Condensation, Termites and other Timber pests
When air is cooled, its ability to retain water vapour is reduced and it condenses (merges into large drops of water) on colder surfaces. In most cases this condensed water evaporates again, but sometimes a wall, floor or ceiling may remain wet for days, or even months. This may lead to corrosion (rusting) of nails and fastenings, sagging of ceilings, rotting of framing timbers and encouragement of mould growth or timber pest activity such as termites.
There has been a trend in the warmer climes over the last 30 years to retrofit roof void insulation and air conditioning to older homes. During our Building and Pest Inspections we are seeing an increasing number of problems stemming from this combination of “improvements”. For Example: In a hot humid summer the occupier of a home has their air conditioning running. The ceiling sheeting is cooled buy the cooler air in the room below. The humid air in the roof void is then cooled by the cooler ceiling sheeting and condensation occurs. This condition in turn increases the moisture content of the ceiling joists and battens to a point were condition conducive to fungal decay, borers, and termites are created. When ceiling insulation is then introduced, evaporation of the condensation is prevented making the situation worse.
So why is this a termite issue?
When timbers moisture increases above 15% a commonly present soil bacteria called “Actinomycetes” multiplies rapidly in the damp timber environment. This bacterium produces CO2 and other gaseous compounds. These compounds also responsible for the present smell of freshly turned soil. Termites have evolved over millions of years to be attracted to these compounds and will move from the ground towards this damp timber and establish a feeding site. It is not uncommon for termites to bypass many meters of timber framing to establish a feeding site where there is a warm, high moisture and stable environment under the insulation in the roof void.
Building design must take many interrelated factors into account if condensation is to be eliminated. Such factors include orientation, temperature and relative humidity differentials, shading, ventilation, methods of heating and cooling, building techniques, use of materials of low or high thermal capacity, function of the building, and living habits of the occupants.
The aim should be to always:
- Keep materials and structures above the "dew point" temperature
- Keep air spaces well ventilated to allow re-evaporation
Moist air should be kept away from cold surfaces (e.g. by using a vapour barrier, i.e. a thin membrane of low water vapour permeability). Ventilation which encourages warm moist air escape is critical and an effective solution that minimizes condensation. This is provided in roofs with airspace above the insulation systems by ridge vents or end wall louvers.