Adding sarking for an existing tile roof?

Looking at beefing up the insulation in my place. With the "free" bulk insulation from the government recently proposed it's a no brainer however I'm also looking at reducing the radiant heat that enters the cavity for the next summer and hence we come to sarking.

I have an old terracotta tile roof as seen below
Attachment 74925

1) How would the sarking be attached to this roof? Can it run along the beams (that the battons and tiles sit on) which would leave it about a 80mm airgap between the tile and sarking or does it need to be right up near the battons the tiles sit on?

2) Would sarking run vertically or horizontally.

3) Should the foil be facing the underside of the tiles or facing the cavity. Rumor has it that it should be facing the tiles but it's often put the other way around due to OHS as reflections into the eyes of a roofer make it unsafe. Since this will be a DIY I'd like to do it the proper most efficient way so the insulation works as designed. It's tough finding the answer.

I've tried finding articles on how to do it for an existing roof but haven't had any luck. If anyone has any links I'd much appreciate it.

The house is in Sydney.

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