I've been thinking, which is nearly always a bad sign, especially whenever there's math involved along with various conversions. My nature or perhaps fault is to look hardest at the positive aspects and, that method sometimes unintentionally circumvents a few other issues (like accuracy). Although, unless you're another village idiot such as myself or perhaps much worse off as being a NASA Borg, you should easily get my drift and, for all of your helpful feedback of ideas and/or corrections, I'll certainly provide the fullest of credits wherever I can, as unlike our NASA, I have absolutely nothing to gain by burning such bridges.
I recall that of several months ago, when I was pondering over the positive consideration as for H2/N2 rigid airships, as well as for my subsequently considering the thermal insulation attributes associated with applied H2/N2 spheres, I could instantly tell by the sorts of impudent reactions and replies of continued discovery bashings, that most of the pro-NASA types were entirely dumbfounded. As I went about my asking upon the characteristics and/or potential of applied spheres and, except for receiving some intentional disinformation flak from time to time, I continually received either nothing of worth or merely further bashings on every flank. I guess those pro-NASA wizards of "spin" and "damage control" simply didn't have a bloody clue of what buoyancy or R-factors were all about, sort of an intellectual black hole, as usual.
In addition to my limited but somewhat first hand knowledge of silica/ceramic insulation pads or blankets that can easily exceed R-100 (BTW; that's stuff right off the shelf), my focus was then as it is now, further regarding upon how to go about obtaining an R-256 insulation factor by means of H2 filled spheres, lets say initially of applying relatively large 100 mm diameter spheres and of 1 mm wall or shell thickness (a Venus buoyancy of roughly 0.071 kg each):
Obviously the smaller these spheres become, the better off, as for whatever their wider application range as well as for overall endurance, although for starters, I thought of initially understanding something one can easily see and perhaps best comprehend, as the actual size is certainly not of any criteria factor unless you're attempting to stuff big spheres into lesser areas, as then the smaller the better, all the way down to those micro spheres that could be easily incorporated into clothing, shoes or of portable shelters.
To start things off; I've selected upon a relatively robust sphere constructed of 1mm thick glass (Pyrex grade if need be), which I've hopefully correctly calculated at 0.071 kg or roughly 2.5 oz apiece. Again, I've not been fully reassured of the actual implosion worth, although I'm of the opinion we're easily exceeding 345 bar, which is only a worthwhile consideration if the interior were of H2 contained at 1 bar (1 bar of H2 is simply a whole better off than of 75 bar H2, especially when it comes down to insulation [thermal conduction] factors, as well as a slight buoyancy advantage, especially if we can create 0.1 bar or less worth of H2)
In order to fill or displace a cubic meter worth of space or void, I believe that'll require roughly employing 1150 of these nearly buoyant glass or ceramic spheres.
On Earth; I believe each sphere alone will weigh roughly .071 kg
On Venus; that same glass sphere element weighs in at .065 kg
Thus the mass associated with the m3 volume as that filled by such a sphere is in the neighborhood of 1150 X .065 = 75 kg
Being that a 100% volume in H2 buoyancy of a cubic meter is worth 65 kg/m3, that places the insulation spheres at just 10 kg more, although the actual internal sphere volume is occupying but roughly 57%, then roughly +40% of that remaining cubic meter is comprised of N2 offering the total potential of 26 kg * 40%, thus becoming 65 -10 kg worth of H2 sphere buoyancy plus the 10 kg buoyancy offset factor of N2 is remaining zero kg worth of internal buoyancy per cubic meter structural unit, of which smaller spheres could otherwise be made so that the otter shell or structural considerationss still achieved nearly zero weight per unit.
I'm thinking of a structural block/wall sort of modules having to be associated with the 625ºK exterior nighttime environment, as needing a full meter worth of such insulation barrier could be a realistic goal. Along with the remaining voids in between these internal spheres being of N2, at say 75 bar associated with the elevation of 5 km, where N2 as certainly another insulator, especially if bone dry and effectively trapped as gas pockets in between and throughout the maze of glass/Pyrex H2 spheres, thus also another excellent worth of buoyancy.
