Large apartment windows – desirable or deadly?

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

However, in the temperate climates of Australian metropolitan cities such as Sydney, Melbourne, Canberra and Adelaide (climate zone 5 – warm temperate and 6 – mild temperate, as defined in the National Building Code), it is the cold that constitutes a significant health risk. . More people die of hypothermia in Sydney than in Sweden!

If you live in a newly built apartment with large windows, you are probably using your heating system for much of the winter and wearing warm clothes to try and stay comfortable inside.

It’s also likely that you have to put up with the condensation that drains onto window surfaces every night and the temperature drops to single digits, which is not uncommon in winter. To add insult to injury, if you’ve bought your “code-compliant apartment” in the past few years, chances are you’ve paid the best part of a million dollars for being so uncomfortable!

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

The accommodations (houses and apartments) are classified by stars on the basis of NatHERS (the Natat ion scale HHouse Eenergetic Reat Schemist). Originally designed for homes, NatHERS assigns a rating, out of a possible maximum of 10 stars, based on the expected annual energy consumption for heating and cooling of a home design, when it is used. tested in a particular climate. This energy consumption is a function of the “fabric” of the building (also called the building envelope). The “better” the building structure, the lower the expected annual energy consumption and the higher the number of stars. The “star bands” energy ratings are given in MJ / m2 per year and can be found on the NatHERS website.

In a home, 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), exterior walls in three or more orientations, windows, and either a ceiling / roof / roof or a skillion roof (no roof).

On a cold night, these exposed building elements become conduits for heat loss. To achieve a particular star rating, each of these elements in such a “fabric dominated” home must have appropriate thermal performance to mitigate both winter heat loss and summer heat gain. (Let’s leave air leaks out of the discussion for now, as their impact is not tested by current rating scales).

In a typical apartment, on the other hand, the fabric of the exposed building that interacts with the weather outside is quite different from that of a house. A corner apartment can have exterior walls and windows along only two orientations. This can be reduced to a facade along a single orientation for a ground floor apartment sandwiched between two other apartments. In other words, the number of walls and floor slabs shared between apartments results in a relatively smaller area of ​​exposed building fabric.

With 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.

This simply means that due to its small area, the fabric of the building on display, especially for those middle apartments, does not need to work as hard as the fabric of a house to achieve the same number of stars.

This translates into the unacceptable result where the average NatHERS rated six star apartment in these metropolitan areas can get away with large single glazed windows!

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

Large areas of single-glazed windows mean that there are large areas at almost the same temperature as the outside ambient temperature.

Thus, in cold weather, if it is 12 ° C outside, then the temperature of the large single-glazed windows facing 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 cooler than the air temperature (measured by a thermometer). And when it gets even colder at night, the now cooler glass causes moisture to condense inside. Pulling a thick curtain over the window at night may alleviate your discomfort a bit, but it won’t prevent water from flowing inside the glass surface, which can increase the risk of mold and associated health issues. . In addition, these large single glazing also lose heat at least 10 times faster than an insulated wall per square meter!

So now you know. A 6-star NatHERS apartment with large bay windows does not offer the same thermal comfort as a 6-star NatHERS house.

Current use of predicted annual energy consumption for heating and cooling as an indicator of thermal comfort, while perhaps acceptable for homes, leads to undesirable results for these apartments.

The advantage of the compact form of the apartment is not passed on to the owner / occupant, but rather is traded for a less efficient facade, which leads to an inequality in thermal comfort performance between a house and such an apartment with the same energy rating.

Although the extent of fabric or exposed building facade is relatively smaller in apartments, improving the thermal performance of that facade, especially windows, is essential to ensure thermal comfort for occupants. This is all the more important as the actual energy consumed for heating (and cooling) in dwellings is strongly influenced by the thermal comfort experienced by the occupants.

In a scenario where apartment development remains at the heart of the housing strategies of most Australian cities, attention to improving thermal comfort, associated thermal performance and actual energy use in residential areas. apartments is a matter of urgency.

PC Thomas is the director of Team Catalyst, specializing in the energy efficiency of large buildings. He is also Assistant Associate Professor at the University of Sydney.

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