How to double your old home’s energy rating: Lessons from two efficiency retrofits

In early 2021, Tochi carried out thermal performance assessments (NatHERS star-ratings) on two very similar two-bedroom villa units in Melbourne’s inner north.

One home is my own place, located in Westgarth (refer to this blog post for more about our home). The other, located in Northcote, belongs to a client, Rob.

Both homes were built in the late 60s/early 70s using brick veneer construction and both suffered from dismal thermal performance – they were freezing cold in winter, hot in summer, and mould would form on the windowsills where condensation could be seen collecting every winter’s morning.

The purpose of the thermal performance assessments was to determine the energy-efficiency potential of the two homes, and to understand the likely costs involved with upgrading them.

First, we modelled the existing houses – The Westgarth home came in at a low 4.2 stars, while the Northcote home was an appalling 2.2 stars. This is due to differences in orientation and layout, and lower insulation levels in the ceiling of the Northcote house.

The thermal modelling software provides an opportunity to fine-tune insulation levels in the ceilings, walls and floor, and to optimise window sizes, specifications and shading, etc, to determine what improvements are worthwhile and how far to take them.

Unsurprisingly, the optimum upgrades for both houses were very similar: Seal up the obsolete wall cavity vents, weather seal the doors, add insulation to the ceilings, exterior walls and under the floors; replace the existing aluminium single glazed windows with double-glazed windows. While the Westgarth home used timber frames and only some of the windows had low-e coatings, the Northcote home used uPVC frames with low-e all round.

The cost to make the thermal performance upgrades for the Northcote house was approximately $22,000 and for the Westgarth house about $17,000 (this number is skewed by the fact that we did most of the work ourselves, including the installation of windows and most of the insulation).

In both cases, the improvements are spectacular. The Westgarth home goes from 4.2 stars to 8.1 stars, a theoretical 72% drop in energy needed for heating and cooling, while the Northcote home leaped up from 2.2 stars to 7.1 stars, a 78% drop!

More than a year later, we can see the impacts of most of these improvements in real life.

In the Westgarth home, we still need to insulate one exterior wall and replace the last two single glazed windows. However, there is already a clear difference in comfort and energy use, with the added bonuses of no more mould on windowsills and a much quieter home. We can’t yet make reliable comparisons of energy used before and after the changes as they have not been finished, were very gradual, and the occupancy patterns in the household have changed a number of times since we started.

The Northcote home also has some work left to do, and occupancy patterns have changed over the last couple of years, but Rob has been able to make some useful before-and-after comparisons by looking at the energy bills year to year. Interestingly, after only doing some basic draught-sealing and simply replacing the old electric heater panels with a single efficient split-system unit in 2021, the electricity usage went down by 44% (even as the home was kept much warmer) – ably demonstrating the importance of air-tightness and efficient appliances.

We will have to wait until the upgrades to the Northcote house are complete and another winter has gone by before we can get some clean data to make comparisons with. Although the home is markedly more comfortable after Rob installed the insulation and the double-glazed windows, there is still some fairly significant gap-sealing left to do, as well as some work to the skylight in the bathroom.

Lessons learned

The importance of design:

Because these two houses are so similar and the upgrades nearly identical, it is easy to see the impact that orientation and layout can make.

The Westgarth house enjoys sunlight from the north into the main living area and one of the bedrooms, while the Northcote house only has one smallish window facing north. The Westgarth house shares walls with units to the east and west, while the Northcote house only shares a wall with a neighbouring garage to the north. These advantages give the Westgarth house a full star above the Northcote house, theoretically needing 32% less heating/cooling energy per square metre.

This highlights the importance of design when building a new house or undertaking a major renovation to an existing home.

It is good news that even in Melbourne’s challenging climate, an old poor-performing house with bad orientation can be retrofitted without any design changes and beat 7 stars. And a very similar home with better orientation and a few small design tweaks can beat 8 stars. You can do it too!

Sensible window sizing:

7.1 stars is a good rating. If all it takes is some insulation, double-glazing and draught-sealing to achieve such a rating on an existing home with poor orientation, why are there people struggling to hit 7 stars even on a new home design? The answer is usually to do with over-glazing.

There is an increasing tendency to design homes with huge windows, especially in the living areas. Combined with soaring high ceilings these homes are difficult to regulate thermally, regardless of whether the windows are facing north, and no matter how high-spec those windows are.

Once you get over a certain glass-to-floor area ratio (above around 30%) a room just won’t rate very high – this is because even the best windows are about five times less effective at resisting heat flow than an insulated wall (and do very little to prevent heat gain in summer if the sun shines through them). Both of our homes, however, were built with a more sensible amount of glass – nevertheless we get plenty of daylight and airflow in our homes.

Models vs reality:

Both of these homes seem to be performing as the modelling predicted – heating and cooling is turned on infrequently and for short periods, energy bills are low, and the homes feel dramatically more comfortable to live in. But it is unlikely we will actually see the 72% or 78% drop in energy usage predicted by the modelling. Why is that?

It isn’t that 7 or 8 stars is not what it promises to be in real life, but for different reasons:

1) For both houses, the actual amount of energy used before the upgrades was much lower than predicted by the star rating. When a house is given a rating of 2.2 stars, the assumption is that certain rooms will be kept at certain temperatures at certain times of the year, and the energy use is calculated accordingly.

However, when you live in a poorly-sealed 2.2 star house, you usually can’t keep your rooms at those assumed temperatures, and Rob certainly didn’t. Even if you could, the energy bills would be exorbitant. The reality is that you simply suffer, or find other ways to get by – like spending time elsewhere, huddling around a heater wearing a blanket, or putting on a beanie to go to bed. (Yes, I’ve been there.)

2) NatHERS star ratings currently only model the amount of energy needed for heating and cooling the home. Other energy uses like hot water heating, cooking and lighting are not affected by thermal performance upgrades, so we don’t expect the overall energy usage to drop as much as the heating/cooling usage on its own.

On the other hand, NatHERS does not currently take into account the efficiency of the heating/cooling appliances. Both of us installed energy-efficient split-systems, which means we will use less electrical energy than predicted, and bring us closer (but probably not quite) to the modelled drops in electricity use.

Both of us aim to replace all gas appliances with efficient electric versions like heat-pump hot water and induction cooktops which will increase our energy-efficiency further.

3) Obviously the energy use in a home will be affected by how many people live there, how long they spend in the house, and their behaviour (do they crank the heating all the time, or just put on a jumper on mild winter days?). NatHERS makes standard assumptions about occupancy and behaviour. In both of these homes however, occupancy patterns have been inconsistent over the last few years – at different times there were one, two or three people living there, sometimes working from home, sometimes not; and every person, of course, has different behaviour.

Not just lower energy bills

The lower energy usage (and associated greenhouse gas emissions) following the efficiency upgrades is great, but both Rob and I agree that even better are the health and comfort benefits. Living in a cold, mouldy house poses real health risks, and many heatwave deaths could be avoided with better thermal performance of homes.

In an energy-efficient home, the heating and cooling equipment is running much less often, while windows and doors are open more of the time, connecting us better with the outdoors. And as a big bonus, an energy-efficient house is also much quieter.

As Rob says, we need to remove the focus of energy-retrofits away from ‘payback periods’ and look at all the other reasons Australians can benefit by improving their homes.

Jeremy Wells runs Tochi Workshop, an architecture and energy efficiency workshop based in Melbourne. This article was originally published on the company blog, here. Reproduced with permission.