What an energy assessor taught me about my 1960s home

Bhree Lowry discovers why a large solar system can’t compensate for a poorly performing building envelope—and how an energy assessment helped her work out what to tackle next.

When I arranged a NatHERS Home Energy Rating assessment for our 1960s Sydney home, I was hoping for a roadmap towards energy independence.

We already have a 9kW solar system and an electric hot water system that uses excess solar through a SolarEdge controller. We have added ceiling insulation where we can and are currently upgrading our sunny, east-facing front entry to reduce draughts and heat transfer. We upgraded to a solid timber front door and are currently finishing the DIY secondary glazing using polycarbonate panels fitted to the windows on either side of the door. Our ambition was to create a home that could remain comfortable without relying heavily on heating and cooling.

The assessment gave me a roadmap, but it also delivered a reality check. Our home achieved a Thermal Star Energy Rating of just 1.4 out of 10. Yet its Home Energy Rating, which includes fixed appliances and solar generation, was 81 out of 100. Without solar, that second rating would have been only 27.

It was a striking demonstration of why both ratings matter. Our solar system makes the home look relatively good from an overall energy perspective, but it does not fix a poorly performing building envelope.

What does the new assessment measure?
NatHERS has traditionally been associated with assessing the designs of new homes and major renovations. The scheme has now expanded to provide nationally recognised ratings for existing homes through an on-site assessment.
Existing homes receive two complementary ratings.

The Thermal Star Energy Rating, from zero to 10 stars, assesses how well the building maintains comfortable temperatures. It considers its orientation, walls, floors, roof, insulation, glazing, shading and air leakage. Appliances and solar panels do not affect this rating.

The Home Energy Rating, scored from zero to 100 or higher, combines the home’s thermal performance with its fixed appliances, lighting, heating and cooling, hot water, solar panels and batteries. A score of 100 represents a net zero energy-value home under the standard assumptions used by the rating tool.

My certificate estimated that our home would be comfortable without heating or cooling for around 45 per cent of the year, require heating for 40 per cent and cooling for 15 per cent, which feels accurate to our experience.

It also broke down our modelled appliance energy use, solar generation and export, emissions, heating and cooling loads, building materials and calculated air leakage. These figures use standard occupancy and energy-use patterns so homes can be compared consistently. They are not a prediction of a particular household’s electricity bill or a judgement on how efficiently its occupants behave.

What happened during the assessment?
Before the visit, I supplied information on our DIY insulation, appliances, solar system and hot water. Evidence matters: if an insulation product or R-value cannot be verified, the assessor may need to apply a conservative default value—so DIYers, keep your receipts!

Our assessor, Liliana Zreik from TPA Studio, inspected every room. She recorded construction materials, windows and their coverings, home orientation, shading, heating and cooling equipment, lighting, oven and cooktop type. She also took photographs so the information entered into the assessment could be documented.

Those photos formed part of an extensive supporting record. My complete report and supporting attachments ran to 89 pages, although the level of additional reporting may vary between assessors. It would be useful information to pass on if we sold the home, so remember to pick up that dirty laundry first!

The standard certificate itself is much shorter. Liliana also provided a detailed upgrade-variations table and zone-by-zone analysis showing how different parts of the house performed.

One thing I had expected, but which was not included, was thermal imaging. I particularly wanted to check the quality of the DIY insulation my husband had installed and identify any gaps so I could put him back to work—doing what he describes as “the absolute hardest job in the world”.

A thermal camera can be very useful for identifying missing or uneven insulation and gaps, but it can’t tell you whether the insulation present is R2.0, R3.0 or R4.0, which is what the NatHERS assessor needs to include it in the assessment and contribute to the thermal rating.

Thermal imaging is not a standard requirement of a NatHERS existing-home assessment. The assessment models the home using observed and verified inputs. I still think thermal imaging would be a valuable optional addition, particularly for households investigating missing insulation or workmanship. Before booking, ask whether the assessor offers thermal imaging, blower-door testing or other diagnostic services, what they cost, and if they’d suit your situation.

The result I didn’t want
I was surprised not only by our 1.4-star result, but by how difficult it would be to shift.

Our variations table modelled 13 upgrade stages. The full package included draught proofing, floor and roof insulation, honeycomb blinds, double glazing, wall insulation and targeted external shading, and would lift the home to 2.7 stars and increase its Home Energy Rating from 81 to 91.

Although this almost doubles the numerical Star Rating, 2.7 stars still sounds low after such extensive and expensive work.

The limitations are partly structural. Our living areas have a challenging orientation and west-facing glazing. The house is exposed through its external walls, roof and suspended floors. Its split-level design also includes a double-height void, allowing warm air to rise into space we do not occupy.

