion and the HITEX Diamond interconnecting cavity allow moisture to escape to the outside atmosphere and hence dry out the wall.

3.       No timber in the fibre cement system reached the target dryness of 18% during the time of the test.

4.       The HITEX Diamond wall sections hold less water than a fibre cement wall section with fibreglass insulation batts in the event of a leak.

5.       The sole plates take up the most water during wetting and are the last part of the wall to dry out or begin drying out.

6.       The gib board both takes up and releases moisture during a wetting event.

7.       Mould and fungi are able to become established in less than 33 days in the fibre cement wall sections, but were not able to be found in the HITEX Diamond wall sections.

6. REFERENCES

1.       HITEX Research Bulletin 301
2.       University of Auckland report.
T.A. Oxley and E.G.Gobert, “Dampness in Buildings”. 2nd Edition published 1994

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Page last updated Tuesday, 08 March 2005