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

	Early colonisation of microbial growth was found on the fibre cement test panel materials only.
	No microbial growth was found on the Hitex text panels.
	Of concern is the establishment of both Stachybotrys and alternnaria that can produce microtoxins that cause ill health.
	It is patently obvious that building materials intended to be used to construct walls need to be treated to resist microbial growth so they can remain durable.
	It is also obvious that the wall itself must remain within acceptable moisture levels otherwise microbial growth can occur.
	There is a serious lack of knowledge regarding moisture and wall cavity construction in New Zealand.

2. Background

A test rig was constructed, by Hitex Building Systems and University of Auckland, to examine moisture and drying out conditions of external wall cavities, under different conditions. (Ref 1).  Experimentation using the test rig included the simulation of water leaks into the external wall cavities.  At the conclusion of the testing it was found that microbial colonisation, in particular fungal growth, had occurred within the fibre cement test panel.   Sections of the materials exhibiting growth were sampled to examine the extent and type of fungi present.

3.  Methods

Samples of fungal growth present on the building materials (insulation material, timber, gypsum board, building paper) used to construct the test panel were obtained by scraping fungal growth off into plastic bags with a scalpel blade as well as removing pieces of the contaminated materials directly. 

These samples were plated (Fig 1.) onto microbiological media to obtain the fungal species into pure culture.  Plates were then incubated at room temperature for 7-14 days until fungal microcolonies appeared.  Microcolonies were then examined by microscopy to identify species present.  Some cryptic microcolonies were further subcultured onto additional microbiological media plates and incubated for a further 7 days under UV light to enable identification to be completed.

Cellotape mounts were also used to remove fungal growth from the contaminated test rig timber frames. Cellotape mounts were then directly examined by way of light microscopy and identification of species made where possible.

4.  Results

A range of widespread contaminant fungal species was encountered in this pilot study.  Table 1 below summarises the fungi obtained from the contaminated materials in the fibre cement test panel.

Table 1:  Fungi observed on water damaged FIBRE CEMENT TEST PANEL

Table key

+  = low contamination (at least 1-2 isolates obtained)
++ = moderate contamination (3 -10 isolates obtained)
+++ = high contamination (>10 isolates obtained)

Trichoderma viride was the most commonly encountered species on the moist timber frames, but colonisation by both Cladosporium and Penicillium species was also frequently observed (Table 1). 

Water damaged gypsum board was most commonly colonised by the black wet moulds Stachybotrys chartarum and Alternaria alternata, both of which have been reported to produce mycotoxins in indoor environments. Of note was a species of Corynespora which was relatively common but which has not been recorded contaminating building materials until now.  Another fungus obtained, Cheiromycella microscopia, is also unrecorded in building materials.  Sporulating structures (conidiophores) of Stachybotrys chartarum were observed on all building materials examined, which is consistent with previous results that showed this fungus to be widespread in a survey of water-damaged buildings in the Auckland region. 

There was low-level fungal contamination of both the insulation and paper materials but fungi were still present on both.

Identification was unable to be completed for three species as they were obtained in culture and pertinent diagnostic structures were absent from the cellotape mounts.  A species resembling a Cylindrocarpon was obtained in culture from the timber frames but more taxonomic work is required to determine its species classification.

5.  Discussion

There was no evidence of microbial growth on the two Hitex Test Panels.  This is assumed to be because the panels remained below 65% RH where fungi growth can establish.

The contaminant fungal species identified in the fibre cement panel materials is classified as early colonisers.  Longer test durations would expect coniophora puteana and possibly serpula lacrimans to establish.  These later fungi are the serious wood destroyers and generally establish following the early colonisers.

Most of the fungi present on the water-damaged test rig are widespread saprophytes and frequently reported as indoor contaminants.  The elevated level of fungal growth and infestation on these materials is indicative of relatively high moisture/humidity present in the fibre cement test panel materials due to the presence of elevated water.  Most fungal species are usually not present below 80% RH (Ref 3).  Some Penicillium species have been reported to be xerophilic (i.e. active between 65%-80% RH).  From previous overseas research, an abnormally high frequency of the wet-rot fungus, Stachybotrys chartarum, indicates a water activity in excess of 95%RH, which in buildings is only usually due to water-damage.  

The exact moisture and water activities of some of these NZ fungi is largely undetermined to date, therefore more research into this question, as well as the presence of species such as Cheiromycella and Corynespora, must be undertaken to determine the limiting growth factors of these fungi so that appropriate materials and building methods can be employed to eliminate nuisance and harmful microbial contamination.

Most indoor moulds 90-99% RH

Yeasts  88-99% RH

Xerophilic fungi (eg some Penicillium species)   65-90% RH

No fungal growth 65% RH

Fig 2

Fig 2 represents a plot of the equilibrium moisture content of wood (Ref2)  in relation to the relative humidity of air.  The Hitex humidity lines intersect the emc below the necessary conditions for moulds and fungi growth.  The Fibre Cement humidity lines intersect in the upper range between common and toxic mould establishment.

Also plotted on the emc curve is the NIWA (Ref 4) typical Auckland summer and winter days showing why we expect microbial growth even in summer conditions at 65%RH with more prolific growth in winter as RH approaches 90%.

References
 

1. Hitex Research Bulletins 301-306

2. Figures taken from

3. Water-humidity (water activity a_w), available moisture of building materials = vapour pressure of water in substrate ¸ vapour pressure of pure water.  For this report it is reported as % RH (Relative Humidity).

A. Gosai and S. Fisher of NIWA.  “Typical Temperature profiles for Auckland”.

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Page last updated Tuesday, 08 March 2005