Risks with retrofitting wall insulation


The greatest unknown, and potential risk, when insulation is installed into the external walls of existing houses is the effect on moisture transfer within walls, and the effects any increase in moisture will have on fungal growth in a wall.

Sources of moisture within a wall cavity may include: external water passing through a porous envelope (eg rain on brick veneer); external moisture entering around openings (eg a defective or ineffective window flashing); moisture generated within the building (eg clothes drying or cooking); or moisture rising from inadequately ventilated subfloor space where the ground is poorly drained.

Moisture transfer into and out of walls occurs in a variety of ways through the movement of liquid water and water vapour or from diffusion through building materials. The amount of water that transfers through a wall via these different mechanisms will vary with different types of construction. Moisture transfer through walls is not, in itself, a problem, but it can be problematic when drying and ventilation is inadequate to remove moisture and moisture accumulates inside walls, providing suitable conditions for fungal growth.

Potential problems with fungal growth inside walls are:

  1. timber decay, which reduces the strength of framing and other wall components, and
  2. production of mycotoxins, which are deleterious/harmful to human health.

There has been a substantial amount of effort to develop robust ‘weathertight’ designs for new houses, but little guidance is available when altering existing houses. Timber treatments and drainage cavities that are usual in new houses are often not present in older houses making them more vulnerable to fungal growth and the problems that arise from this. On the other hand, older houses may have mitigating factors such as;

  • native heartwood timber that is more resistant to fungal growth/decay than radiata pine
  • simple designs (e.g. wide eaves) that are less prone to moisture leaks
  • high ventilation rates from relatively large air-leakage, both within wall cavities and within the interior of the house.

Specific consideration should be given on a case-by-case basis to:

  • the amount and source of moisture that may be transferred into a wall (i.e. the existing weathertightness, defects and subfloor conditions), and whether such defects should be repaired
  • the extent to which ventilation of the framing cavities in a wall, and drainage cavity if included, would be affected by the installation of insulation
  • the resistance to fungal growth (e.g. timber treatments)

Fire safety can be affected if insulation is installed over or around appliances that dissipate heat. There have been reports of fires in buildings where ceiling insulation has covered downlights. In these situations heat builds up within the insulation, downlight and adjacent parts of a ceiling until it reaches combustion temperatures. Similar problems could occur in an insulated wall that has recessed luminaries or internal flues. The use of highly flammable insulation could affect the performance of fire separation walls, such as external walls of houses that are close to boundaries.

Electrical safety can be compromised if existing electrical wires are covered by thermal insulation and, as a consequence, the current-rating of the wire is reduced below the electrical loads on the circuit. Old electrical circuits are most at risk, as the electrical insulation covering the wires11 can be fragile and prone to fail. Old circuits also tend to have hard wired fuses, which provide less overload protection than modern mini circuit breakers. Mini circuit breakers help to mitigate the adverse effects of covering electrical wires with thermal insulation.

This information is published by the Ministry of Business, Innovation and Employment’s Chief Executive. It is a general guide only and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case. Expert advice may be required in specific circumstances. Where this information relates to assisting people: