Roof Insulation: any advice for insulation retrofit?

Houses in FNQ are built with sisalation directly under the iron sheeting.

Our home was also fitted with Rudd’s Bats by the previous owners and it is a very cool house.

It also has 2 rotary ventilators near the gable and a very high ceiling.

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Thanks Fred, and hello to FNQ, where I grew up. Yes, I am replacing the decrepit sisalation that’s under the metal sheeting on my place down here. It is entirely inadequate. The roof cavity gets incredibly hot in summer. For example, I measured the temperature of water coming out of the “cold” tap at 65 degrees celsius one very hot day, a result of the heat trapped under the metal and sisalation heating the PVC water pipes that run via the roof cavity.


The roof blanket is fairly cheap when installed at construction and is compulsory in some cases. Consider if the money you spend on adding this after construction would be better spent elsewhere, for example:

  • heavier batts in the ceiling,
  • sunshades over important windows,
  • double glazing important windows.

Look at the whole house not just the roof.


Yes, wise words, S. Look at the whole house. I’m going to respond to your wisdom with even more questions though: Do you know, what is the best way to get an accurate calculation of the cost/benefit ratio - For example, is there an online calculator that will tell me what measurable benefits can I expect from spending $10,000 on a retrofit roof blanket vs. spending $10,000 on double glazing? Given the extreme heat build-up in the roof cavity during heatwaves here, how else can I reduce the heat-energy burden on the ceiling batts? For instance, a roof ventilation system like Bradford Odyssey can blow out the hot air at night; is that an adequate solution?
I’m a member of Renew Australia (the Alternative Technology Association) and I have signed up for an “Energy Consult” from one of their experts.


Looking at your whole house there are other factors such as the overall construction, timber, brick, stone etc, floor system, aspect …

We often compare notes between family who live in an older house in urban Melbourne and up here in coastal Qld. The general discussion has been that Melbourne is really cold and wet and horrible most of the year. Really hot weather is far less common.

It’s worth considering just how much of the year will be spent heating, for how many hours a day and at what temperature differential. The ceiling batts may be all you need for winter so why add more through under tin blankets at great expense. As @syncretic suggests it is likely more cost effective to add another lay of ceiling insulation and cheaper. Adding the blankets or ventilating the roof space may or may not be necessary for the relatively small number of really hot days each year.

For many houses there may be high summer heat loads from north and west facing walls that should be a priority. Leakage losses under floor, at windows and doorways are common to both cooking and heating.
Double glazing may be a waste if for less money there are other big losses to fix next.

There are apps and programs that estimate heating and cooling requirements for different room and household constructions. I’ve not found a free version that I can recommend. In determining cost benefit you may need to manually try different options to give you the targets you require. The cost of double glazing or other strategies varies so much you may need to get quotes to decide on the cost vs benefit. An experienced professional consultant should have sufficient prior experience to suggest what is essential, of limited benefit or nice to have.

You can get thick walled medium density foam insulation from a plumbing supplies wholesale to insulate your in ceiling copper pipe work. I’m not recommending it as DIY, as there are risks with working in the roof space. It’s worth the thought if you leave the roof space mostly as is. The copper piping if it is only in a few small areas may alternately be cheaper to insulate from the heat with an extra layer of insulating wool in those areas. We insulated the underfloor HW piping of our house on stumps. It’s easy to access. Our ex Townsville house had the same hot cold water problems all summer long until we put batts in and ensured the copper pipe was also covered.


There are many. google such as ‘double glazing saving calculator’ and same for insulation, etc. My house has lots of Windows in NE Melbourne, all with honeycomb blinds. It is a 1998 build with typical insulation in the walls and roof, 2 whirly birds. I forget which calculators I used but my payback would have been a century+ ! Even the cheapest style retrofits would have been 30-40 years to do them all (not suggesting one has to do them all, but).

Be wary that most calculators you find will be US centric, almost all by businesses, and thus will be optimistic rather than conservative in payback time.

A non-quantifiable aspect is the improvement in comfort level double (or triple) glazing provides, so each to their own budgets and values of order of importance.


A number of whirly birds/winds as well to vent any hot air which collects in the ceiling cavity. This will cool the ceiling space and make your existing insulation perform better.

Removing the roof sheeting seems like a lot of work and could increase the risks of roof leaks if the roofing is damaged or not refitted properly.

I would be looking at some of the alternative, less impacting options.

It would also be worth looking at the R-rating of existing ceiling batts as they may not be sufficient for the climate where you live. A higher rating batt may better than adding another layer of insulation on the roof.

Another option from left field is to shade the roof. This can be done by planting trees (potentially deciduous so you get the benefit of winter warming). Even installing a solar cell system on the norther/western roofs may also provide some shading and also have the added benefit of making power for your house.


Thanks @phb. Using a PV system to shade the roof would definitely be a win-win. I’ll look into it.


Thanks Phil. There definitely are parameters beyond money that need to be considered. I’ll continue to work at home, and I like to have a quiet office to work in, so the added acoustic insulation of double glazing would be a real benefit.
I’ll do some googling for calculators, thanks.


Thanks @mark_m. Sadly, despite your comparison of notes with family in urban Melbourne, it’s not really true to say that

Landlocked Castlemaine has a different climate to Melbourne, 120kms to the south, and my experience living here is unambiguous: extreme heat is a problem. And, tragically, it will become a greater problem as we march inexorably into our self-created climate disaster.
You suggestion about having lagging placed around the water pipes (which are PVC in my case) is an excellent one. I’ll definitely get onto that. And good advice about leaving it to the professionals to work in the roof space.


You might find the cost cheaper than removing the roof and replacing it. Is this is the case, would be a triple win for you.


