Guidance on Building Code compliance for retrofitting insulation in external walls
This document is issued as guidance under section 175 of the Building Act 2004.
On this page
Appendices
Introduction
This guidance provides information on the Building Code1 and Building Act 2004 (the Building Act) requirements that relate to retrofitting insulation in external walls of buildings. It highlights the main considerations when assessing work for compliance and lists the relevant Building Code performance criteria for common types of insulation retrofits, such as retrofitting rigid or semi rigid segments and sheet insulation, injected loose fill insulation or injected foam insulation.
Decisions on this work can be complex. Insulation is usually retrofitted into older houses that have stood the test of time but are now too cold and draughty for modern lifestyles. Old houses often have simpler designs and are built of more rot-resistant timber than houses built during the 1990’s, which are now stigmatised by moisture problems. Insulation retrofits deliver tangible benefits, but there can be unintended problems if the insulation or the installation is unsuitable.
This guidance does not provide solutions or recommend installation methods for particular types of insulation or house construction. Insulation specialists should be consulted if you have doubts about how best to retrofit insulation.
Who is it for?
The guidance is intended for building consent authorities (BCA’s), to help them decide whether to exempt wall insulation from a building consent or to approve/decline building consent applications for retrofitting wall insulation. Designers and installers may also find the document a useful guide to the relevant Building Code requirements for retrofitting wall insulation.
Back to Top
Is a building consent required?
Schedule 1 of the Building Act now exempts underfloor and roof insulation retrofits from building consent2. However, retrofitting insulation into a wall cavity is not exempt; it either requires a building consent or specific approval from a BCA that a building consent is not required3.
Building consent applications for retrofitting wall insulation will need to cover all the associated building work, show that it will comply with the Building Code and that it will not adversely affect the performance of the existing building.
Why is retrofitting wall insulation not included in Schedule 1?
Retrofitting insulation to walls involves more work than simply placing or injecting insulation in walls. All the associated work must be assessed to see if it complies with the Building Code, whether it is as small as drilling and repairing holes in interior linings or as significant as removing and reinstating exterior claddings. Even if the building work is relatively simple, the effects on the existing building may be complex and potentially problematic.
Retrofitted wall insulation may affect moisture transfer inside timber framed walls and change drying rates, which in turn may cause moisture to accumulate in the wall and affect the durability of timber framing and cladding. Poorly installed insulation may also affect the fire and electrical safety of houses. Houses must be assessed on a case-by-case basis to determine if they are suitable for retrofitting wall insulation and if the proposed methods and materials are appropriate.
Back to Top
Compliance with the law
Complying with the Building Act
Retrofitting insulation into the external walls of an existing building is “building work”4 and therefore must comply5 with the Building Code. When a building consent is required (see previous section), it should not be granted until it is shown that the alteration (i.e. the insulation retrofit) will not reduce the extent to which the existing building meets the Building Code performance criteria6.
It is important to distinguish between the need for “building work” (i.e. retrofitting insulation) to comply with the Building Code (as required by section 17 of the Building Act) and the need to ensure retrofitted insulation does not reduce the extent to which the existing building complies with the Building Code (as required by section 112(1)(b)7 of the Building Act). These two requirements relate to different parts of the building (i.e. new part versus existing parts); the extent of Code compliance is different; and they can relate to different Building Code performance criteria.
Complying with the Building Code
When buildings, or parts of buildings, are constructed or altered they must meet Building Code performance criteria8. Retrofitted insulation material, sitting within the wall cavity, is required to meet relevant Building Code performance criteria. The associated building work, such as repairing cracks in claddings, fixing holes that have been drilled and reinstating linings or claddings must also comply with the relevant Building Code performance criteria.
Performance criteria that relate to the existing building (particularly the wall framing, cladding and internal lining) and with which the building must continue to comply to the same extent9, may or may not be the same as those relating to the insulation retrofitting work. The Building Code performance criteria relevant to the retrofitting work and the Building Code performance criteria relevant to the existing building are analysed in Appendix A and B.
How do I show the work complies?
There are no acceptable solutions or verification methods specifically for retrofitting insulation into wall cavities. The current acceptable solutions provide useful benchmarks when deciding if relevant performance criteria are met, in some instances. However, not all the performance criteria relevant to retrofitting wall insulation have corresponding acceptable solutions, which means judgements will need to be made on each ‘specific design’.
Decisions on the Code compliance of retrofitting wall insulation, in particular the effects of the retrofit on the existing building, can be complex. These decisions are not easily distilled into a one-size-fits-all solution. Appendix A and B highlight relevant factors to consider when deciding whether insulation retrofits comply with the Building Code. A number of different approaches may be used to support claims that work complies with the Code including Determinations, compliance with Standards, test results, expert opinion, appraisals or in-service history10.
Back to Top
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
- timber decay, which reduces the strength of framing and other wall components, and
- 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.
