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 and to measure temperatures within the building wall to assess the thermal insulation properties of the wall. Results showed that the HITEX Diamond systems gave superior insulation to the fibre cement wall. Both HITEX Diamond systems retained their insulation properties when the walls were wetted inside to simulate leaks. The fibre cement wall did not retain its insulation properties due to the fiberglass insulation batts shrinking and slumping within the wall cavity. It is concluded:

	Superior insulation is achieved when the insulating material is polystyrene and attached to the exterior to the timber wall.
	HITEX Diamond systems retained their insulating properties even when the timber cavity was wetted.
	Fibreglass batts lose their insulating properties when wetted by moisture, eg from leaks.

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 rates of drying out of a wet wall. Other tests carried out included measurements of temperature profiles through the walls and the thermal insulation properties of the walls, and this bulletin reports on these temperature tests.

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 “exterior” temperature was increased with a bank of lights and the temperatures at various locations within the wall were monitored.

3. TEST RESULTS

3.1. Temperature Profiles in a “DRY” Wall: The temperatures within a “dry” wall (not wetted) were monitored as the simulated exterior temperature was increased. The results are shown in fig 1.

Fig 1: Temperature Response in Dry Wall to Heating Event

The graphs in fig 1 show that the internal wall temperatures in both HITEX Diamond systems did not respond to the external temperature event within the time period measured. The temperatures remained constant at close to the laboratory temperature and were uniform throughout the wall cavity. On the other hand the fibre cement system showed a response quite quickly with the timber wall cavity temperature increasing in relation to the outside temperature. The fibre cement system developed a significant temperature gradient within the timber located in the wall cavity during the test.

3.2. Temperature Profiles in a “WET” Wall: The wall timber cavities were wetted to simulate a leak and the drying out test started. During this test the temperature profiles within the wall were recorded, and the results of one time period are given in fig 2.

Fig 2: Temperature Response in “Wet” Wall to Heating Event

The “inside gib” lines in fig 2 show that the fibre cement system (right) has a much more significant response to the heating event. The slope of these temperature lines indicates the overall insulation value for the wall. The steep slope for the fibre cement system indicates that the insulation is much reduced, whereas both HITEX Diamond systems have retained their insulation properties. Fig 3 shows the temperature profiles through the walls at time 13 hours from fig 2.

3.3 Temperature Profiles through “WET” Wall

Fig 3: Temperature profile through wall at time 13 hours from fig 2.

Fig 3 shows how the wall cavity temperatures in the timber wall cavity on the fibre cement system have increased markedly just one hour after the heating event started. Further the graph shows that the batts have retained little of their insulation value. On the other hand both HITEX Diamond systems have retained their insulation value and the temperature within the timber wall cavity has only marginally been affected by the exterior temperature increase. From the graph it can be seen that the “D All” has performed better than the “D150”. Note that the “D All” includes a full layer of building paper between the HITEX Diamond expanded polystyrene whereas the “D150” does not.

When the test rig was opened up after the completion of the drying out tests, it was found that the fiberglass insulation batts in the fibre cement system had slumped and shrunk in size by more than 50% (see photo opposite). This explains why the insulation properties of the fibre cement system were much reduced as found in fig 3. Note that during the process of wetting the wall, the fiberglass insulation batts were completely wetted. The technicians doing the work commented that the batts acted like a sponge.

4. DISCUSSION

The test results show there are significant benefits placing the insulation on the exterior of the timber wall of the building. This is a feature of the HITEX Diamond cladding system where the expanded polystyrene at 50 mm thick is placed external to the timber frame. Thus the entire timber framing is insulated from the exterior.  With the fibre cement system (and other claddings relying on batts) the insulation is within the timber wall meaning the outer edges of the timber framework have almost no thermal insulation from the exterior conditions. This was demonstrated clearly in all the tests as temperatures inside the fibre-glass batts cavity responded quickly to the external temperature event.

The conditions inside the timber wall cavity on the HITEX Diamond systems remained close to the laboratory or “interior” conditions. This means that in this case the cavity remains fairly stable and warm, ie they are not fluctuating widely from low to high as the “external” conditions do. This is considered desirable for the dimensional stability of the timber which may reduce cracking of internal gib and external cladding. It is also good for the avoidance of interstitial condensation.  Note:  Air conditioned rooms require special knowledge.

The tests were for a heating event similar to the diurnal summer day.  It is conceivable that if the test rig was subjected to a cooling event, then the temperatures inside the cavity would also respond quickly with the fibre cement system and then there would be the possibility of condensation within the wall and this will be reported in upcoming tests.

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

5. CONCLUSIONS

1.       The HITEX Diamond expanded polystyrene systems gives better insulation than systems using fibreglass batts, particularly if there has been a leak into the timber wall cavity of a building.

2.       The HITEX Diamond systems with polystyrene on the exterior of the timber framing, reduces the rapid temperature changes causing interstitial condensation.

3.       The HITEX Diamond polystyrene is not affected by moisture from leaks meaning the R-value is not lost.

6. POLYSTYRENE REFERENCES

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

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Page last updated Tuesday, 08 March 2005