Energy Matters Forums

[TerryB]

I am about to replace the roof on our house with a new colorbond roof (old one was done badly when an extension was added to the house before we purchased it and is leaking in multiple places). I am trying to find out how much difference the colour we choose actually makes to the inside temperature of the house. I have seen the data on the colorbond website that shows how much each colour absorbs but there seems to be very little information on how much difference that actually makes inside the house if the roof is lined with the Air-cell (or similar bubble/reflective) insulation. Does anyone know of any studies that have been done on what the real-world difference is between lighter and darker roof colours on internal temperatures assuming correct insulation? The reason for the question is my wife would prefer a darker grey colour and I am trying to convince her that the lighter colour will save us money but she has asked the ackward questions of "how much per year" are we likely to save on cooling costs or "how much cooler will it be" with the light coloured roof and I cannot find any information to back up my argument!

We are located on the Sunshine Coast in Queensland (just north of Brisbane) and tend to be in the house most days so currently in summer the aircon gets a bit of a workout (I think this is partly due to poor insulation under the current roof but as we have no ceiling space I can't do anything about that until the roof is replaced at which time new Air-cell will be installed)

[Gordon-Loomberah]

Hi Terry, it would be extremely difficult to say exactly how much difference it would make to the room temperature, there are way too many (mostly unknown for forum members here) variables to take into account. However, I would think the published Colorbond data would be believable. Perhaps you can say the heat load contribution from above will be x% less (from their tables) if you use a light colour over a dark colour.
You will definitely be wanting to put a lot more insulation under it than just air cell type insulation though, I think from memory is only about R~1.5?). I've got 75mm insulwool with reflective foil under my colorbond (I think it was called Wheaten -a light cream colour), plus 150mm polyester over the ceiling, I think in total about R5.9, and that seems to work quite well.

[bpratt]

I'm currently painting my plain zincalume roof which has been there for some 30 or so years, so it's not as shiny as it used to be. The colour I have chosen is Thredbo White, which is a very bright white.

The parts that I have done are significantly cooler to touch than the unpainted sections, but until the entirety is painted I won't know for sure.

If touch temperature relates to the internal roof temperature, then I think I'm on to a winner.

[Gordon-Loomberah]

If touch temperature relates to the internal roof temperature, then I think I'm on to a winner.

I'm pretty sure it is related. What light (and IR and UV) isn't reflected, is absorbed. That heats the iron up, then it re-radiates in all directions as long wave IR, ie heat, including down into your house.

[bpratt]

I now wish I had've recorded the in-roof temperatures versus outdoor temps, solar before I painted it, then do the same afterwards to make a fair comparison.

[TerryB]

You will definitely be wanting to put a lot more insulation under it than just air cell type insulation though, I think from memory is only about R~1.5?). I've got 75mm insulwool with reflective foil under my colorbond (I think it was called Wheaten -a light cream colour), plus 150mm polyester over the ceiling, I think in total about R5.9, and that seems to work quite well.

Unfortunately we have no ceiling space to speak of with sloped ceilings through the property which means we only have about 10 cm or so between the roof iron and the ceiling. I will be speaking to the roofing company about what they can do but I don't think we have a lot of options to work with in that space (further complicated of course by the downlights installed by previous owners).

Thanks everyone for their feedback - I am definitely going to try to persuade my wife on a light colour for the roof as I figure any little bit is going to help and due to the flat pitch on our roof you actually cannot see it from most of our property so I can't see why the colour even matters to her!!!

[Gordon-Loomberah]

Unfortunately we have no ceiling space to speak of with sloped ceilings through the property which means we only have about 10 cm or so between the roof iron and the ceiling. I will be speaking to the roofing company about what they can do

Some more suggestions
Get rid of the downlights and put ceiling mounted fixtures in, the wires should reach and the holes can be covered with the new fittings or plastered over.

When you replace whatever roofing is there now, maybe replace the battens with higher profile ones to give you a bit more cavity space for insulation... no idea if its feasible in your case, but worth discussing with the roofing company.

[Inspector]

Terry, I only just replaced my sunroom (approx 16 sq-m) zincalume roof with colourbond. I specifically chose a light colour (I think it was "mist white" - a cream colour but not stark white) and retained the original end capping (which was dark brown colourbond) - by the time i finished the job around midday, the dark brown was too hot to keep my hand on for more than about 1 second. However, the new colourbond roof I could easily leave my hand on indefinitely and it was still only warm to touch. I also replaced all the insulation that was in there (loose-fill blown-in fibreglass which was compacted down (due to age) to as little as 1inch in most places) with green batts (dacron) R3.5. As it was only last week, and now I've returned to work, it's too difficult to say how much the temperature in the room has dropped was caused by the insulation or the roof.

I believe Colourbond also has a painted additive/option to help reduce temperature slightly (3% less I think I recalled reading somewhere).

[zzsstt]

Think about the physics. The heat transferred to the inside of the house is done by conduction through the roofing iron. That heat is then transferred to the air on the other side (inside) of the iron by radiation or conduction. In order for the iron to transfer heat to the inside, the iron must be significantly warmer than the space below it.

Quite simply, the hotter the iron feels to the touch, the more heat it will transfer through to the inside. A finish or colour that reflects heat back outside will prevent the iron getting hot, and that in turn will prevent heat being transferred through to the inside. It is also worth noting that white also radiates less heat, which means that in winter a white roof will transfer less heat from the inside to the outside - though of course your insulation should prevent the inside of the iron from being warmed by the house anyway! Interesting to consider that "old" dark coloured roofing iron that has been painted a lighter colour "in situ", if it is still dark coloured on the inside will not be as effective as iron that is white on both sides, as the dark inside surface will absorb more heat from the house when it's cool, and be more efficient at radiating any heat absorbed from the sun when it hot. New, white both sides iron on the other hand will reflect heat back in to the house when it's cold, and radiate less heat downwards when it's hot.

So, a white roof will result in a cooler building during sunshine, and a warmer building during cool times.

Of course under enough sunshine, even a white roof will get hot!

The other thng to consider is an air gap between the iron and the insulation. Any "reflective" insulation, such as the aluminised bubblewrap or fibreglass roof blanket, requires at least 50mm of space on the shiny side to be effective. Shiny fibreglass roof blanket (or in fact any insulation that relies on trapped air) should not be compressed. So a house with shiny fibreglass roofing blanket compressed tightly between the iron and any internal roof/ceiling material cannot be considered to be usefully insulated! The best approach is to use white (both sides) iron on 2" battens, thus creating a 2" air space above the insulation which in turn should have the shiny silver side pointed out towards the iron and should otherwise fill, without any compression, the entire space between the rafters. Of course few builders will do this, as there are "OH&S" issues with shiny silver insulation pointed upwards......

In early summer I dropped my wifes car (white!) off for a service, and was given a dark grey Hilux as a loaner. I parked it right next to my identical, but white, Hilux. Two cars, exactly the same but one white and one dark grey, parked next to each other under exactly the same conditions. After an hour or so the grey vehicle was HOT to the touch, whilst my white one was still only about air temperature (it felt cool). I must admit it surprised me just how big a difference it made.

[karlajensen]

I'm getting there slowly
http://www.nearmap.com/?ll=-31.949687,1 ... d=20110215
roof turning white -cheaper than more insulation

so far hand on the inside reveals a massive improvement by feel which is pretty good if you ask me.

Karl