Appendix B - Retrofitted insulation



B1.3.1 Buildings, building elements and sitework shall have a low probability of rupturing, becoming unstable, losing equilibrium, or collapsing during construction or alteration and throughout their lives.

Guidance: Retrofitted insulation could affect the structural performance (i.e. B1.3.1) of an existing house if moisture were to accumulate in a wall cavity and cause timber studs to rot and collapse. This could only occur if the weathertight performance of an existing house was compromised, as discussed below in relation to the effect retrofitted insulation has on compliance with the Building Code performances E2.3.2 and E2.3.5. The impact of retrofitted insulation on B1.3.1 is considered in the analysis for E2.3.5 below.

The performance of structural claddings or linings may be affected, when claddings and linings are altered as part of retrofitting insulation. Any holes, patching or reinstatement of structural claddings or linings will need to be done to a standard that doesn’t reduce the overall structural performance of the house.


B2.3.1 Building elements must, with only normal maintenance, continue to satisfy the performance requirements of this code for the lesser of the specified intended life of the building, if stated, or:

(a) the life of the building, being not less than 50 years, if:

(i) those building elements (including floors, walls, and fixings) provide structural stability to the building, or
(ii) those building elements are difficult to access or replace, or
(iii) failure of those building elements to comply with the building code would go undetected during both normal use and maintenance of the building.

(b) 15 years if:

(i) those building elements (including the building envelope, exposed plumbing in the subfloor space, and inbuilt chimneys and flues) are moderately difficult to access or replace, or
(ii) failure of those building elements to comply with the building code would go undetected during normal use of the building, but would be easily detected during normal maintenance.

(c) 5 years if:

(i) the building elements (including services, linings, renewable protective coatings, and fixtures) are easy to access and replace, and
(ii) failure of those building elements to comply with the building code would be easily detected during normal use of the building.

Guidance: Durability applies only to the extent that other Building Code performances apply. The effect retrofitted insulation has on the durability is considered in the discussion of the other Building Code performances.

Outbreak of fire

C1.3.2 Fixed appliances shall be installed in a manner that does not raise the temperature of any building element by heat transfer or concentration to a level that would adversely affect its physical or mechanical properties or function.

Guidance: Retrofitted insulation can affect compliance of an existing house in relation to outbreak of fire (i.e. C1.3.2) if the insulation covers appliances that generate heat. Sufficient clearance must be provided between thermal insulation and a heat source to prevent an undue build up of heat in adjacent building elements. The Acceptable Solution C/AS1 specifies clearances for insulation around downlights in ceilings, but no specific details are given for wall insulation.

Spread of fire

C3.3.5 External walls and roofs shall have resistance to the spread of fire, appropriate to the fire load within the building and to the proximity of other household units, other residential units, and other property.

Guidance: Retrofitted insulation can only affect compliance of an existing house with respect to spread of fire (i.e. C3.3.5) if the retrofitted wall is required to be fire rated. Insulation that is retrofitted into walls of detached dwellings that are lined with plaster board will comply with C3.3.5, by complying with the Acceptable Solution C/AS1. C/AS1 allows insulation materials to be used in walls provided they are protected from ignition, which in turn means that the insulation must comply with flame propagation criteria and be protected by a flame barrier16.

In addition, many insulation materials are either inherently non-flammable or have fire retardant additives improving their performance in a fire.

Moisture penetration

E2.3.2 Roofs and exterior walls must prevent the penetration of water that could cause undue dampness, damage to building elements, or both.

Guidance: The effect retrofitted insulation has on compliance of an existing wall with E2.3.2 will depend largely on the existing wall and the physical properties of the insulation. Walls with porous claddings, poorly maintained claddings, walls with pre-existing leaks or moisture problems and walls exposed to high wind and rain are more likely to have water penetrate through the cladding. Insulation materials that are porous or hydrophilic will tend to hold and transfer moisture through the insulation. In these situations, retrofitted insulation may help transfer moisture from the cladding to other building elements, thereby reducing compliance with E2.3.2. Conversely, walls with well maintained impermeable claddings, which have low exposure to rain, and include building paper inside the cladding are unlikely to have water penetrate the cladding. In this situation insulation will not reduce compliance with E2.3.2

The penetration of water through wall claddings is unlikely to be affected by insulation, unless the cladding is damaged and compromised by poor installation. Insulation has potential to reduce the penetration of wind driven water through claddings by increasing the air tightness of the wall and reducing the pressure differences that are generated across the cladding. However, any increase in air tightness will adversely affect the drying rate within the wall cavity, as discussed below in relation to NZBC E2.3.5.

The largest potential problem for retrofitted insulation in relation to E2.3.2 is the bridging effect of the material, which allows moisture on the inside of the cladding to penetrate further into the wall cavity, to framing and the lining. This bridging effect may be mitigated where well defined gaps exist between the insulation and the framing, cladding and/or lining. Small gaps between the insulation and the framing, cladding and/or lining will act as a physical barrier to moisture transfer, similar to the drainage cavity that is designed into new timber framed walls. However, such gaps generally diminish the thermal effectiveness of the insulation and should be avoided when insulation is installed unless they are specifically designed. Therefore, any mitigation from small gaps in the insulation should not be relied upon when assessing the building performance for a building consent, unless specifically designed.

Building paper, between the cladding and framing, will help to mitigate any moisture bridging by insulation as it will reduce the amount of moisture that comes into contact with the insulation.

Moisture accumulation

E2.3.5 Concealed spaces and cavities in buildings must be constructed in a way that prevents external moisture being accumulated or transferred and causing condensation, fungal growth, or the degradation of building elements.

