by self locking I mean that many foam boxes, if you stay in the one design, will stack together with little bumps and grooves and crenulations and whatnot so that when they are stacked on a truck they don't move around. My preferred box is called a 'brocolli box' here and is 1'x1'x2' without holes and with a lid. They have 8 pegs moulded into the tops and 8 matching dimples in the base, and you can stack vertically, or any kind of stretcher bond (because the pegs make two squares). They are strong enough that you can fill them with water and stack them 5 high without collapsing. They're getting hard to find though, as growers seem to have switched to some kind of reuseable polyethylene or polypropylene crates. The subsoil I use is clay with granity chunks in it, and it sets fairly hard without cement, but slumps when wet. Adding 10% cement powder makes it rock hard. Trying to remove the surface often resulted in the polystyrene pulling apart rather than the surface. (When we moved I had to dismantle the wall, and I ended up using a hammer to break the surface up, even then I only removed enough to get them apart) Part of the magic is the technique of splatting the mix on rather than troweling. The force drives the cementy mixture into the cracks and forms a strong bond with the foam. You also get a far more natural finish than trowelling approaches. This process could be automated with a concrete sprayer. I found it helpful to rub over the surface with a cloth with some mixture on it. Originally I thought this was to wet the surface, but I now think that what was happening was that the gritty mixture scratched the surface giving a better key. I think polystyrene and a tensile element is probably one of the best structural materials out there for this kind of work - PS and mylar film is still the material of choice for ultralight mirrors used in cloud chasing. Look up 'tensegrity structure'. My prefered method is PS with fibreglass mesh and acrylic-cement basecoat. Then you can go nuts with whatever tufa you like. They make buildings out of PS foam with plywood coatings now http://en.wikipedia.org/wiki/Structural_insulated_panel and cementboard coatings http://www.thermasave.us/

that is the correct foam to use. Assume expansion of roughly 1:10, so you do not need very much at all. However if you can not fit it in there, then for your climate I might recommend simply installing a foam backer rod and caulking the joint instead. Air tight drywall is just that...creating an interior air barrier to prevent conditioned air movement from inside or outside of the gyp layer. Basically concepts use rubber gaskets or a layer of caulk around the edges of each sheet prior to being screwed on. Gyp is caulk too and around electrical boxes/lights. For can lights, I actually prefer building a plywood box in the attic around the can light fixture and simply taping the edges of the box and caulking it to the gyp. This creates a tight seal around the fixture since it is hard to seal up the fixture to the gyp alone. Basically any and all penetrations through your ceiling needs to be sealed. Think of it just as that..."air tight". Google "air tight drywall" for more information. Assuming you use r5-10 insulated duct and blow the cellulose completely over them, they should be ok in your climate. Remember you MUST seal the duct work to prevent air leakage as well. This includes sealing the ductwork to the registers and the registers box surrounds to the sheetrock. Caulking and air sealing is the single most important part of efficient and tight construction. It happens to be also the cheapest way to drastically change the performance of the house. It is more important than u values, etc. On that topic, it is also important for the rest of your house as well. The more caulking/air sealing you can do the better. On the EMHE, I assume you are asking about my house. It airs first of January I think.

The best advice is to have a frame shop mount the posters onto foam core for you. I have had desperately bad luck using adhesive foam core, and I am fully qualified to have done better! It isn't terribly expensive, and will save you much frustration. The posters will stay very nice and flat. I definitely would not varnish them (I assume you mean gloss medium), as that would very likely cause them to warp. I don't know why you aren't putting glass in them, but if it is because of the weight, most lumberyards will cut thinnish plexi for you for a reasonably cheap cost, and it is very lightweight.

I'll add this then: Durock? It's an approved substrate for any of the surface applied membranes. Sure it generates dust when cut with a power saw or angle grinder. But if you score and snap with a blade? It's pretty clean. It is heavy when compared to foam...but you'll be installing a half-dozen sheets max. Foam board? Sure, it's even lighter in weight. Here's all I recommend. Find the foam you want to use. Choose a membrane, example, Hydroban sheet membrane. Then call Laticrete's tech department and ask what thinset/adhesive they recommend for adhering their membrane to the EPS/XPS/foam of your choice. If you get a solid answer, by all means move onward with the project! If not, repeat for each foam/membrane/adhesive combo you can think of. Honestly, and again...I'm a guy who thoroughly appreciates the DIYer mentality...I built my own houses, boats, kayaks, canoes, airplane, swimming pool, I've drilled my own wells for water, I've done a couple of frame off car resto's...all solo. Trust me, I embrace the DIYer mentality. But showers? Here, on this forum, I do worry about compromised showers. Because houses are the most valuable investment most people on these forums will ever make, and water damage from a shower can just muck things up in tremendous fashion. Plus, it's near impossible to properly repair a shower short of tearing it out and rebuilding it. If your primary concern is weight and dust? Then I'll wave the bugger flag. And I'm waving it in an attempt to help you out. Just buy USG foam board and be done with it. Hydroban sheet is maybe $1.25 a foot? Kerdi about $2. Foam board? Maybe .40 cents. Adhesive? Who knows. Best case, you're saving yourself $1 a sqft by trying to make your own versus buying USG's product. For a shower of your size, that's maybe $75 you're saving. On an expenditure of how much for the entire shower? It's a fairly small percentage. Again...I THOROUGHLY embrace the DIY spirit. But with you being motivated by weight and dust? I'd spend the extra $75 to get the USG foam board. Lightweight. No dust. And you KNOW it's going to work. Plus you'll avoid the pain of having to lug a heavy and dusty bag of thinset home and the pain of trying to thinset or adhere the membrane of your choice to the foam of your choice with the adhesive of your choice on your own. Just my opinion. Worth the price paid. Good luck!! Seriously. Whichever way to proceed, I'd love to hear how things turn out.