1970s case studies

These case studies show how to choose and apply a compliance path for a 1970s house renovation.

They give examples of the compliance path options that may be used demonstrate performance with relevant clauses of the Building Code.

They follow the six steps set out in the page on selecting compliance paths.

The case studies cover:

  • Case study 1: An extension is proposed with a large area of glazing, but this will reduce overall thermal performance. Additional insulation may be required in the existing structure to compensate.
  • Case study 2: An extension is proposed for a weatherboard-clad house. The owners want to install new cladding without a drainage cavity so it aligns with existing cladding.
  • Case study 3: The proposed renovation of a 1970s house is to include a new extension and to insert a new double-glazed aluminium window (R-value = 0.26) into a wall where there is currently no window.
  • Case study 4:The proposed renovation of a 1970s house is to include a first floor addition.
  • Case study 5:The owners of a 1970s house with a reinforced concrete floor want to add an extension maintaining a consistent floor level throughout, but the existing floor is too close to the ground to provide the clearance required under E2/AS1.

1970s houses – compliance case study 1

An extension is proposed to add a family room to an existing, open plan kitchen/dining room area of a house built in the 1970s.

There is no insulation in the walls or underfloor (concrete slab) of the existing house, but some insulation was installed in the ceiling in the 1980s.

The owners want to have a large area of glazing in the extension (approximately 60% of the total new wall area). Although it will be double glazing, the BCA is asking for the existing walls of the kitchen/dining room space to be insulated to improve the overall thermal performance of the new space.

The owners prefer not to do this as they had not planned to re-line existing walls and to do so will add significantly to the cost of the project.

Step 1. Identify the aspects of the proposed design that fall outside the scope of Acceptable Solutions The large amount of glazing means that the performance requirements for energy efficiency of the extension cannot be met without additional insulation being installed.
Step 2. Identify the Building Code clauses for which performance must be demonstrated by the design and supporting documentation.

The Building Code clauses to be addressed by this Alternative Solution are:

• H1 Energy efficiency

Step 3. Identify the performance criteria that apply.

The performance criteria that apply include:

• H1.3.1 The building envelope must provide adequate thermal resistance and limit uncontrollable air flow.

Step 4. Select the most relevant compliance path(s).

The most relevant compliance path options could be:

• Compliance path 9 – use of an Acceptable Solution

Step 5. Determine what type of information is required to demonstrate compliance.

To demonstrate compliance it is necessary to provide evidence of adequate thermal resistance, i.e. the Calculation Method from NZS 4218 Energy efficiency – small building envelope.

The calculation method of determining minimum R-values allows for a range of wall construction types and applies to buildings where the glazing is over 30% of the total wall area.

If, by increasing the amount of underfloor and roof space insulation of the new extension, the performance requirements of NZBC Clause H1 are not met, additional insulation could be installed in the ceiling space over the existing kitchen/dining room. If this is not sufficient, the house could be looked at as a whole and insulation added to the ceiling space of the entire house.

In the worst case scenario, the owners may have to insulate some existing walls (which is probably a good solution anyway).

Step 6. Provide the evidence.

As it is an Acceptable Solution, when they meet the performance requirements of NZBC Clause H1, it must be accepted by the BCA.

 

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1970s houses – compliance case study 2 

A house has direct-fixed rusticated weatherboard cladding over building paper. It is to have an extension added and the owner would like to use weatherboards to match the existing. Having to install a drainage cavity as required if following E2/AS1 will make alignment of old and new cladding difficult, so the owners would like to omit the drained cavity.

Step 1. Identify the aspects of the proposed design that fall outside the scope of Acceptable Solutions The proposed cladding material is outside the scope of the Acceptable Solution E2/AS1 because the building has a weathertightness risk score of 8 which is just above that in E2/AS1 which allows direct-fixed rusticated boards.
Step 2. Identify the Building Code clauses for which performance must be demonstrated by the design and supporting documentation.

The Building Code clauses to be addressed by this Alternative Solution are:

• B2 Durability

• E2 External moisture

Step 3. Identify the performance criteria that apply.

The performance criteria that apply include:

• B2.3.1 Building elements must have a not less than 15 year durability

• E2.3.2 Roofs must prevent penetration of water

Step 4. Select the most relevant compliance path(s).

The most relevant compliance path options could be:

• Compliance path 3 – comparison with in-service history

• Compliance path 4 – expert opinion

Step 5. Determine what type of information is required to demonstrate compliance. To demonstrate compliance it is necessary to provide evidence of the performance of the existing cladding in terms of durability and preventing water ingress.
Step 6. Provide the evidence.

The condition of the existing framing behind the existing weatherboard without a cavity would provide evidence of the in-service history and satisfactory performance. Removal of a small part of the wall to demonstrate the lack of damage to the wall behind the cladding would reinforce the evidence of the satisfactory performance.

A report by a building surveyor on the state of the framing behind the cladding would provide the expert opinion that the performance of the existing wall cladding has been satisfactory for the life of the house, even if the risk of water ingress is slightly higher risk than allowed under E2/AS1.

In a similar situation the designer proposed that a flexible air barrier be used behind the new cladding (which allowed the boards to be aligned) to provide an additional level of protection over that provided by a wall underlay that was not an air barrier.

