Eelco

https://www.hotmeltstyle.com/eva-hot-melt-adhesive-web-film-product/ https://www.shanghaixinxu2016.com/product/honeycomb-holes Found two hot melt films specifically made for this purpose. EVA isnt breathable itself but that does not matter much when its perforated.

Thinking more about it, I think the most practical method would be a premade thin glue sheet, with tiny say 0.1mm holes punched through in a dense pattern. You could set up a simple desktop laser cutter to do that in the absence of an off the shelf product. PVB would be a nice base material since you could either melt it on with a heatgun or mist on some ethanol to activate it (not many other materials that are top-tier adhesives, match the mechanical properties of leather, can be hot melted and are soluable in a bunch of eco-friendly solvents; but not in water; infact I think only PVB fits the bill). If you get the hole pattern right it should add minimal moisture resistance to the total of your shoe. The total effective extra length that the moisture needs to travel through the leather should be on the order of the hole spacing; a few ten's of a mm or so. It should easily be strong enough to bond linings to outers; and maybe also leather outsoles; but again if you add a rim of a proven shoe cement on the outside it should be totally fine I imagine.

Found some numerical values for water permeability of PVAoh here in Figure 76 and some values for leather here. To put that in perspective, even 20mu of the most water permeable glue I can think of, has a 10x greater barrier to moisture than the most permeable leathers; and is kinda on par with a less permeable leather. So I think that pretty much proves that if you care about permeability, you better stay away from linings at all and stick with the single layer leather; and also if you want your sole to contribute, it better be a single layer. Also a single male foot thats active can produce some 20g of sweat per hour. Even with a leather shoe on the more permeable side of things, like a single layer thin mocasin, itd only barely keep up. Thats of course not taking into account the ability of your leather to store/buffer moisture; not like im going to be working out 8 hours continuously. But yeah... stay away from glue if you care about moisture, seems indeed like it should be the moral of the story here, at least if im reading these numbers correctly. Not super happy with that conclusion because I kinda like the idea of replaceable outsoles; but perhaps only glueing the outer rim of an outsole with a strong cement would suffice? Also it is quite possible I am not reading these numbers correctly. If you take a thin layer of permeable glue and squeeze it out as much as possible, you dont get a homogenous 20mu line, but rather a variable thickness with both sides of the leather in near contact in many places. So what the net resistance of such a glue line is can really only be established empirically I think; not hard to imagine that in practice things could be an order of magnitude better; in which case properly applied glue would be back on the menu as a viable option...

Acetone isnt all that different from ethanol in its toxicology. And I frequently handle both by the gallon. Not taking any baths in it though; and proper ventilation never hurts. Seems like these would be the least of my VOC-worries as a leatherworker though. I personally dont let any chloro-anything into my house; or any glue with an unspecific solvent composition; so its not like im not paranoid about these things. But yeah I acetone ethanol propanol and ethyl-acetate... those are the only ones I allow into my house really. But to each his own.

Im hearing mixed things about it; depends on the timing and type of leather I guess. But the melting point of the PVB + plasticizer can be tuned to be super low, like 50C or so; plus water also acts like a further plasticizer on PVB. So if you just soak the leather outsole in warm water, I think you could just tear it right off with a bit of force, without meaningful heat exposure to your upper.

Google seems to have a hard time deciding on what a Crepida is exactly; but the term has led me to some other interesting historical designs. The term calceus seems to come pretty close to what im thinking of. Great that its already been done; not trying to win any originality points; I just want a shoe that will hold up to hours of indoor walking, while looking good in the process. In a perfect world id both find the time to give a decent shot at doing it myself; and find someone capable of executing on it professionally. The more theyd have done it before the better.

Yeah good question about the working properties; I imagine that makes all the difference in practical shoe making. I think PVB is kinda nice since you can use it either as a low melting point hot glue, or as a solvent goop, or a mix of the two. You can clamp a sheet in a frame, put it over an IR heater, squeeze something in between and it will stick right away (which is how carglas is made); or alternatively use a slow vaporizing solvent to get a long working time. But thats just theory... I dont have much shoemaking practice. I made a 20%PEG PVB sheet as well which is very flexible; and does not have the same oily feeling as the PEG-PVAc, suggesting that the plasticizer is bound better. It barely needs it anyway to be sufficiently flexible so I dont think it matters much. I put a piece of wet paper towel folded inside, and put a wet paper towel next to it as control. The PVB sheet is about 0.05mm thick I guess. In the same time that the control sample lost 20% of its weight, the PVB/PEG wrapped sample lost 10% of its weight. Thats pretty good id say; in terms of diffusion-resistance, we are comparing R_air_boundary_layer to R_air_boundary_layer + R_glue, suggesting the diffusion resistance of the water through the adhesive is of a similar order as the transfer to the surrounding air. In a real shoe we have of course a whole chain, of R_sock + R_lining + R_glue + R_outer + R_air. And im guessing normally in that chain, the R_leather dominates, since it sure is noticeably different from the pure R_air youd experience barefeet. So I think in total a boot glued like this would not be terribly different from the same amount of leather without any glue. I should buy some latex adhesive and pour a sheet of that. I think my paper would lose some moisture eventually but probably not enough to be measurable after an hour.

Yeah; single layer upper leather shoes have a good reputation for breathability; which is also my experience for single layer tough boots I own. Though its definitely not as breathable as a fabric upper... so that made me curious how much breatability we could be getting from our soles. But im having a hell of a time trying to find my ideal indoor all single layer leather boot. I should buy some type of single layer mocasin type shoe though, just to have something to compare against, even if its not stylistically what im looking for. Note what im looking for is the ideal all natural material desktop threadmill shoe. Its going to see some serious marching so it needs to stand up to that in terms of construction and comfort, as well as breathability. I kinda want ankle height because I hate heel slip or tight laces on top of my foot. And im not sure I want to larp as a roman soldier although to be fair the right kind of roman sandal would be kinda ideal for my purposes... but id rather want something more boot-looking, less sandal looking. So yeah...

Tested out some PVB today; made a thin sheet which is really nice and thin and uniform since its fully dissolved in ethanol. Looks and feels like snake skin. Didnt add any plasticizer since in pure form it should have a shore hardness similar to leather. Was kinda worried about it being brittle; though it isnt even in the freezer. In pure form it does not have a ton of stretch before breaking though... but enough to match leather I suppose. You can notice that it is water permeable; the thin sheet pretty much instantly becomes more rubbery when put into water, showing that water dissolves into it fast. I kinda like that it seems like itd work without a plastizicer; since PEG is also used as a leather conditioner and its a fairly small molecule; so if the PEG would diffuse out of the PVAc into the leather over time the glue line would become progressively more brittle. Also I the PVAc/PEG sheets leave a bit of a greasy residue on my fingers, indicating that there is PEG weeping out onto the surface I suppose. Its interesting that PVB glue manufacturers frequently cite their product as being suitable for leather; but ive never heard of a leatherworker actually using it.

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.