Disclose NatHERS Before Mandating Disclosure

Sourceable gains an industry viewpoint of the proposed change in energy efficiency assessment by the Victorian State Government from Jessica Hogg, Senior Building Physics Engineer from Inhabit

Disclose NatHERS Before Mandating Disclosure

It has recently come to light that the Victorian Government, in their desire to improve the energy efficiency of residential properties, is considering introducing mandatory disclosure of the energy performance of dwellings.

This will require that potential tenants would be provided with information on how much their prospective new home is likely to cost them in energy bills.

I think this is a fantastic idea; it would certainly put pressure on both developers and landlords to improve the status quo of what’s being built. There are so many social benefits to having an improved building stock, particularly when it’s targeted at residential properties of lower income households.

But first we need to ensure that the methods in place for assessing the energy efficiency of these developments is up to the mark. Otherwise, we run the risk of producing misinformation with undesirable consequences.

Existing dwellings would need to have sufficient energy audits completed; it won’t be enough to collect energy bills from past tenants since there is so much uncertainty around whether that resident was a ‘typical’ consumer of energy or had (for instance) a solarium in their cellar. The energy audit would need to investigate the thermal performance of the build construction and determine the expected energy bills based on climate data.

New-build dwellings would also rely heavily on modelling, based on the hypothetical thermal performance of the yet-to-be-built construction.

The existing methodology for assessing the energy efficiency of a new-build dwelling, NatHERS, is currently far from ideal. The calculation assessment is a black-box insofar as even accredited energy modellers have no idea on the calculations behind it. There needs to be far more transparency in the process before we can introduce a mandatory disclosure process, otherwise how can we be capable of providing suitable design advice?

There are numerous things that are challenging around the current methodology of determining thermal performance, but my top three recommendations for improving the process would be:

Introduce audits of new-build dwellings, including air pressure testing
Air pressure testing involves an on-site test to determine how ‘leaky’ the building is, which can have a huge impact on the energy consumption of a building. If you cannot control it, you waste heated air during the winter and lose cooled air during the summer. The tightness of the building is determined by the quality of build, which can be greatly improved with sufficient sub-contractor coordination. Thermographic imaging upon completion could also be an extra driver to encourage quality of build.

Account for thermal bridging of building fabric
Thermal bridging is a term used to describe areas of higher heat transfer in the building fabric. It takes place at junctions between different constructions or different fabric materials, such as the aluminium frames of spandrel panels, the metal studs in a plasterboard wall, or the connection between a wall and floor. Currently, these types of extra heat flows are not taken into consideration in the NCC, and they can have a large impact on not only the energy consumption, but the thermal comfort of the dwelling.

Release the NatHERS calculation for review and revision
We need to ensure that the calculation procedure is fit for purpose; there may be some unclear and questionable assumptions in NatHERS, including the mysterious ‘floor area correction factor’ and the additional factors related to windows that are invisible to the energy assessor. In addition, the different software options need to be equivalent; it is wholly unacceptable to have different software providing different results, which can mean the difference between a pass and a fail (as is currently the case).

Clearly there’s a vast amount of room for improvement to be had in the process, but it’s encouraging that a state government is stepping up to drive energy efficiency of building stock. If it’s implemented correctly, I’m all for it.


The above is reproduced from the Sourceable website.
Date: 15th September 2015
Author: Andrew Heaton
Interviewed: Jessica Hogg, Inhabit.