Back to Top
Appendices
Appendix A - Building Code performances that retrofitted insulation must comply with12
The Building Code performance criteria listed below are the relevant provisions of the Code to consider when assessing retrofitted wall insulation and whether it complies with the Code. Other Building Code performance criteria may also need to be considered for the building work associated with retrofitting insulation. For example, removing and reinstating structural wall linings or drilling holes through studs would require compliance with the Building Code clause B1.3.1. However the insulation itself does not need to comply with B1.3.1 as it is not part of the structural system of a building.
It is useful to highlight upfront that the R-value of the retrofitted wall does not need to comply with the Building Code clause H1 Energy efficiency. There is no doubt that insulation is retrofitted to improve the wall R-value, but from a regulatory point of view retrofitting insulation is simply an alteration of the ‘thermal envelope’ described in H1.3.1(a). Unless the cladding, framing and linings of the wall are also reconstructed it is hard to consider how the ‘thermal envelope’ has been reconstructed, which would trigger compliance with H1.3.1(a). Therefore, the energy efficiency provisions of the Building Code are not mentioned in this section, which lists the performance criteria that insulation retrofits must comply with.
Durability
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:
(iii) failure of those building elements to comply with the building code would go undetected during both normal use and maintenance of the building.
Guidance: The durability requirement in the Building Code applies only to the extent that other Building Code performance criteria apply. The Building Code requires 50 year durability for building elements that are difficult to access or replace, or where failure of the building element to comply would go undetected.
The durability requirement is not relevant to E2.3.6, which relates to moisture at the time of construction (see External moisture below). Common types of insulation are likely to meet the performance criteria F2.3.1 for a period of 50 years. Unusual types of insulation or very harsh environments may cause insulation to degrade over time and produce hazardous materials, though such a possibility could only be assessed on a case-by-case basis.
External moisture
E2.3.6 Excess moisture present at the completion of construction must be capable of being dissipated without permanent damage to building elements.
Guidance: Moisture levels in most types of insulation should be at acceptable levels when installed. Moisture levels in insulation that is installed wet are likely to drop over time, provided the existing wall is vapour permeable and does not have pre-existing moisture problems. However, compliance of insulation that is installed wet with NZBC E2.3.6 will be difficult to assess given the variability in drying rates that occur and would need to be assessed on a case-by-case basis that could involve measurements.
There is no acceptable solution for the dissipation of construction moisture from retrofitted insulation. Although not directly applicable, the Acceptable Solution E2/AS1 does provide a useful upper limit of 20% for timber moisture levels in timber framed walls13. The water content of some types of insulation that are installed wet is approximately 75% by weight, so the insulation must dry out after it is installed. The moisture content of adjacent timber framing should not exceed 20%, as suggested by the Acceptable Solution.
A study of moisture levels in cavity walls show that drying rates vary widely depending on the type of wall construction, temperature and ventilation rate14. Drying times of 600 hours (i.e. 25 days) were measured for timber framing in south-facing, direct fixed walls with insulation installed in the framing cavities. In a separate study, moisture readings of timber framing in a brick veneer wall took approximately 60 days to drop below 20% moisture content after urea-formaldehyde foam was injected into the wall drainage cavity15. It was noted that the drying rate, which was measured in summer, would be significantly worse in winter and would likely result in south facing walls staying ‘wet’ throughout winter.
Factors that will affect the drying potential of insulation in a cavity wall include,
- the vapour permeability of the wall linings and claddings (including any building wraps, paints and surface coatings)
- the rain and wind environment (i.e. the wetting potential)
- the ground conditions and foundation connections to a wall
- the condition of the existing cladding (e.g. cracks and gaps)
- the ventilation rate within the wall cavity
- temperature of the external and internal wall surfaces
Hazardous materials
F2.3.1 The quantities of gas, liquid, radiation or solid particles emitted by materials used in the construction of buildings, shall not give rise to harmful concentrations at the surface of the material where the material is exposed, or in the atmosphere of any space.
Guidance: Provided insulation is properly manufactured and installed, in accordance with manufacturers’ instructions, it is likely that it will comply with F2.3.1.
There is no Acceptable Solution covering hazards associated with insulation. However, off-gassing and small airborne particles are the primary hazards to consider with insulation.
A number of different chemicals are used in various types of insulation materials and in the binders that hold some types of insulation together. While such chemicals can be hazardous in high concentrations, generally the concentrations that are associated with thermal insulation are not high enough to be considered problematic. Formaldehyde is such an example, and while relatively common in many different building products it is generally not found in sufficiently high concentrations to be considered hazardous.
Back to Top
Appendix B - Building Code performances that should not be compromised by retrofitted insulation
Retrofitted insulation may effect the way existing parts of a house perform, particularly parts of a house that are adjacent to the insulation such as wall frames, claddings and linings. The performance of an existing house should not be reduced in relation to the following provisions of the Building Code.
Structure
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.
Durability
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 air18,19 .
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.
Back to Top
Footnotes
Back to Top