Liliana compared our result with two other assessments. A similarly sized detached home in Mortdale started at 1.9 stars but could reach 4.1 through upgrades, helped by the orientation of its living areas and absence of west-facing windows.

A Redfern apartment began at 3.6 stars and could reach 7.9 stars with upgrades. Although its living room also faced west, it had only one external wall, with apartments above, below and beside it acting as thermal buffers. Most revealingly, further improvements could take that apartment to 9.1 stars but a Home Energy Rating of only 39 because it had no solar. Our house had the reverse problem: a weak building envelope but a relatively high Home Energy Rating because of its large solar system.

Turning a poor result into a plan
The upgrade-variations table was the most useful part of my assessment. It showed what individual measures could achieve and how their effects changed when combined with all the other options.

The draught-proofing and window shading package, for example, lifted the home from 1.4 to 1.6 stars and increased the Home Energy Rating from 81 to 85. Floor insulation alone did not change the Star Rating but increased the Home Energy Rating to 83.

After a series of nine staged upgrade measures, the model reached 1.9 stars and 85/100. Adding R5.0 insulation to the rear flat metal roof increased this to 2.0 stars and 86/100. Upgrading all windows, insulating the external walls and adding appropriate external shading eventually lifted it to 2.7 stars and 91/100.

This staged information is particularly valuable when your budget is limited. A higher star rating is not the only measure of whether an upgrade is worthwhile. Draught sealing may produce a modest numerical change but quickly improves how a room feels. An improvement may also make an uncomfortable bedroom much more usable without greatly shifting the rating of a 195-square-metre house.

When I asked Liliana about the most common misconception people have about home energy assessments, she said: “Upgrade costs—people are surprised when they understand that draught proofing is the least expensive upgrade to do but one of the most beneficial to home comfort and energy efficiency.”

For our home, the immediate priorities are insulating the flat metal roof, finishing the draught proofing and installing adjustable external shading on the west and north-facing windows. We hope to complete these by the end of November, before what’s expected to be a very hot and uncomfortable summer.

The modelling also produced counterintuitive findings. When modelled alone, double-glazed low-e windows to the east-facing bedrooms and entry did not change the Star Rating and actually reduced the Home Energy Rating from 81 to 79. Low-e glass uses a microscopically thin coating to reduce radiant heat transfer. Liliana explained that the particular glazing modelled blocked some useful winter morning sun as well as unwanted summer heat.

External shutters across both the west-facing kitchen and family-room windows reduced the Home Energy Rating to 78, also because they blocked useful winter sun. Combining those shutters with honeycomb blinds reduced the Star Rating to 1.2.

By contrast, adjustable external shading to the kitchen—while retaining winter solar access to the family room—improved the result. This does not mean double glazing or shading is inherently bad. It demonstrates why orientation, glazing specifications and interactions between upgrades must be considered rather than assuming every commonly recommended measure will benefit every home.

The role of household behaviour
Behaviour does not alter the official rating because the software applies standard usage patterns. However, the certificate includes some practical behavioural guidance.

Our solar system was estimated to generate about 11,900kWh annually, with 51 per cent used by the home. Moving hot water, dishwasher and washing machine use into the middle of the day allows us to use more solar directly instead of exporting it cheaply and buying electricity back later.

The assessment initially identified our electric storage hot water system as a priority for replacement. Once our hot water solar diverter was correctly recorded, that advice changed and so did our energy rating from 78 to 81. Much of our hot water was already being supplied by excess solar, so replacing a functioning system with a heat pump no longer made financial sense. We will wait until the existing tank reaches the end of its life.

That correction reinforced another lesson: keep talking to your assessor and question anything that does not reflect how your home actually operates.

Before choosing an assessor
Ask whether the assessor is accredited specifically for existing homes and clarify what is included in the fee. How many upgrade scenarios will be modelled? What evidence should you provide? Are thermal imaging or blower-door testing available? Can the assessor prioritise upgrades according to your budget, renovation plans and most uncomfortable rooms?

I wanted an assessment because we had been going around in circles trying to decide what to do first—it was that domino effect that leads to decision paralysis. We needed evidence about which improvements would give us the most bang for our insulated buck.

Our assessment cost $255 after the NSW Government rebate. Prices vary according to the size and complexity of the home and the amount of additional modelling and advice included, so obtain a written quote. Eligible NSW homeowners can currently receive $330 off through the NSW Government’s Subsidised Home Energy Ratings program zapcat.com.au/subsidised-home-energy-ratings

My assessment did not reveal a simple route to a high-performing house. What it gave me was something more realistic: an understanding of the home’s limits, evidence to challenge my assumptions and a way to spend our renovation budget where it will make the greatest difference.