Guidance: There are many contributory factors that affect condensation, fungal growth or degradation of building elements. This complexity means it is difficult to provide simple rules for assessing compliance with E2.3.5 that are not overly conservative in many situations. Installers, designers and BCAs need to consider the merits of each installation when assessing the affects of retrofitted insulation on the compliance of existing walls with E2.3.5.

Retrofitted insulation has potential to affect the accumulation of water in wall cavities, causing condensation, fungal growth or degradation of building elements. Its affect will depend to a large degree on the design and condition of the existing wall/building, the climate and the physical properties of the insulation.

Insulation will reduce the air movement in wall cavities and in doing so will reduce the rate at which moisture dries out of wall cavities. However, the drying rate depends on a number of factors, including climate, wall design and the permeability of the claddings, linings and insulation, as well as the ventilation rate. In addition, the ventilation rate and moisture exposure are not the only factors affecting condensation, fungal growth and degradation of a wall. The materials used to construct the wall and the treatments applied will also impact on compliance with E2.3.5.

Factors that will tend to lessen the impact of insulation on compliance of an existing wall with E2.3.5 include:

  • impermeable claddings that are well maintained,
  • low exposure to rain (e.g. wide eaves and low wind)
  • building paper inside the cladding
  • steel or treated timber framing
  • permeable wall linings
  • warm climate

Factors that may lessen compliance of existing walls with E2.3.5 when insulation is retrofitted include:

  • porous claddings,
  • high rain exposure
  • untreated timber framing and cladding, including native sapwood
  • vapour barriers
  • high humidity
  • cold climate

Noise performance

G6.3.1 The Sound Transmission Class of walls, floors and ceilings, shall be no less than 55.

Guidance: Insulation will have no affect on the compliance of an existing house in relation to noise (i.e. G6.3.1), because the Building Code does not control the noise transmission through external walls.

Only when insulation is retrofitted to intertenancy walls is there potential for it to affect compliance with G6.3.1. In these unlikely situations, the insulation materials and the installation will have a large bearing on the noise performance of the wall, and so must be considered on a case-by-case basis.

Electrical safety17

G9.3.1 The electrical installation shall incorporate systems to:

(d) safeguard people from injury which may result from electromechanical stress in electrical components caused by currents in excess of the installation rating,

(e) protect building elements from risk of ignition, impairment of their physical or mechanical properties, or function, due to temperature increases resulting from heat transfer or electric arc,

Guidance: Retrofitted insulation can affect the continued electrical safety of an existing house (i.e. G9.3.1) if either, the thermal insulation reduces the heat dissipation from the wires, the thermal insulation and electrical insulation are incompatible, or if the thermal insulation causes electrical circuits to short.

The safety of electrical systems may be compromised by retrofitted insulation if electrical wires that lie within a wall cavity are encased/enclosed by the insulation. Both the current carrying capacity of the wiring (i.e. G9.3.1(d)) and heat dissipation from the wires (i.e. G9.3.1(e)) will be reduced by the thermal insulation.

The safety of an electrical wire enclosed by insulation will depend on the original rating of the electrical circuit, the proximity of the wire to combustible building elements and the electrical load on the circuit. The current-carrying capacity of electrical wires completely surrounded by thermal insulation is half that of wires surrounded by air 18,19.

The compatibility of thermal insulation with the electrical insulation protecting the wiring must also be checked. The common material incompatibility is polystyrene thermal insulation and PVC insulation on electrical wires. Care must be taken to avoid contact between these materials, as polystyrene has the potential to embrittle PVC.

Liquid-applied and loose-fill insulation may cause short-circuits if it migrates into unsealed electrical flush boxes or contacts surface mounted electrical switches and fittings. Old electrical circuits which use VIR (vulcanised indian rubber) or TRS (toughened rubber sheathed) cables may also short-circuit if insulation is pushed onto and around these old cables. This is because the electrical insulation on VIR and TRS cables tends to degrade and become fragile and brittle over time.

When the safety of electrical circuits is likely to be compromised by retrofitting insulation, or when electrical wiring is modified, a registered electrician should check and certify the electrical safety. Miniature circuit breakers can be used to improve protection from overload currents, on circuits when the electrical wiring is surrounded by thermal insulation. Alternatively, the size of electrical wiring may be increased to safely carry the current and limit temperature build up. VIR and TRS cables may also need to be replaced, depending on their condition.

Energy efficiency

H1.3.1 The building envelope enclosing spaces where the temperature or humidity (or both) are modified must be constructed to—

(a) provide adequate thermal resistance; and
(b) limit uncontrollable airflow.

Guidance: It is almost certain that retrofitted insulation will improve both the thermal resistance and the airtightness of an existing wall, so will not adversely affect the compliance of an existing house in relation to H1.3.120.

The compliance of retrofitted insulation with H1.3.1 is not a consideration for retrofit situations where the thermal envelope of the building (which H1.3.1 relates to) is not being replaced.

For example, the thermal envelope of an old timber framed house would comprise the weatherboard cladding, the still air in the framing cavity and the interior wall linings. Installing insulation into the framing cavity would not alter the thermal performance of the weatherboards or linings, and would improve the performance of the still air, which would by and large be replaced by insulation. Replacing the still air with insulation, while altering an element within the thermal envelope, does not trigger any requirement to upgrade the thermal envelope to meet the Building Code. This principle of not having to upgrade to Code compliance when alterations are undertaken on only part of a building is described in section 112(1)(b) of the Building Act.

The thermal performance of retrofitted insulation can vary considerably, depending on the type of insulation, the quality of installation and the in-service conditions. While the thermal performance is not relevant to issuing a building consent, it is likely to be fundamental to any contract between a homeowner and the supplier/installer. In this regard, the relevant legal mechanisms that ensure retrofitted insulation delivers good thermal performance are; the contract, the implied warranties in the Building Act and the Fair Trading Act.

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: