Risks with retrofitting wall insulation

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Existing building compliance level after retrofitting insulation also discusses how to ensure risks relevant to building code compliance are not increased after retrofitting insulation, and some others that may be applicable.

Fire and electrical safety with retrofitted insulation

Fire safety can be affected if insulation is installed over or around appliances that dissipate heat.

For example, fires have occurred in buildings where ceiling insulation has been installed over downlights. In these situations, heat built 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 a flue penetration.

NZS 4246 provides recommended safety clearances between thermal insulation and such heat sources for common residential construction. Further requirements for luminaires can be found in AS/NZS 3000.

Retrofitting insulation to fire separation walls (such as external walls of houses that are close to boundaries) should be considered on a case-by-case basis. Many such walls will already contain insulation as part of the fire rated system. The use of highly flammable insulation could also affect the performance of the fire separation.

Electrical safety can be compromised if existing electrical wires are covered by thermal insulation. Electrical
cables and fittings could be damaged or short-circuited if disturbed when retrofitting insulation. Old electrical
circuits are most at risk, as the electrical insulation covering the wires can be fragile and prone to fail.
Additionally, foamed polystyrene insulation can damage the PVC insulation on most modern wiring, creating a
new electrical safety risk.

Another safety consideration is that both the current carrying capacity of electrical wiring, and the heat dissipation from the wires, will be reduced when these are covered by thermal insulation.

Miniature circuit breakers (MCBs) can help to mitigate the harmful effects of covering electrical wires with thermal insulation, by cutting power off when a circuit is overloaded and the wiring could overheat. Electrical outlets could also be rewired with thicker wiring or with additional subcircuits to reduce the subcircuit load. Older electrical installations tend to have hard wired fuses, which provide less overload protection than MCBs.

Residual current device (RCD) protection can also be added to subcircuits at the distribution board, to mitigate against any short-circuits.

An electrician can advise on suitable measures to improve electrical safety during insulation retrofits.

Moisture transfer and retrofitted insulation

The biggest potential problem for retrofitted insulation is that the insulation material will create a pathway for external moisture to migrate to where it could damage moisture-sensitive materials in the wall.

This occurs when the insulation material creates a pathway for moisture present on the back (inside) face of the cladding to penetrate further into the wall cavity. That moisture could then come into contact with the wall framing and interior lining and could also be held within or adjacent to the insulation for a prolonged period. Any increase in moisture can cause or increase fungal growth in a wall. Moisture could also cause or exacerbate
any electrical hazards where there are electrical fittings in the wall.

Sources of moisture within a wall cavity may include

• External water passing through a porous or absorbent cladding (for example rain on brick veneer).
• External moisture entering at joints in the cladding or around openings (such as at laps in weatherboards or at a defective or ineffective window flashing).
• Moisture generated within the building (for example clothes drying or cooking).
• Moisture rising from a damp and inadequately ventilated subfloor space.

Moisture transfer into and out of walls occurs in a variety of ways through the movement of liquid water, through the movement of air which carries water droplets and water vapour, and through the diffusion of vapour through building materials.

The amount of water that transfers through a wall via these different mechanisms will vary with different types of construction, different climatic conditions, and different ways that a building is used.

Moisture transfer through walls can be beneficial as well as problematic. Problems arise when drying and ventilation is inadequate to remove moisture and it accumulates inside walls, providing suitable conditions for fungal growth. The presence of insulation tends to inhibit ventilation and drying in comparison with an uninsulated wall.

Potential problems with fungal growth inside walls include:

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

Older houses often do not have the cladding drainage cavities and timber treatments that are common in new houses, making them more vulnerable to moisture entry, 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
• designs that are less prone to moisture ingress (eg simple forms, wide eaves, single cladding type), or
• high ventilation rates from relatively high 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(s) of moisture that may be transferred into a wall (eg the existing weathertightness, air and water permeability of interior linings in wet areas, and subfloor conditions), and whether any defects will be rectified
• how the proposed method of retrofitting wall insulation manages the risks of moisture transfer and accumulation (see BRANZ articles linked in the box above)
• 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, and
• the resistance of wall materials to fungal growth (eg timber treatments).

Asbestos

Older buildings may have asbestos-containing materials present. These will need to be identified and managed as part of the retrofit planning.