A reminder of what's in between all those H2 spheres; of the remaining containment displacement being of N2 (-10.5 kg/m3 worth of buoyancy), thereby the actual insulated weight impact upon the overall structural elements is likely going to be in the range of 10 to 20 kg/m3. As such good insulation/structural materials go, even a 20 kg/m3 impact simply isn't all that bad and, those spheres could certainly become less than half the weight of the .0575 kg version, like 0.025 kg/sphere is still offering sufficient body, bringing their m3 impact down, which obviously places the insulated m3 module into a potentially negative displacement or buoyancy contribution of at least -10 kg at ground zero and, perhaps that's still remaining just slightly buoyant at the 5 km elevation, which is a darn good thing if one was attempting to construct really big sorts of things, like large thick walls or other facets associated with fending off the toasty CO2 exterior of 625 to 650ºK and, didn't want to be dealing with any appreciable weight, where saving 10+ kg of less overall structured weight seems like that would be a worthwhile outcome (just for the heck of it, lets plan upon that m3 structure or H2 sphere containment unit or module weighing typically at somewhat more then 10 kg/m3).
A slight problem obviously arises whenever our glass spheres start weighing in at something less then the potential buoyancy of their contained 1 bar worth of H2, as that's a serious dilemma if those insulation and presumably valuable spheres should commence drifting off into the Venus stratosphere, thus any such sphere mass of at least a gram or more so above the H2 buoyancy would be in order, where as smaller spheres would simply be requiring a ratio of perhaps 1.1 X H2 buoyancy, so as to remaining manageable, as any lesser mass might be ill advised unless you have devised upon an effective method of retrieving lose micro spheres.
BTW; Obviously smaller and thinner glass or even of certain metallic spheres may also represent even lesser thermal conduction issues, as well as another million+ fold increase worth of trapped N2 voids, so perhaps those thinner sphere shells and of smaller diameter (like 1 mm) could become the ultimate in structural insulation (obviously being more robust, less likely for breakage and, of those structural exterior walls could all be all that much thinner or perhaps better yet, under such bone dry conditions the thermal conduction R-factor can technically be pulled down to as little as R-1024 (that's better than 99.9% thermal efficiency).
As with insulation and of any rigid airship H2 considerations; Containment of H2 is simply not an option on Venus, as unlike Earth, H2 is relatively illusive and much more valuable then on Earth. H2 is certainly safe and, as such it can be easily contained within spheres, especially those under the external force of the CO2 environment and/or with regard to the N2/O2 interiors of airships as well as for whatever surface dwellings. H2 simply can't leak out (even if there's a few micro holes in the sphere), especially when it's contained under essentially a vacuum. It's relatively simple (low tech) to seal N2/O2 out of something containing H2. The airship spheres might be very large metallic sorts, as controlled ballast via compressing H2 back into those spheres and/or releasing the H2 into the outer containment zone of the primary or outer hull, as N2 would otherwise surround these extremely large H2 buoyancy spheres and, subsequently a little O2 that's merely sufficient for a lizard sort of chap to survive upon would be distributed throughout the lower levels of the occupied portions of this global metro transport and, most likely astronomy platform.
As always, I'm absolutely certain I've made a few more mistakes and, as such I'll eventually be back to either correct them or make everything much worse and/or post your better ideas and expertise into this equation. Obviously with all the correct data and, displaying everything in a more graphical format along with correctly knowing of the resources at hand, both on Earth as well as for anything Venus, I'm certain the right sort of individual(s) will either educate the likes of myself or better yet, accomplish their own superior calculations for this phase of the adventure. Too bad our Club NASA can't become involved, as they seem still more interested in or in demand as for cloaking on behalf of NSA/DoD agendas then of accomplishing one darn thing on behalf of humanity. ESA on the other hand has not been crawling about on all fours and, as such should be fully capable of introducing this challenge into the appropriate hands and brains of those interested in making a difference.