Bad analogy, but think of a sponge. When you look at a sponge, you can't see through it. To a certain extent it looks solid. But you know that the sponge is porous, that things can pass through it. Some things are too large to pass through the porous structure of the sponge. Some things are small enough to pass through the sponge. Same with PEX. PEX looks solid. For all intents and purposes, it IS a solid. It holds liquid water. But things can still pass through it. Oxygen is one of those things. Water has dissolved gases in it, oxygen being one of them. In something like a closed heating system with a cast iron boiler, oxygen can be a bad thing. Oxygen plus cast iron? Not a good combination. Iron plus oxygen? Rust. You don't want to rust out your cast iron boiler. In new installation, dissolved gasses in the system will eventually work their way out of solution and be purged from the system, especially if the water is heated. If you have PEX in this system, say with radiant floor heat? If your PEX does not have the O2 barrier, then atmospheric oxygen can pass through the wall of the PEX and enter your closed water system. The circulating water will then carry the oxygen to your cast iron boiler, potentially causing damage to your cast iron boiler. The oxygen barrier in the PEX prevents additional oxygen from getting into your closed system, so once the initial dissolved gasses in the water come out of solution and are purged after system start-up, you have "dead water" in your heating system. "Dead water" will not rust your cast iron boiler as there is no free oxygen in the water to react with the iron in the boiler. Hope that makes sense.

My solution to the cats is a door at the bottom of the stairs with the cats down there. This is partly humor and partly just the truth. We built with a relatively open stairwell and now we have to put the cats outside or sleep is interrupted. For some reason, my wife stresses about them getting cold out there when it is 35 or less. I am understanding for the few nights below 25. But they were born outside and survived as new born kittens although to be fair it was springtime. But their parents did ok. I would build a door at the bottom of the stairs. The cat door would help of course but having to be quiet is really annoying - I'm the early riser. I am a big fan of 2 doors to separate public space from primary bedroom. But you need to decide which side the cats are on unless you leave the bottom open at night and they come down with you.

The planter box trellises I posted are very affordable with your budget--get several. Also this came up when I searched for "privacy screening." Do a search on that.

Don't know how much you're up for . . . This shows a 4-foot wall, with dwarf lantana in front and a hedge of columnar podocarpus behind for more height. I read that can be kept to 3' wide; however, I doubt there's really room for all three layers plus space to open the doors of parked cars. I would probably skip the hedge and just do the wall (if budget allows one), with low, easy-care plantings at its base on both sides. The wide driveway opening will let sound in anyway, even if the wall-plus-hedge muffles it a little on the sides. You could do tall plants in the corners, past the parking area, I suppose; there they wouldn't have to stay narrow. Would the city let you set planters (not part of the structure) on top of the wall? That could get the visual separation above eye height to at least block some view. Caveats: It looks like the driveway is wide enough to be split as shown to break up the paved expanse, but I can't tell for sure without dimensions. And I have no personal experience with Florida plants!