Understood. Per @phb suggestion of rooftop PV solar power. We found that for summer our solar PV provides enough output to deliver the majority of our aircon daytime cooling. That benefit is available even if the feed in tariff is not very high.

We have one set of PV panels on our high pitched tin roof facing west to maximise solar generation in summer and the hottest part of the day - the afternoon. Our previous home was a more modern slab on ground masonry block house in Nth Qld (higher thermal mass). We found that we could supercool the house a few extra degrees in the daytime and minimise aircon needs in the early evening using just ceiling fans to move the air.


I don’t know if there is a whole house evaluation package but there are separate ones for glazing and other treatments, you have been given some refs already. Here is a general treatment of the subject.

From my qualitative experience double glazing is very effective in both winter and summer and exposed walls with windows add much to heating. Once again much cheaper to do at construction than later.

I have recently built a trellis on the east side of my house which would get very hot by 10am on summer mornings as the sun fell on it. It has a deciduous vine (ornamental grape) so we get sun in winter and shade in summer. We have verandahs all along the north and west which were built at construction but can be added for a reasonable price afterwards. Shading walls in summer is very important especially if your walls are not well insulated.


I think fitting “Whirlybirds” (rotary ventilators) will probably provide the relief from heat you need. Just making sure you get enough fitted for the roof space you have (this means not just one as that will be insufficient). With those venting the roof cavity the heat load will move off the ceiling and much closer to the roof. We have in the past fitted them to our father in law’s house when he complained his Air Conditioning wasn’t very good, after fitting them he then complained it got too cold in the first Summer after that, we then just increased the temp to 24 C as per the recommendations. When fitting them we ensured there was some under eave vents to allow good airflow, many don’t get this done but from past experience living in the very hot central Australia area it does pay dividends.


Yes you need something under the roof iron to block radiant heat. That silver stuff up used to be always put immediately under the roof to reflect radiant heat(I have owner built major renovations multiple homes using subcontractors). Rock wool is a better insulation product if you find it, made from basalt, does not hold water. I agree with others and don’t like the idea of the roof coming off and laying the blanket. Whirly birds good idea imo. Good roof insulation is important for whichever way the climate goes and I agree Castlemaine is damn hot. Rock wool, can be blown into wall cavities too. Again it does not hold water or let mould grow,


This linked Australia and NZ focused handbook provides a useful guide on residential insulation. The tables and guide notes illustrate differences in effectiveness of common strategies for home insulation.

Refer to pages 14,15 for metal sheeted roofing with a standard pitch and flat ceiling space.

One observation, insulating a ceiling has a much greater potential to improve insulation than under metal. Both combined is better, however a single layer of R3.5 ceiling batts will out perform the highest rated under metal blanket in the table. Best to do both if necessary at new build or roof replacement. Condensation is an important concern in humid climates.

A second observation is that ventilating a roof space has a benefit in summer (equivalent to approx R0.2 improvement) but is detrimental in winter (decreases effectiveness by approx R0.2). It may reduce the temperature in the roof space, but the science is suggesting the overall benefit to room temperature in the house is not significant in this example.


Some ventilators these days come winteriised ie they can be closed to maintain roof heat. Many of the tropical type are also designed to be safe in cyclones so rooves are not removed due to pressure differences. We found them very effective but it does require more than one to effectively remove heat build up and depending on a roof size this may mean 3 or 4 or more.

A darker roof colour can also speed up heat build up in sunny weather, so not getting dark colours can help reduce heat loads in the cavity during summer but also means a reduction in warming during winter months. I think slightly colder is easier to deal with than much hotter in summer (as I put it you can only get down to skin when it is hot but when it is cold you can keep adding clothes).


Light roofs do help with keeping out heating in summer through reflection of radiation. The current fashion of charcoal grey roofs and the policy of some councils (mainly in the past) of forbidding pale and silver roofs is just madness. Blue Mountains used to insist on darker ‘heritage colours’ I don’t know if they have caught up to the 20th century much less the 21st.

In winter light coloured roofs do reduce heat gained from radiation but also reduce heat lost through radiation. This is the old polished kettle principle. So on dull cold days you are much better off. Overall light roofs give better passive temperature control in Oz.


Some info on adding insulation foil under the rafters from within roof space and other changes to enhance comfort/reduce energy needs.


I live in Canberra where 50 deg fluctuations (eg -8 to +42) are normal throughout the year. The house was 30 year old double brick with uninsulated concrete slab, thin windows albeit timber framed, no insulation in the walls, poor ceiling insulation and older sarking under the roof sheets. Cool in summer but like an icebox in winter, noting it’s colder here more often than hot. Stage 1 was blow in wall cavity insulation which made a big improvement in comfort, the cavity was quite large so lots went in but when I checked it with a mate’s thermal camera they were back to redo a fair bit soon after. Stage 2 was top up ceiling bats and cross hatched bats over the top again, transformed the house but it’s a nightmare trying to move in the roof space as all you see from the manhole cover is a solid sea of bats. Again I checked the job carefully for gaps but this time no redo. Stage 3 is…nothing yet as I haven’t found anything extra that is worth doing from a cost benefit perspective.

Taking roof sheets off would be expensive (a roofer won’t do a smaller high warranty risk job cheaply when there’s a queue of expensive full roof jobs) and runs the risk of leaks as others have said. Also new sarking without better ceiling insulation is a waste of money.

As for double glazing it’s the last thing I’d do because it’s stupidly expensive and way down on the bang for buck list, and unless you get the expensive thermally broken good stuff it’s less effective than a standard timber window. And a poorly designed ally double glazed window is worse than single glazed timber.

IMHO the best bet is to spend your double glazing money on good wall and ceiling insulation, an appropriately sized solar set up, split systems and some good window covering like honeycomb blinds. House will be comfortable and power bills low.