Point to note ......
The Renault OEM bars have a load capacity of 80kg and the weigh 3kg, as Renault wont tell you what the roof load capacity is you must assume its 83kg.
If you fit Thule "Classic" bars (the black square ones) they are rated at 75kg. These bars and the fitting kit weigh 7kg, this gives you a maximum load on the roof of 76kg, the bars can only support 75kg so the safe load is 75kg.
The Thule Aero (aluminium oval) bars are rated at 100kg. These bars and the fitting kit weigh 4kg, this gives you a maximum load on the roof of 79kg as the bars can take 100kg the safe load is 79Kg.
This type of calculation can be applied to any manufacturers kit.
You must also remember to take the weight of the roofbox or other carrier off the safe load figure as well.
Most roofboxes are approx 15kg, unless its the 2mtr + ones which are approx 20-25kg.
Most roofboxes have a max load of 50kg, the better quality ones its 75kg.
So what am I rambling about ....
The most you can safely put on the roof is approx 75-83kg if using Thule kit, LESS the roofbox or carrier.
The reason I mention this ..... I've seen 2 Espace IV's with snapped OEM roofbars, and I've heard of 1 that rolled on a corner because the roof was overloaded, there is also a forum member (not me) that has had OEM bars snap!
DO YOUR SUMS and be dont take risks, whats inside your Espace is worth a lot more than anything you'll ever put on the roof.
Sermon here end'ith (sorry if it was a bit grannies and eggs
Hope it helps