Remedies: moisture

Houses built during the 1940s-60s generally have few weathertightness issues. With renovation, attention will be needed to ventilation to ensure that internal moisture does not become a problem.

Weathertightness and external moisture

The housing built during the 1940s-60s generally had few significant weathertightness issues.

However, some houses built later in the period, particularly those without eaves or with new cladding materials and glazing systems, may have had some weathertightness problems.

Once insulation or modern windows are installed, the house becomes more airtight, so consideration must be given to how the principles of Acceptable Solution E2/AS1 External moisture can be incorporated into any renovation that is carried out.

Other issues to consider during renovation include:

  • detailing windows where an existing window requires replacement or a new window is to be inserted into an existing wall
  • dealing with minimum roof pitches if they are below the current minimum slope requirements
  • the lack of a wall and roof underlay 
  • the need for a cavity when matching existing work (see Compliance paths).

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Window flashings

Traditional window and other head flashings have simply been inserted into the lap or joint of the weatherboard immediately above the opening. As long as the flashing remains in good condition, the detail appears to have generally worked well at protecting the top of the window.

Acceptable Solution E2/AS1 requires the flashing to be lapped under the wall underlay; this cannot be applied to an existing wall unless the cladding is being removed. BRANZ also believes that, where a window matching existing is being inserted into an existing wall, and the existing windows have performed well, to use the same detail is a logical solution.

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Internal environment

Internal moisture

Older, less airtight houses usually have fewer internal moisture problems than more airtight houses – air leakage through gaps around windows and doors generally removes internal moisture generated inside the house.

As a house becomes more airtight through renovations and alterations, internal moisture can become a problem. Renovation work will generally make a house more airtight. Systems to remove moisture must be included in the design solutions. Options include:

  • installing extract fans in kitchens and bathrooms that are ducted to the outside
  • installing insulation where possible to help maintain higher indoor temperatures 
  • installing security catches to windows to allow them to remain open and provide ventilation without a security risk
  • installing trickle ventilators into window systems to provide continuous ventilation.

If there is a musty smell indoors, it may also be the result of:

  • a damp subfloor and the migration of the moisture through the floor
  • a leak through the roof or wall cladding.

Occupants can help create a drier, healthier home by not using unflued gas heaters and by not drying washing inside. BRANZ testing has also shown that opening windows wide for just 10 minutes each day can reduce indoor moisture levels.


Houses can let in cold outside air and lose heat from the interior by air passing through gaps between weatherboards, tongue and groove board floors, around window sashes and doors, and through chimneys. Heat loss through draughts can be reduced by:

  • foam stripping doors and windows
  • installing carpet over foam underlay
  • removing or blocking open fireplaces or installing closed firebox inserts.

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Other problems


See Common problems and remedies: Roofing and cladding.

Rot and mould

Rot and mould are common when moisture is present. Rot can cause significant structural damage, and mould can be harmful to health. Read more.