Topping up ceiling insulation question?

I have a three year old house in Canberra, with R5.5 insulation in the ceiling which is not really adequate to cope with the winter cold (sub-zero) or summer heat (superheated roof). So I would like to add more insulation, topping up with another R3 or R4 batt.
Unfortunately, the builder decided that aesthetics had a primacy so the roof has a multiple hip and valley construction with almost a latticework of trusses cris crossing each other. These means that some parts of the roof are very difficult to get to and very low! This means I can only push batts into these areas.

My question is, assuming the sarking provides the vapour barrier: Can I install insulation batts so they press against the sarking? Or should I use "underfloor insulation?


I live in the central ranges of Vic and ‘enjoy’ similarly cold winters and IMO you roof insulation is adequate and any further additions will not make a lot of difference.
In Australian homes the biggest enemy of thermal efficiency is the type of windows and the glass chosen. Most Australian homes have single 3mm glass panes and aluminium frames without thermal breaks and this amounts to little more than tent fabric in insulation properties.
By world standards most australian homes built before the mid 2000’s are thermally little better than tents.

Double glazing installed in window frames with thermal breaks and using high efficiency glass are big factors in improving modern homes.

A typical suburban home with single 3mm glass and aluminium frames this glass combo (depending on window sizes) can amount to thermal leakage/or gain depending on the prevailing climate of equivalent of approx 6-7kw/hr.

Don’t take my word for it there is a lot of info on Australian window thermal performance on the net only a simple search engine inquiry with have you reading the good oil.


Thanks ‘tndkemp’, I agree completely about the windows that are standard in Australia, as you say about as good as tent cover! I actually have a quote ready to start replacing the windows in my house.
However, we are currently sweltering in summer heat and I want the roof space reaching well above 40℃. This affects the house internal temperature as the ceiling gets warm, even with a full layer of R5.5 insulation. The ducting insulated at R1 runs through the superheated roof space, so the cooled air is constantly fighting against the hot air, I can actually see the roof space temperature drop when the aircon is running (the same in winter when it is warmed).
So, I am looking to reduce the superheated air in summer and the frozen air in winter!

Hi @allandorrington

Have you considered putting one of those whirlybirds to remove the summer heat? I’m not sure if they are still available, but you used to be able to buy ones that were temperature controlled. That is flaps on the underside opened if it was hot allowing hot air to be drawn out, and closed when cold stopping the warm air from escaping.

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Yes they are still available and in Qld you see a lot of houses with them.

Thanks. Yes, you can still buy the whirlybirds, but I was not sure if you can still buy the temperature controlled ones.

I installed two whirlybirds and an electric, thermostat controlled roof vent in my house in Newcastle. The were desperately needed as the roof did not have sisalation so the roof space got very hot, the thermostat was set at 50℃ and ran all day and well into the night.
These whirlybirds are still available, another option is a “universal tile ventilator” which fits in place of a roof tile. they work well for exhaust vent and can also be used as a roof vent.
The problem is that installing vents does not address the main problem of the roof space overheating in summer and freezing in winter. In summer the concrete tiles get so hot I cannot work on the roof. I did some work during autumn and still managed to get blisters on my knee from burns - I now wear knee pads!
I would really like to mitigate the heat problem in the roof space, which follows the outside temperature with only a few minutes delay, except in summer when it gets up to 15 to 20 degrees hotter!

I am not an expert, but I don’t believe that adding more insulation over the top of R5.5 (if I understand your 1st post correctly) will make much of a difference.

Are your walls insulated? In Canberra they need to be.

The trite answer would be to have deciduous trees to protect your east/west aspects from the sun, but they take years to grow.

In the short term, I believe you need to create convection currents to vent the heat from your roof space in summer. The use of closable under eave vents will assist in creating a flow of air to the temperature controlled whirly birds will remove more heat in summer.

Have a look at:
for information & ideas.

It appears that there is a common misconception that adding to R5.5 ceiling insulation is not worthwhile. I have friends who live in Canada, they look at the insulation standards in Australia as a joke. In Canada and for that matter most northern hemisphere even R10 would be considered a minimum and a very basic level of insulation. They are more likely to have insulation levels of R20 or more, and that includes the walls.

