This guide is intended to help designers, builders and building consent authorities reach a common understanding of which building work is required to comply with the Building Code clause H1 Energy efficiency, and what is needed to comply with clause H1.1 when an existing building is repaired or altered.
This information was confirmed as current in September 2023.
Thermal resistance of repaired or replaced building elements
Clause H1 of the Building Code does not specify the thermal resistance that is required for each repaired or replaced building element. The relevant acceptable solutions and verification methods are designed primarily for new buildings.
The size and extent of the building envelope that is repaired or replaced will influence how closely the repair/replacement should match the R-values specified for new buildings in H1/AS1.
Repairs or replacement of a small part of the building envelope should simply ensure the overall thermal resistance of a building is not diminished.
For example, if an old wood-framed window is replaced, you can use any of the following:
- A new wood-framed window of the same size ('like-for-like' replacement - the material is the same, therefore the thermal resistance is the same).
- A new PVC-framed window of the same size (PVC window frames have similar thermal resistance as wooden window frames).
- A new aluminium framed window of the same size, plus additional insulation added elsewhere (roof, wall or floor) to offset the lower thermal resistance of the aluminium frame.
- A double-glazed aluminium framed window of the same size (this will exceed the thermal resistance of the original window).
Repairs or replacements of large parts of the building envelope are similar in effect to additions and extensions, and compliance with clause H1 should be approached in a similar fashion (you can find details in additions and extensions).
Repairs or replacements that reduce the thermal performance
Provided it has a minimal effect, repaired or replaced building elements that do not strictly meet the energy efficiency performance of the old building element may be acceptable.
Slightly less performance might be accepted when:
- there is limited availability of an "identical" replacement (such as standard sizes/dimensions may have changed slightly since the original element was constructed)
- other considerations such as the use of second-hand parts have slightly different sizes/properties.
When considering the compliance of the thermal envelope with clause H1.3.1, the requirement "to limit uncontrollable airflow" is also relevant.
The overall energy efficiency of the building envelope may be the same after the repair or replacement if a decrease in thermal resistance is offset by an increase in air-tightness.
Additions and extensions
Additions or extensions to a house involve new construction and the alteration of an existing building. The new construction could be as small as cutting a window into an existing wall, or could be as large as adding a whole new storey to an existing building
The new construction must meet the requirements of clause H1 but the rest of the building does not have to be upgraded to comply with H1. The altered building must comply with clause H1 to at least the same extent as it did before the alteration. The overall energy efficiency of the building cannot be made worse by the alteration.
A simple way to comply with H1 is for the new construction to comply with an acceptable solution or verification method. For compliance with H1.3.1, if any of the proposed construction R-values are lower than those required in the Schedule method found in H1/AS1 or H1/AS2, or the other requirements are not all met, you can use the Calculation or Modelling methods to evaluate options and to come up with a design that will comply.
For all housing, and buildings up to 300m2, you can find further guidance on how to use the Schedule, Calculation and Modelling methods for additions in the section 'Application of Acceptable Solution H1/AS1 and Verification Method H1/VM1 to additions and extensions'.
The only time existing parts of a building would need upgrading is if the energy efficiency performance of the building is reduced because of the addition or extension. However, any required upgrade does not have to reach the level set in the relevant H1 acceptable solution, but must bring the building back to the level of thermal performance it enjoyed before the addition or extension.
You can find two examples below of an addition to an existing house where upgrading is and is not required.
Example - extension with no upgrade
If a new room is added to the side of an existing house, then the extension (the floor, roof, walls and windows) will need to comply with clause H1.
The untouched existing parts of the house will have the same performance (thermal resistance) after the alteration as they did before.
Once completed, the thermal resistance of the altered house will be at least as good as it was before the alteration (it might be better, given the extension complies with H1), therefore no upgrading is required.
Any parts of the house that previously formed the thermal envelope, but are now enclosed by the alteration (such as the side of a house onto which an extension is built), are no longer part of the thermal envelope.
