Eelco

True; any glue will clog the pores; which probably does matter, since there is also of course such a thing as wicking, which plays a role in leather, whereby the large surface tension in microscopic hydrophilic pores will rapidly suck small channels of bulk liquid water around in a way that the much smaller atmospheric air pressure differences never could. But the glue line would interrupt those continuous channels. So im having a hard time imagining a glued piece of leather to be quite as vapor permeable as a single piece of equal thickness; but the open question to me is how close you can approximate that ideal in practice.

Yeah indeed the holes in a coarse canvas to allow for a direct pumping effect; like if youd hold it to your face you could quite easily blow through a significant amount of air (unlike leather). But I mean it here in the sense of all water transport; not just bulk convection. Clearly, a single layer of leather, while more closed than canvas, is far superior to a gum boot or a latex glove.

Goretex is a little different still; its a thin hydrophophic membrane with tiny holes. The hydrophobic nature makes that liquid water will bead up on the outside, while gaseous water can diffuse through the holes by virtue of it being such a thin membrane. Waxed leather is similar in that regard really

Im not sure breathable is a very well defined term; but if you want to get technical about it; yeah there is a difference between convection, as you would get from bulk a bulk gas being pumped around, versus molecular diffusion through the material. Subjectively the net result is the same though; how fast does a given amount of moisture go from one side to the other, and how much moisture will build up inside the shoe. While a solid piece of leather technically is porous, the holes are so tiny that I think the amount of bulk convection through an unbroken piece of leather can be ignored. The way water gets through then, is by dissolving into the leather, diffusing along the leather fibers, and evaporating out at the other end. Same with PVAc; its a solid sheet of rubber without holes, but its a very hydrophilic rubber so water can diffuse through it. Hopefully fast enough to make a difference. Something like neophrene is super hydrophopic though; close to zero water molecules will dissolve into a rubber like that, and close to zero will make it to the other side.

I did some experiments with PVAc; made a small sheet out of pure PVac, 10wt% and 20wt% PEG400. The pure material is not very flexible and can be cracked even at room temperature, as expected. The 10% can be cracked right out of the freezer, the 20% remains flexible in all conditions tested. Both are very tough and ductile and seem like they would make for perfectly nice leather glue (as is the conventional wisdom). Ill guess Ill make a bigger thin sheet on a piece of silicone sheet so I can fold it into a little bag, then put in a damp sponge, plus a paper and PP bag as controls, then weight them in over time to see if it makes any real world difference in water permeability. I got one shoemaker to talk to me about what they use for their boot linings, and its latex. Again not sure what it matters in practice but it might explain why the lined leather boots I have are about as breathable as a latex glove. I also have PVB resin that I think in theory might also work great for leather with some PEG400 added, and its also a highly hygroscopic polymer, capable of absorbing almost 10% water; even before mixing in any PEG. Probably should also add that to my experiments. Havnt been able to find anyone directly commenting on this topic; except for this guy; though he does not offer much in the way of real world tested solutions; more a general observations of 'yeah adhesives can ruin your shoes breathability'.

True; or I could make a sandal instead . My interest here is somewhat theoretical though, to figure out how much glue matters to vapor permeability in practice.

Ive been wanting to make my own indoor leather minimal shoe for a while, really optimizing for breathability. Ive noticed that the leather boots I have vary a lot in breathability. Some of that makes sense; thicknesses and leather types; but in part its a bit of a mystery to me. Especially lined leather can be a real disappointment in breathability; even when the total thickness isnt that much. Im inclined to explain this in terms of shoe cement used. If you look at the chemistry of the most popular shoe cements, they are not vapor permeable at all. Infact they often prize themselves on their ability to block water. But that goes both ways. The construction im currently considering is an upper sticheddown to the insole, and then a glue only leather outsole. This being a barefoot indoors design, I want to keep the total sole thickness minimal. And ideally, the outsole should be easily replacable. What im hoping is that the right use of PVAc glue will serve me for these indoor soling purposes. What im hoping is: * the glue I find wont make my sole fall off all the time * if I want to replace the outsole, it should be easy to get off by soaking in warm water / maybe acetone? * the thin leather sole plus water permable glue will noticeably contribute to overall breathability * I can find a PVAc sufficient flexible for this purpose in the first place (adding 15%wt PEG400 should help with that?) Does this sound at all like a reasonable experiment, or should I save myself the time? Im having a hard time finding shoe cements explicitly advertizing themselves as breathable. If it sounds like this is unlikely to work, perhaps a good option would be thin rim of a stronger cement, with the bulk interior of the sole being PVAc?