I had an assessment done on my house insulation and the expert advised me to install an additional layer crossing the beams. This will reduce the loss through the beams as well as improve the overall insulation level.

Firstly it appears you have the answer already to your question, before you asked.

Secondly adding another layer across the beams won’t stop the heat coming into the cavity in summer it will still above the insulation but it will likely slow down the transfer into the living areas.
From a personal experience I added another 2.5 layer around 20 years ago to my 3.5 batts in the roof (house was built in the 80’s) and it made no real appreciable difference from a comfort feel point of view. I did not do a before and after temp measurement comparison. But our average firewood consumption did not noticeably change. It does naturally vary a bit from year to year as some winters are milder than others.

The biggest improvement happened when we fitted thermal close fitting drapes with pelmets, the house was noticeably more comfortable and average firewood consumption dropped by a couple of cubic metres per winter. In summer we found awnings on the western windows and deciduous climber plant on a pergola on the northern aspect bay window made big noticeable impact. We regulate our opening of the drapes also in the warmer weather to slow heat build up on stinker days.


Just a note of care about adding extra insulation I was warned by a friend in the electrical trade that you need to be careful about covering electrical power cables running through the ceiling cavity particularly the ones that carry heavy loads like to the aircond units or hot water heater stove etc as they can overheat and be damaged if covered by insulation


I recommend double glazing. If getting double glazing first please check the insulation value of them.

We live 100 km north of Perth so have very hot summers and cold winters (not as bad as Canberra). When first looking at double glazing (in 1996) to replace the windows in the old 2 room 1975 house on our property (which had the two rooms facing west), we discovered that some were a joke offering very little insulation; these were mainly Australian companies who appeared to be cashing in on double glazing vs providing a good product.

We went with (a then) newer small company who were making double glazing to European standards the result was fantastic. Not only for their insulation properties but for their sound barrier as well, incredible how quite the inside is.

When we built a new home we also had ceiling fans in all rooms with our double glazing; when we have a run of 39-40+ degree days the inside temp does rise but the highest it has ever been is 28 degrees which was after 4 days of 40+. Average is 22 - 24 on very hot days.

We did built with all main windows facing north with a small veranda, no windows on the west and only three windows facing South.

I cannot believe that in 2016 the hundreds of homes being put on the northern coastal strip of Perth that double glazing isn’t compulsory. Would definatly reccomend for both heat and cold insulation. A good investment for many years.

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I have travelled to UK and Europe many times and am so disappointed that in Australia proper double glazing to windows is not even a consideration.
In South Australia where I live this should be compulsory as heat is lost through windows in winter when heating the house, and heat enters the house through windows in summer.
I also believe that insulation is extremely important and I do agree that our insulation levels are a joke.
The problem goes back to the inferior quality of Chinese made products and the fact that Australian companies are quite happy for the public to buy inferior Chinese products which eventually will become our landfill.
Australian builders are extremely lazy and only build for profit not quality. And so the problems continue.

There has been much talk about how to insulate a home, but one VERY important issue has not been mentioned. It is too late for established homes, but should be mandatory for new homes. I am talking about Solar Passive Orientation.

It seems that in Australia Solar Passive Orientation is almost never a consideration when planning a new home, yet it determines the thermodynamics of the home. Solar Passive Orientation makes a huge difference to the temperatures inside a home, and ultimately the cost of heating and cooling.

In Australia the basic Solar Passive principles are:-
Building on a concrete slab allows the ground to act as a heat sink. It will absorb heat in summer and release it in winter.
The broad sides of the home should be facing North & South.
The narrow ends should be East & West.
The North side should have large (floor to ceiling) windows to allow the sun in.
The North side should have a row of tiles just inside the windows to absorb the sun, pass the heat to the concrete slab, and down to the ground. (It also stops other types of flooring from fading and perishing.)
The North side eaves should be wide enough to block or minimize the summer sun from hitting the windows.
The North side should also be protected by deciduous trees
The South side windows should be small.
The East/West windows which get the most heat should be minimized to as few and as little as possible.
The East/West sides should be protected by evergreen trees.

For more information research solar passive principles.

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Insulation references in the US are different from those in Australia. The numbers are about 5.7x in the US for the same insulation property as an Australian value.

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