Photo by Med Badr Chemmaoui

Poorly designed and constructed houses or apartments can be freezing cold in winter or blazing hot on summer days.

However, in the temperate climates of metropolitan Australian cities like Sydney, Melbourne, Canberra and Adelaide (climate zone 5 – warm temperate, and 6 – mild temperate, as defined in the National Construction Code), it’s the cold which is a significant health hazard. More people die of hypothermia in Sydney than in Sweden!

If you live in a recently built apartment with large windows, you’re probably running your heating system through much of winter, and wearing warm clothes to try and keep comfortable when indoors. 

It is also likely that you are having to put up with condensation running down the window surfaces each night that the temperature drops to single digits – not uncommon in winter. To add insult to injury, if you’ve purchased your “code compliant apartment” in the last few years it’s very likely you’ve paid the best part of a million dollars to be this uncomfortable! 

Although they receive seemingly similar thermal performance scores to a house under the current rating processes, apartments often do not deliver the same level of thermal comfort! Why is this happening? To figure this out, we need to understand some fundamentals of building physics and anomalies in the energy star rating of apartments and houses.

Dwellings (houses and apartments) are given a star rating based on NatHERS (the Nationwide House Energy Rating Scheme). Originally designed for houses, NatHERS assigns a star rating, out of possible maximum of 10 stars, based on the predicted annual energy use for heating and cooling a dwelling design, when tested in a particular climate. This energy use is a function of the building’s “fabric” (also called the building envelope). The “better” the building fabric, the lower the predicted annual energy use, and the higher the star rating. The energy rating “star bands” are given in MJ/m2 per annum and can be found at the NatHERS website.

In a house, the building fabric that is exposed to the vagaries of outdoor weather is extensive and typically includes the floor (either suspended, or in contact with the ground), external walls on three or more orientations, windows, and either a ceiling/roof space/roof or a skillion roof (no roof space).

On a cold night, these exposed building elements become conduits for heat loss. To gain a particular star rating, each of these elements in such a “fabric-dominated” house needs to be of a suitable thermal performance to mitigate both heat loss in winter and heat gain in summer. (Let’s leave air leakage out of the discussion for the moment, since its impact is not tested by the current rating schemes).

In a typical apartment, on the other hand, the exposed building fabric that interacts with the outside weather is quite different from that of a house. A corner apartment may have external walls and windows along just two orientations. This can reduce to one façade along a single orientation for a middle floor apartment sandwiched between two other apartments. In other words, the number of shared walls and floor slabs between apartments results in a relatively lower area of exposed building fabric.

With the NatHERS star bands set to be the same for a house and an apartment, the energy budget against which the thermal performance of the fabric is tested is the same. 

What this means, quite simply, is that by virtue of its reduced area, the exposed building fabric, particularly for these middle apartments doesn’t have to work as hard as the fabric of a house to achieve the same star rating. 

This results in the unconscionable outcome where the average six-star NatHERS rated apartment, in these metropolitan locations, can get away with large, single pane windows!

This brings us back to the question of thermal comfort in such apartments in winter. 

Large areas of single pane windows mean there are large surfaces at almost the same temperature as the outside ambient temperature. 

So, on a cold day, if it is 12C outside, then the temperature of the large single pane windows facing into the room is also close to this value, and will dominate how cold you feel. 

This is because the radiant temperature (what you feel) in such situations is colder than the air temperature (measured by a thermometer). And when it’s even colder at night, the now colder window pane causes moisture to condense on the inside. Drawing a thick curtain across the window at night may alleviate your discomfort a little, but it isn’t going to stop water running down the inside of the glass surface, which in turn can increase the risk of mould and associated health issues. Moreover, those large single panes of glass are also losing heat at least 10 times faster than an insulated wall on a per square metre basis!

So now you know. A six-star NatHERS rated apartment with large single pane windows doesn’t provide the same thermal comfort as a six-star NatHERS house. 

The current use of predicted annual energy use for heating and cooling as a proxy for thermal comfort, although perhaps acceptable for houses, is leading to undesirable outcomes for such apartments. 

The advantage of the compact form of the apartment is not being passed on to the owner/occupier, but is instead being traded-off against a lower performing façade, leading to an inequity in the thermal comfort performance between a house and such an apartment with the same energy star rating.

Although the extent of exposed building fabric or façade is relatively smaller in apartments, improving the thermal performance of this façade, particularly the windows, is critical to ensuring the thermal comfort of occupants. This is all the more important, as the actual energy consumed for heating (and cooling) in dwellings is greatly influenced by the thermal comfort experienced by occupants.

In a scenario where the development of apartments remains central to the housing strategies of most Australian cities, attention to improve thermal comfort, associated thermal performance and real energy use in apartments is a matter of urgency.

PC Thomas, Team Catalyst

PC Thomas is the Director of Team Catalyst, specialising in the energy efficiency of large buildings. He is also an adjunct associate professor with the University of Sydney. More by PC Thomas, Team Catalyst

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  1. We have project in Toowoomba QLD where going to three passive house buildings although not certified. We waiting on the developer approved 2250m2 PV panel and 4MW of batteries.

    This is spreadsheet where I worked out how much power produce 1.7MW a day for 250 resident of retirement village. I’m using Adobe Acrobat.
    You can view “GLENVALE VILLA QLD-R01.pdf” at: https://documentcloud.adobe.com/link/track?uri=urn:aaid:scds:US:65fbfb13-6807-4d3f-80dc-13f965162b10

  2. “apartments often do not deliver the same level of thermal comfort” is there any source or test data to backup this statement and article’s premise?

    Regarding radiant temperatures, these will be contributed to by all the surfaces of the room, including those adjacent to corridors and other apartments which will typically be warmer than equivalent houses.