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Housing
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Condensation & Mildew


One of the most common complaints concerning housing, is that of condensation and/or mildew growth on walls, ceiling, in cupboards etc. Mildew growth occurs when the atmosphere within a dwelling remains damp and when little ventilation is provided.

Condensation and subsequent mildew growth can be caused by faults in the dwellings construction, such as lack of insulation, but more commonly is caused by moisture generated by activity within the dwelling, such as cooking, showering, failure to ventilate etc,.

Controlling it in My Home

Keeping Condensation and Mildew at Bay

Condensation and mildew in homes are mainly winter problems but they can occur year round in some situations. The Environmental Health Department of Central Hawke's Bay District Council is regularly asked for advice on controlling condensation and mildew in the home. The purpose of this pamphlet therefore is to provide some simple guidelines to minimise the moisture burden inside your home, therefore reducing the level of condensation and mildew.

Condensation occurs when the air becomes saturated with water vapour to the point where it cannot hold any further moisture, i.e. high moisture burden. In areas where there is a temperature difference between two surfaces, such as with windows, some of the moisture will condense on to the colder surface. Condensation can leave water stains and if persistent, can lead to mildew growth and will help rot set in.

Mildew and other fungal growths grow from spores that abound everywhere. Mildew needs moisture in order to grow so the more persistently damp a situation the more likely mildew will grow.

How can you reduce the level of Condensation? There are two ways to reduce the level of condensation in your home - by heating and ventilation. Heating will raise the air temperature and allow the air to hold more moisture, ventilation then removes the moisture to the outside.

Tips to Remember

  • Keep the house warm, generally 5-7°C warmer than the outside is recommended. A little heat constantly throughout the day is more effective than a lot of heat in the evenings.
  • Space heaters, open fires or electric heaters are best to provide warmth. Gas or oil heaters give off moisture as they provide heat, thereby contributing to the air moisture burden.
  • It is better to have many windows slightly open for long periods than a few wide open for short bursts, as this prevents the home from losing too much heat as well as providing a more constant level of ventilation. However, there are times when opening windows wide helps to remove large quantities of moisture rapidly, the most obvious times being during cooking or using a shower or bath.
  • Keep doors closed when using bathroom, or kitchen to prevent the steam spreading through your home. Extract fans over a shower or stove for example, will also assist to remove air moisture from the room but do ensure it is ducted outside, not into the ceiling or wall cavities. If you have a clothes drier, ensure it is also ducted to the outside.
  • Use heavy curtains that cover the windows completely so they 'seal off' the cooler surface from the warmer air.
  • Wipe any moisture off glass or cold surfaces when condensation has occurred.
  • Avoid hanging wet clothes indoors to dry out.
  • Limit the number and size of houseplants as the plants themselves, along with watering, can add to the level of moisture in the air.
  • Provide ventilation to wardrobes by such means as louver doors or cupboard heaters. Products like Damprid remove moisture from the air but they must be changed regularly.
  • The use of dehumidifiers in particularly cold and damp areas is recommended.

Carrying out building repairs or alterations

If carrying out building repairs or alterations, you should also:

  • Insulate the ceiling and wall cavities to keep these surfaces warm, and thereby lowering the risk of condensation forming.
  • Install the correct building papers on the outside wall frame and under roofing to keep the rain and wind out and to absorb moisture if necessary.
  • Prevent water leaks in the walls and roof.
  • Do not use wet timber or particle board during construction. If the materials are wet allow a drying out spell before finally closing in the structure.
  • Adequately ventilate the basement or underfloor area and insulate with perforated reflective foil,
  • Repair any faulty drainage so water does not lie around the house.

Dealing with Mildew

Mildew can usually be washed out of clothing and curtains; however, if this is not done in time, it can leave permanent stains.

For mildew growing on walls and ceilings, clean the area down with a damp cloth and household bleach. 1 part bleach to 4 parts water is recommended. Use a test-patch to make sure the bleach does not effect the paint or wallpaper colour. If the colour is damaged, try a fungicide solution, which you can purchase from most paint or hardware shops.

If re-wallpapering, strip the old paper off the wall first. Next paint the wall with a fungicide solution. When it is completely dry, re-hang the wallpaper using a paste containing a fungicide agent.

If re-painting, use a paint containing a fungicide agent but never paint directly over existing mildew.

When the mildew has been removed, ensure it does not recur by keeping your home warm throughout and moderately ventilated.

So remember to prevent condensation and mildew, it's a bit like real estate, the golden rule is

"ventilation, ventilation, ventilation"

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