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. The sole plates of three test walls were wetted to simulate a leak or being left wet during construction. The moisture content of the sole plate was measured against time when the wall was subjected to Auckland summer conditions. Results showed that the sole plates in the HITEX Diamond systems dried out faster than the sole plates in a fibre cement system. The sole plates in the HITEX Diamond systems reached the 18% target dryness in 20 to 25 days. The sole plates in the fibre cement walls never reached the 18% target dryness. It is concluded that:

	HITEX Diamond systems are able to dry out wet sole plates to 18% moisture content.
	Molecular diffusion is the drying out mechanism.
	The fibre cement system was not able to dry out a wet sole plate to 18% moisture content.

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 the rate of the drying out of a wetted wall.  The target dryness was 18%, below which it is reported that decay of wood does not occur (ref 5).

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 7.5mm 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 sole plates of three test walls were wetted.

3. TEST RESULTS

3.1. Drying Out of Wet Sole Plates: The sole plates were wetted by opening up the wall cavity and literally pouring water on to the sole plates over a period of 2 days. The walls were closed then left to dry out when the test rig was exposed to Auckland summer conditions.

Fig 1: Drying of Wet Sole Plate (Middle Probe)

The lines in fig 1 show that the moisture content of the sole plates in both HITEX Diamond systems reached or would reach the target 18% in 20 to 25 days,. The “D All” system with its full layer of building paper dried out slower than the “D150” system. The sole plate in the fibre cement system never reached the 18% target and was not expected to reach it for 30 days or more.

Fig 2: Drying of Wet Sole Plate (Left Probe)

The lines in fig 2 again show that the moisture content of the sole plates in both HITEX Diamond systems reached or would reach the target 18% in 20 to 21 days. The “D All” system with its full layer of building paper dried out slower than the “D150” system. Again the sole plate in the fibre cement system never reached the 18% target and was not expected to reach it for 30 days or more.

When the fibre cement cavity was opened at the end of the test it was evident from wood moisture readings that there had been substantial redistribution of water within the cavity where the sole plate was wetted. The adjacent studs and nogs had increased in moisture content as had the fiberglass batts and the fibre cement cladding in the fibre cement system. Thus some of the measured drying was not a reflection of water being removed from the cavity.

4. DISCUSSION

Wet sole plates are typical where there are leaks and are common on new building sites. One of the key aims of the drying out test was to reduce the moisture content of the sole plate to less than 18% as it is reported that decay will not occur if the wood moisture content is less than this (ref 4,5).

It was found to be difficult to wet the sole plates uniformly due to the presence of irregularities in the wood including knots. The sole plates soaked up the water only slowly making the task even more difficult. It was decided to start the drying out tests even though each sole plate did not have an equal starting point. This was the first drying project undertaken and one of the key aims was to get information so that a second drying test could be better designed for obtaining more accurate data. Full data is included in the University of Auckland report when published (ref 3).

In section 3 of HITEX Research Bulletin 301 (ref 1), the possible drying out mechanisms are listed. With this test there was no ventilation holes provided for any of the systems so this mechanism is clearly not responsible for the drying out that was measured. The HITEX Diamond system has small holes at the base of the cladding and molecular diffusion through these holes was the expected drying mechanism. The only way the fibre cement system could have dried would have been through diffusion/permeation through the internal and external surfaces.

The test results show that the wet sole plates in both HITEX Diamond systems were dried to less than 18% moisture content in a time of 20 to 25 days. This is seen as being desirable in order to prevent the growth of mould and fungi (refs 4,5). The tests also showed that the “D150” dries out faster than the “D All” system, and this is most probably due to the full layer of building paper in the “D All” system. This suggests that the moisture in the “D All” sole plate needs to first evaporate into the cavity, then permeate through the building paper, before traveling to the outside air by molecular diffusion through the small holes at the base of the polystyrene.

With the fibre cement system, drying out of the wet sole plate was measured by the instruments. On opening up the test wall at the end of the test the researchers found and measured that there had been substantial moisture redistribution from the sole plate to the studs and nogs.  One measurement on a stud found its moisture content had increased from 12% pretest to 18% after the test.  So there is substantial doubt as to how much moisture was physically removed from the wall during the test.

The comments applied to the fibre cement system would apply to other cladding systems where the outer cladding has little to no insulation value.

5. CONCLUSIONS

1.       The HITEX Diamond expanded polystyrene systems allow a wet sole plate to dry out to less than the target 18% moisture content in an acceptable time of 20 to 25 days.

2.       Molecular diffusion through small holes at the base of the wall was the drying mechanism for the HITEX Diamond systems.

3.       The fibre cement system did not allow the wet sole plate to dry to the target 18% moisture content during the duration of the tests.

6. REFERENCES

1.      HITEX Research Bulletin 301
2.      HITEX Research Bulletin 302
3.      University of Auckland report.
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

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Page last updated Tuesday, 08 March 2005