Example - alteration where upgrade is required
If you are inserting a window into an existing wall (R0.5-R2.0), then the thermal resistance of the building envelope will usually be reduced. This is because the thermal resistance of a window is usually lower than that of the existing wall, even if the window is double-glazed.
In this case, you will need to upgrade some of the existing building to ensure the building performs at least as well as it did before the alteration.
You will need to add sufficient insulation elsewhere in the thermal envelope to offset the loss in thermal resistance at the new window.
Garages and conservatories
New garages or conservatories that are attached to a house and sit within the thermal envelope must comply with H1.3.1. This includes conservatories that are open to the interior of the house or which are actively heated or cooled. A simple way to comply with H1 is for the new construction to comply with the Schedule method or Calculation method of Acceptable Solution H1/AS1.
Attached garages or conservatories that are unconditioned (that is, not heated or cooled) and are outside the building thermal envelope do not need to comply with H1.3.1.
In these circumstances, any walls, windows or doors that sit between the garage or conservatory and the interior of the house will need to be appropriately insulated as they will form part of the thermal envelope.
Application of Acceptable Solution H1/AS1 and Verification Method H1/VM1 to additions and extensions
For all housing, and buildings up to 300 m2, using the Schedule or Calculation method of Acceptable Solution H1/AS1, or the Modelling method of Verification Method H1/VM1 can be a simple way to demonstrate that an addition or extension to an existing building has adequate thermal resistance to comply with Building Code clause H1.
These methods can be used for extensions to buildings that are within the scope of H1/AS1 and H1/VM1 (refer to these documents for details).
Schedule method for extensions
The Schedule method (contained in Acceptable Solution H1/AS1) can only be used if the building meets the limits for the glazing area, skylight area and opaque door area specified in H1/AS1 Paragraph 220.127.116.11.
For the glazing area limits of H1/AS1 Paragraph 18.104.22.168 a) and b), either:
- calculate the glazing ratio using the wall and glazing areas of the entire altered building, or
- alternatively, the glazing ratio may be calculated using the wall and glazing areas of the extension only.
For the limits on the skylight and opaque door areas (H1/AS1 Paragraph 22.214.171.124 c) and d)), the skylight, opaque door, total roof and total wall areas should be calculated based on the entire altered building, not just the extension.
Only the new construction of the extension is required to achieve the minimum construction R-values of the Schedule method. Unaltered parts of the building are not required to be upgraded.
Calculation method for extensions
The Calculation method (contained in Acceptable Solution H1/AS1) can only be used if the building meets the 40% limit for the glazing area specified in H1/AS1 Paragraph 126.96.36.199.
To calculate the glazing ratio, either:
- use the wall and glazing areas of the entire altered building, or
- alternatively, you can use the wall and glazing areas of the extension only.
Only new building elements of the extension should be included in the heat loss equations of the proposed building and reference building.
Only the new construction of the extension is required to achieve the minimum construction R-values determined by the Calculation method. Unaltered parts of the building are not required to be upgraded.
Modelling method for extensions
The Modelling method (contained in H1/VM1) may either be applied to the entire building, or the extension only.
Where the entire building is modelled, the reference building model and the proposed building model both include the existing and new parts of the building.
The existing part of the building is modelled with the same construction R-values in the reference building and proposed building unless the construction R-values of any existing building element are changed as part of the alteration (for example where insulation of existing building elements is upgraded).
The new part of the building is modelled with construction R-values as specified in H1/VM1 Paragraph 188.8.131.52 for the reference building, and as proposed for the proposed building.
Where only the extension is included in the model, it may be assumed that no heat transfer occurs between the conditioned spaces of the extension and the existing parts of the building.
Only the new construction of the extension is required to achieve the minimum construction R-values determined by the Modelling method. Unaltered parts of the building are not required to be upgraded.
Building Performance Index (BPI) compliance when repairing or altering housing.
For housing, Building Code clause H1.3.2E (Building Performance Index or BPI) is an additional requirement.
If the above guidance is followed for ensuring that the overall thermal resistance of an existing building is not diminished when it is being repaired or altered, then compliance with H1.3.2E (Building Performance Index or BPI) is also generally not reduced.