I took the time to measure and weigh a large square of foil and found it to be 29.15gsm. The surplus foil I was using measured up at 29.88gsm. Slightly more, but not significantly (what is that, maybe 1.5g difference for my size of blimp?). Fluffy's foil is just so much easier to work with.

a)
This is the 44" coloured material. 44" is a bit of a tease. The edge of the foil has a good 1/2" where the foil layers drop off. First the colour at 1/2", then the metallized at 1/4" and finally the last 1/4" of the foil is just the clear plastic base. So you have to trim the edges down and then it's closer to 43" width. I must admit, the wider width and trimming edges does add to the amount of work that goes with this foil, not to mention the extra work table area (which I have precious little). In theory, I suppose you could plan a design where the edges over lap, then you' have that 1/4 edge with a heat seal surface on both sides. It was probably manufactured with the intent of joining rolls into huge sheets.

b)
The 1cm strip is easier than it sounds. I cut everything with roller blades (the cutting kind, not the skating kind ). After trying a few different methods I found the easiest way to cut long thin strips is to fold the material over a few layers thick and as wide as my longest metal straight edge. Then slice away. The strips do curl, as does the rest of the foil if you let it get away on you. My gore tool has been lightly dusted with a contact cement to give it some tack. Most of that tack is gone now (dust, old foil coatings, dog hair, what have you), but apply some heat and the foil (non-stick side) still sticks quite handily. Using parchment paper and the iron I start the strip with some light heat and that's enough to hold it down. I sometimes wonder if it's the glue on the gore, or the foil itself that gets sticky when it's heated. Anyway, once the strip end is tacked, it's simple to pull it down the rest of the gore edge (my gore is 1cm wide as well) and tack it again at multiple places so it won't move.

Same goes for the gores. I start with one gore, tack the end and pull it out evenly so the edge follows the middle of the strip using the tip of the iron to tack it in multiple places to hold it in place, then line up the second gore with the edges cleanly butted and do the same. This time I'm actually tacking heat seal side to heat seal side. After it's all tacked down and looking good I take the full flat of the iron and give it a good once over to finish it off. I'm using my iron maximum heat settings now (couldn't do that with the other foil). At maximum heat, the gores and strip will melt into each other and smooth out any any wrinkles. After running the full iron over the seam, it's attached quite firmly to the gore tool. I've done so many seams over the same stretch of gore tool that I'm coming to the conclusion that it's the heated foil that's providing the stick, and the contact cement may not have even been necessary.

Fascinating - I'll have to try it!

Do you think the 'stickiness' of the non-heat seal side has anything to do with the colour coating?

Anyway, I'll give it a shot...

(scuttles off to order more foil before wife wastes all the money buying food for the children)

Ya I'm a little perplexed by colour coating. It's on really well. Smooth coverage and doesn't rub off easily with handling. FAIK it doesn't rub off with the parchment paper during ironing (at least there's no sign of it on the paper), and yet it still disappears on the seams. Or rather, if I use enough heat it disappears. If I don't use enough heat, the colour is relatively un-affected and I'll find the seam weld is incomplete. It's a really good visual indicator of a fully welded seam.

I have a strip of braided cloth ribbon glued to my gore tool. It leaves a nice cross hatch pattern in the seam after ironing. My theory is that the braided pattern makes both pressure points and channels for air to escape. So the seam gets complete welding with a pattern of extra pressure contact points. My gore tool surface was flat wood before I added the ribbon and I kept finding bubbles trapped in the seam. Not to mention the difficulty of trying to apply even iron pressure at exactly the right angle to cover the whole seam. Some times one side of the seam would get a better weld than the other and more often than not the very centre of the seam would be missed. The cloth ribbon gives the gore edge a little give so there's more room for error.

Anyway, back to the foil colour, I think that the heat and pressure of the iron over the patterned cloth base forces the colour coating to push into the pits and expose the metallised coating. If I make test seams on a smooth silicon surface, the colour is essentially un-affected. So perhaps it melts from heat as well.

I can't find any discernible layers in this foil. The salvage foil was clearly made up of multiple layers that separated too easily. This foils appears to be made of only one heat seal able plastic layer that has been metallized and coloured on one side. You can't weld colour side to heat seal side (I've tried). It only has a very very weak grip when you do this. I wouldn't trust it even for decorating purposes . Strangely, when you separate them, the colour stays with the colour and doesn't get picked up by the heat seal side. So even under heat and pressure, the colour side and the heat seal side won't bond. Which kind of throws out the theory that the colour side has at least some kind of stickiness.

I'll just add one more observation.

I've just soaked a sample of fluffy foil in ammonia. As expected it easily stripped the foil of the colour and metallized coating leaving a single layer of clear plastic. When you do this with the salvage balloon foil, not only does it strip the paint and metallized coating, but it also separates the thin heat seal layer from the non-heat seal base. That's the basic problem with the balloon foil. The heat seal layer is too easily separated from the structural layer.

On the other hand, the fluffy foil 'appears' to be a single layer with no indication of two different bonded plastic layers. Rip, it pull it, throw chemicals at it and it's still one piece. But when I tried to heat seal the different sides, it's clear that there's still a heat seal side and a non heat seal side. So what appears to be a single layer is in fact two different kinds of very well bonded plastic.

Where are folks getting the small seal that allows inflation and deflation of these envelopes?

Dollar store. They sell helium inflated foil balloons for cheap (more or less). I go to the counter and ask to purchase the balloon without helium. It's the same cost, but the balloon is fresh out of the box and the foil and valve haven't been handled. Of course you could take it home, filled with helium and play around with it first. The valve should be just as good either way. I look for a balloon design that maximizes foil and minimizes paint. Usually a silver star. I've also bought foil balloons in bulk from eBay. Expect to mess up a few along the way.

Once you've got a foil balloon just cut the valve out and re-use it. The inside surface of the balloon can be heat sealed directly to what ever foil you're using. Be really careful not to touch the valve strip with heat. It's super sensitive and I've accidentally nicked it more than once during construction.

I found a brand of paper straws just the right size to slide through the valve and allow deflation. A vacuum cleaner and straw makes short work of sucking every last drop of air out of an envelope before you fill it with helium.

(I should say that while ruzam is absolutely correct and using a commercial valve is best, if you're feeling lazy it's easy to heat seal your own 'valves' by ironing a double layer of film with a long thin strip of baking paper in the middle - the baking paper doesn't seal, and leaves you with an inflation tube which can be pinched off with a piece of tape or tied in a knot or whatever...)