 

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1970s houses – compliance case study 3

The proposed renovation of a house is to include a new extension and to insert a new double-glazed aluminium window (R-value = 0.26) into a wall where there is currently no window. The wall where the proposed new window is to be installed is timber-framed with bevelback weatherboard external cladding, plasterboard internal lining and is uninsulated, giving an R-value of approximately 0.5.
Step 1. Identify the aspects of the proposed design that fall outside the scope of Acceptable Solutions The extension is able to meet the performance requirements of the relevant NZBC clauses but the installation of the new window will reduce the thermal resistance of the existing wall.
Step 2. Identify the Building Code clauses for which performance must be demonstrated by the design and supporting documentation.

The Building Code clauses to be addressed by this Alternative Solution are:

• H1 Energy efficiency.

Step 3. Identify the performance criteria that apply.

The performance criteria that apply include:

• H1.3.1 The building envelope must provide adequate thermal resistance and limit uncontrollable air flow.

Step 4. Select the most relevant compliance path(s).

The most relevant compliance path options could be:

• Compliance path 9 – use of an Acceptable Solution.

Step 5. Determine what type of information is required to demonstrate compliance.

To demonstrate compliance it is necessary to provide evidence of adequate thermal resistance (schedule method from NZS 4218) and how the air flow is controlled (window manufacturer’s information).

In this situation, some upgrading of the existing wall is required to bring the performance up to at least that of the pre-alteration performance. This could be achieved by adding insulation to the existing wall at the same time as the new window is installed. Alternatively, it may be possible to increase the R-value in the new addition sufficiently to compensate for the reduced R-value of the existing wall.

Step 6. Provide the evidence.

The compliance path option is an Acceptable Solution, so it must be accepted by the BCA. Where the existing wall has insulation added to it, the resulting R-value of the wall is likely to be better than the performance of the wall prior to the alteration work.

 

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1970s houses – compliance case study 4

A proposed renovation to a house built in the 1970s is to build a first floor addition. It is not known whether the bracing capacity of the existing house is adequate to support an addition on top.

Step 1. Identify the aspects of the proposed design that fall outside the scope of Acceptable Solutions The proposed work requires an assessment of the existing bracing to ensure that the lower level structure is sufficiently rigid to support the upper level addition.
Step 2. Identify the Building Code clauses for which performance must be demonstrated by the design and supporting documentation.

The Building Code clauses to be addressed by this Alternative Solution are:

• B1 Structure

Step 3. Identify the performance criteria that apply.

The performance criteria that apply include:

• B1.3.1 Buildings and building elements must remain stable and not collapse

Step 4. Select the most relevant compliance path(s).

The most relevant compliance path options could be compliance path 2 – comparison to other documents.

Step 5. Determine what type of information is required to demonstrate compliance. To demonstrate compliance it is necessary to show that the bracing levels for the existing structure are adequate to support a new upper structure.
Step 6. Provide the evidence.

There are no detailed drawings that provide information about the bracing of the existing floor. However, the building had been lined with plasterboard fixed directly to wall framing for all internal and external walls.

In this case, the building could be assessed in accordance with NZS 3604:1984, which gave generic bracing ratings for nail fixed plasterboard sheet lining. A generic bracing rating can be calculated to establish that the existing ground floor wall structure is adequately braced to be able to support an upper level.

In this case an assessment of the foundations and their ability to carry the gravity loads and resist the earthquake and wind loads would also have to be made, for example: how is the gravity load from the new extension transferred to the subfloor structure? 

 

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1970s houses – compliance case study 5

The owners of a 1970s house with bevelback weatherboard cladding and a reinforced concrete floor slab wish to add an extension and maintain a consistent floor level throughout. However, the height of the floor slab above finished ground level is only 120 mm, which is not enough to provide the clearance as required if following E2/AS1 (150 mm to paved surface).

Step 1. Identify the aspects of the proposed design that fall outside the scope of Acceptable Solutions The height of the floor level of the proposed extension is less than permitted under E2/AS1 and is therefore outside the scope of the Acceptable Solutions.
Step 2. Identify the Building Code clauses for which performance must be demonstrated by the design and supporting documentation.

The Building Code clauses to be addressed by this Alternative Solution are:

• E2 External moisture

Step 3. Identify the performance criteria that apply.

The performance criteria that apply include:

• E2.3.3 Floors and structural elements in contact with the ground must not absorb or transmit moisture that could cause dampness or damage.

Step 4. Select the most relevant compliance path(s).

The most relevant compliance path option could be:

• Compliance path 3 – comparison with in-service history.

• Compliance path 7 – MBIE determination.

Step 5. Determine what type of information is required to demonstrate compliance. To demonstrate compliance it is necessary to show that the construction will not cause damp or moisture ingress.
Step 6. Provide the evidence.

Demonstrating that there has been no deterioration of the existing construction despite the lack of adequate ground clearance can be used to provide evidence of an in-service history and satisfactory performance for clause E2. Three alternative options may be to:

• found the new wall plates on a concrete nib which would lift the framing and the cladding higher above the ground (a solution commonly used in remediation of leaking buildings to solve lack of ground clearance)

• lower the existing ground levels by excavation

• provide a drainage channel around the perimeter of the new extension (and possibly the existing part of the building as well).

    Otherwise a MBIE determination could be used to establish whether the proposed alternative method will be Building Code compliant.