The Challenges:
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Most New Zealanders know the experience of waking up in the morning with the inside surface of the windows dripping with condensation. It’s a very common problem, particularly in older homes with inadequate insulation and single glazing.

But it is only part of the issues with internal moisture in our homes. The continual chronic dampness often leads to problems within the walls and structure – which people may not be aware of until it has created major difficulties, affecting not only the health and wellbeing of families but also potentially impacting the physical integrity and durability of the building. Moisture problems can eventually become evident with:

• Visible mould, or more dangerously, invisible mould growing inside the wall linings or ceiling spaces

• Water stains or damp spots on ceilings, walls or floors

• Warping or sticking windows and doors

• Stubborn musty smells

Internal moisture is generated by normal everyday activities – cooking, washing, showering, even our breathing. A particular problem is drying clothes indoors:

“The 2010 BRANZ House Condition Survey found that 40% of NZ homes had internal moisture problems with 10% considered to be very damp. Health researchers have shown a very clear link between high internal moisture and cold internal environments causing asthma, respiratory tract issues, coughing, wheezing and breathing difficulties. Other associated problems are an increased prevalence of dust mites, bacteria, mustiness and unpleasant odours.”

“A person exhales approximately 200 millilitres of water vapour per hour while awake and approximately 20 millilitres of water vapour per hour during sleep.”

“A family of four in one home can produce about 20 litres a day of moisture.”

Increased insulation and airtightness in our homes and renovations means that effective ventilation is all the more critical to maintain healthy, dry interior spaces.  Without sufficient ventilation and air flow, and without the draughts common in vintage villas and older homes, there is nowhere for the moisture to go.
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Other factors that can contribute to internal moisture include:

• In older homes on piled foundations without an adequate damp proof barrier – moisture rising from the ground, rising from damp soil and creating humidity in the underfloor space which then causes condensation and damage to the timber framing

• Damaged water pipes – even a small leak can disperse a significant amount of water over time

• Leaking roofs or compromised flashings between external walls and a skillion roof or bay window canopy

• Retained construction moisture if timber framing or other building materials became wet and did not adequately dry out during construction.

Managing internal moisture is a requirement under the Building Code clause E3 Internal Moisture including provisions for:

• An adequate combination of thermal resistance, ventilation and space temperature to all habitable spaces, bathrooms, laundries, and other spaces where moisture may be generated or may accumulate

• Disposal of water from accidental overflow

• Impervious floor and wall surfaces and building elements

• Construction to prevent water from penetrating behind linings or into conceal spaces.

How we can minimise internal moisture:
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Key factors to integrate into the design and construction of a new build or renovation are ventilation, insulation, and smart recessed installation of high performance windows.

• Ventilation – either with a Mechanical Heat Recovery Ventilation system or with natural “stack effect” air flow to ensure a healthy exchange of fresh air for stale moisture-laden air throughout the home

• Subfloor ventilation – ensure adequate ventilation and good air flow under suspended timber floors, including vents in foundation walls

• Insulation – BRANZ research shows that homes maintaining a healthy temperature of 18°C experience far fewer periods of high humidity:
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  “For optimum occupant comfort, the relative humidity is recommended to be in the 40-60% range and the air temperature 18°-24°C.”
  
  “Indoor relative humidities… in NZ dwellings [range] from 30% to 65% during the day-time in a dry house, and 50% to 75% in a damp house. Cold bedrooms can have relative humidities of 80%-90% at night-time.”

• Subfloor moisture barrier – thorough coverage with sturdy polyethylene sheets to prevent water rising and condensation forming in foundation and floor structures

• Recessed windows installed in alignment with the insulation in the walls, rather than exposed to cold external conditions