It is important to understand why these balloons do what they do. The latex balloons will rise until they burst. The balloon gets bigger and bigger with the expansion of the gas until the latex fails. They do not float. The superpressure balloon is not stretchy like latex. There is a little stretch but not much.
The superpressure balloon is give a certain amount of gas so it has the correct lift. The balloon will not be full. As it rises the gas expands and eventually fills the balloon. Since the plastic does not stretch (much) the density is now fixed. The balloon will rise a bit more and reach a level where the balloon overall density equals the outside air and it will stay there. The pressure inside the balloon will be higher than the outside.
There are very good articles detailing the behavior of a superpressure balloon and we will give a simplified explaination to get started. Anybody wanting to dig into the math can find great articles to detail the math.
The gases we use can be considered ideal gases. The particles of an ideal gas have no interactions with each other and are so small they never bump into each other. The nature of an ideal gas is that if you have one mole of particles, they occupy about 22.4 liters on the ground. A mole is a convient number for chemists and is 6.02 x 10^23 particles. It does not matter what the weight of the particles are. For air, the average weight is 29 grams and for hydrogen the weight is 2 grams. A mole of hydrogen will produce a lift of 27 grams. As the balloon rises, the air gets thinner and so does the hydrogen. As long as the balloon is not full, the lift remains the same.
The superpressure is an important number. The amount of free lift that the balloon has will determine what the pressure inside the balloon is at float. If the pressure is too high, it will burst. It will not be too low and the balloon will not launch. Over time, gas will diffuse though the plastic and it will rise slightly. The balloon will remain at float until the free lift goes to zero. This can be months so not much of a worry.
There is worry about the loss of gas waiting to launch a balloon. It is a reality and has to be considered. The SBS-13 balloons have a high quality multi-layer plastic that can hold the gas very well. The clear chinese balloons are less capable of holding the gas. They will lose gas at about 6 times the rate of the SBS-13. The simplest way to deal with this is to fill the balloons the night before when you have no time pressure and just roll up and clip the neck. In the morning, check again and add what you think it needs. Then do the heat seal.
The diffusion of gas at altitude is not an issue. There are too many ways to fail and it won’t be diffusion. At ground level, the pressure of the gas inside is 14.7 psi and the outside pressure of the gas is zero. At altitude the inside pressure is 2.3 and the outside still zero psi so the driving force is about 1/7th of that on the grouond but the temperature is dramatically lower so the diffusion through the palstic is lowered exponentially. The net result is, don’t worry.