When we chose to upgrade to colourbond (metal) roof, we were concerned that it might attract and retain more heat than a tiled roof. I asked a friend who runs a roof repair business what he thought. His advice was that a metal roof was fine as long as you had adequate insulation below the metal. He recommended a product called AirCell (which I think is foil-foam-foil) as best. CG use Sarking (foil) which is much better than nothing. Some builders don't use insulation at all, they just bolt the metal roof onto the roof trusses.
We asked CG if it was possible to use AirCell and if so, what it would cost to upgrade and use this rather than Sarking. CG were really great at following through this request, but when the quote came back between $5-6K to upgrade, we had second thoughts. We later talked to some colourbond roof experts who suggested that if the house is 6 star energy rated and has a whirlybird in the roof (to draw out the hot air in the roof space), that Sarking is adequate and absolutely enough to do the job. CG supply a whirlybird as standard with all metal roof upgrades, so we're pleased we saved money by not upgrading to AirCell. Here is a picture of the Sarking insulation.
Last note. Through the week we decided to remove some items from our contract (non-structural, just cosmetic, specifically, built-in shelving systems in the WIRs). CG were very flexible, helpful and understanding when it came to making these variations. They also honoured their word with some credits back to us in relation to our sliding door upgrade. Specifically, we were initially charged an extra $140 because they thought the sliding door would need to be delivered separately to the other windows/doors. This was not the case. They were all delivered together. When this was brought to CG attention, they did not hesitate in passing back a credit. This is excellent customer service. It gives us confidence CG will do what they say. This is what you want from a builder. So far, building with CG feels very collaborative. We have enjoyed the process. Fingers crossed for a similar smooth ride to Pre-Plaster Stage, when we will have our second private building inspection conducted.