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. Data from temperature and humidity measurements was then analysed for dew points and the possibility of interstitial condensation forming within the wall. With the HITEX Diamond systems it was found that there was no approach of the dew points to the temperature within the wall so interstitial condensation was not possible. With the fibre cement system, there was a close approach and interstitial condensation may have occurred. It is concluded that a wall design with the insulation exterior to the timber wall structure gives a much lower chance of interstitial condensation occurring.

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:

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: The cavities of three test walls were wetted to simulate a leak and the temperatures and humidities were measured as the wall dried out when exposed to Auckland summer conditions. Data from a selected period was then analysed for dew points to see if there was any possibility of interstitial condensation forming in the walls.

3. RESULTS

Fig 1: Dew Point Chart for HITEX “D150” System

Fig 2: Dew Point Chart for HITEX D All System

Fig 1 and fig 2 show there is no approach of the dew point temperature in the cavity to the inside temperature of the gib board. Therefore there is no chance of any interstitial condensation forming.

Fig 3: Dew Point Chart for Fibre Cement System

Fig 3 however shows a much closer approach of the cavity dew point temperature to the inside temperature of the gib board between 13:00 and 16:00 hours just after the heating event. While the chart shows that interstitial condensation does not form on the inside of the gib board at the points where the temperatures were measured, any surface that had a temperature of 3 degrees centigrade less than that measured will have interstitial condensation form on it.

4. DISCUSSION

With the close approach of the dew point and surface temperature in the fibre-cement system it is possible that the dew point was dissected and interstitial condensation occurred.  Interstitial condensation will begin firstly on cold surfaces found at thermal bridging points or where penetrations make direct contact with the high humidity within the cavity.  External nails fixing the fibre-cement sheets, the sole plate in contact with dampcourse and concrete floors, bolts holding down the sole plates, through wall fasteners, brackets and beams through the fibre-cement cladding, metal bracing elements within the wall, structural beams within the cavity, metal lintels and aluminium window frames and cold water supply pipes will all become high probability areas for the formation of interstitial condensation.  Condensation forms on the cooler part of the building elements.   Thermal bridging across the cavity by damp batts and damp timber in contact with the external cladding will also be points for the formation of interstitial condensation. Early mould formation evidenced on the lower parts of the gib board, batts and timber suggest this may have been due to interstitial condensation.

In walls thermal gradients will exist due to hot air rising and cooler air settling at the ground.  The colder areas of the cavity will be the sole plate at the base of the wall and this is where interstitial condensation is more likely to form.  This may be one of the reasons the sole plate remained wetter over the duration of the test.  Gravity is also a factor in the collection and concentration of condensation at the sole plate. There is a need in future research to measure the thermal gradients within a cavity, especially in a residential home with thermal gradients within the height of a room. 

Similar or even more severe interstitial condensation would be present where cladding materials like metal siding were being used as these are more conductive than fibre-cement sheets and less severe where the cladding exhibited some insulation properties like bricks.

In HITEX Research Bulletins 304 and 305, it was reported that mould/fungi were found in the fibre cement cavity, but not in either of the HITEX Diamond cavities. References cited (ref 8) state that mould/fungi require free water to grow, and this water could be coming from interstitial condensation or water retained in the wood or both in the fibre cement systems.  Therefore in order to minimise mould and fungi formation and the associated health risks, it is a requirement to design and build a wall free of interstitial condensation. 

Interstitial condensation can only be avoided by keeping the dew point temperature of the air inside the cavity below the temperature of any building element.  Once a wall is wetted with a leak or excess moisture is allowed to accumulate, the risk of interstitial condensation rises rapidly.  This can only be avoided by removing the moisture and keeping the wall cavity at a stable temperature.

A key difference between the HITEX Diamond systems and the fibre cement system is that with the HITEX Diamond systems, the insulation is placed exterior to the timber building wall. This has the effect in the Hitex Diamond system of keeping the timber building wall at a more stable temperature. The fibre cement has the insulation placed within the wall so there is a much stronger thermal gradient across and vertically up the timber wall. With the fibre cement system the temperature within the timber building wall is much more responsive to changes in the exterior conditions resulting in the high humidity recordings when surplus moisture was available.

5. CONCLUSIONS

1.       The HITEX Diamond expanded polystyrene systems showed no signs of interstitial condensation.

2.       The Fibre-cement system closely approached (and may have) interstitial condensation.

3.       Insulation exterior to the timber building wall reduces the chances of interstitial condensation as it protects the wall framing from warming up and cooling down.

4.       Interstitial condensation can only be avoided by removing the moisture and inducing a stable cavity wall temperature

6. REFERENCES

1.       HITEX Research Bulletin 301
2.       HITEX Research Bulletin 302
3.       HITEX Research Bulletin 303
4.       HITEX Research Bulletin 304
5.       HITEX Research Bulletin 305
6.       University of Auckland report
7.      Address by M. Hedley and R. Wakeling of Forest Research at The Science of Building Weather tightness seminar, Auckland, March 2002.

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