What do you mean by "our AC units require fresh air ventilation"? Fresh air doesn't come from the attic. If there's an atmospheric combustion appliance in the attic, such as an 80 AFUE furnace, then you shouldn't be encapsulating the attic anyway. If that's the case, you don't want spray foam on the roofline at all. You can either change out the furnace to a sealed combustion unit or do your insulating and air-sealing at the flat-ceiling level. Insulation Spray Coating
After researching this, Im curious about a method called poor mans sprayfoam where foamboard is cut to size between studs and sprayfoamed around the perimeter. Aside from the labour doing the job yourself, the cost seems comparable, is one better than the other? What about the quality of the canned sprayfoam around the perimeter? When i see sprayfoam done, it is never a full depth of the studs, so you would get 2.5-3 inches in a 3.5" (2x4) wall. Any thoughts? 
In plasma spraying process, the material to be deposited (feedstock) — typically as a powder, sometimes as a liquid,[2] suspension [3] or wire — is introduced into the plasma jet, emanating from a plasma torch. In the jet, where the temperature is on the order of 10,000 K, the material is melted and propelled towards a substrate. There, the molten droplets flatten, rapidly solidify and form a deposit. Commonly, the deposits remain adherent to the substrate as coatings; free-standing parts can also be produced by removing the substrate. There are a large number of technological parameters that influence the interaction of the particles with the plasma jet and the substrate and therefore the deposit properties. These parameters include feedstock type, plasma gas composition and flow rate, energy input, torch offset distance, substrate cooling, etc. Spray Coating Services

Charles spoke of a "huge problem" of convection within cavities not filled with foam. This is a false assertion. The trillions of trapped air cells in foam insulation prevent heat transfer at a rate that would cause those problems. Also the wood framing members are relatively good insulators themselves unless the wood is particularly dense. Convection currents against foam insulation are not the problem he asserts. He also said" the homeowner is literally pissing money away if he pays for this spray foam job - or any job like it - for he will only be gaining a marginal amount of insulation effectiveness." This is a ludicrous statement. The structure pictured above will have a very effective performance despite his assertions. My only concern is humidity. Closed cell foam does not permit moisture migration and trapped moisture could be a problem, especially if the AC is not running due to moderate to cold temperatures.
For over a decade, Synavax™ has served as the preferred industrial thermal barrier insulation, energy saving, surface protection and asset protection coating, meeting the needs of multiple industries around the world, including pulp and paper manufacturing, food and beverage manufacturing, U.S. military, oil and gas, textile manufacturing and many others. Insulation Spray Coating
The criterion for whether your crawl space or attic is occupied is often the size of the entrance. Entry to the crawl space or attic should be a small hatch or door, not a normal-sized door. Additionally, if you cannot get to whatever area you are insulating without cutting through a drywall layer, then you do not need to use an ignition barrier, because the drywall already serves the purpose of the thermal barrier.
In cold spraying, particles are accelerated to very high speeds by the carrier gas forced through a converging–diverging de Laval type nozzle. Upon impact, solid particles with sufficient kinetic energy deform plastically and bond mechanically to the substrate to form a coating. The critical velocity needed to form bonding depends on the material's properties, powder size and temperature. Metals, polymers, ceramics, composite materials and nanocrystalline powders can be deposited using cold spraying.[4] Soft metals such as Cu and Al are best suited for cold spraying, but coating of other materials (W, Ta, Ti, MCrAlY, WC–Co, etc.) by cold spraying has been reported.[1]
During the 1980s, a class of thermal spray processes called high velocity oxy-fuel spraying was developed. A mixture of gaseous or liquid fuel and oxygen is fed into a combustion chamber, where they are ignited and combusted continuously. The resultant hot gas at a pressure close to 1 MPa emanates through a converging–diverging nozzle and travels through a straight section. The fuels can be gases (hydrogen, methane, propane, propylene, acetylene, natural gas, etc.) or liquids (kerosene, etc.). The jet velocity at the exit of the barrel (>1000 m/s) exceeds the speed of sound. A powder feed stock is injected into the gas stream, which accelerates the powder up to 800 m/s. The stream of hot gas and powder is directed towards the surface to be coated. The powder partially melts in the stream, and deposits upon the substrate. The resulting coating has low porosity and high bond strength.[1] Insulation Spray Coating
When intumescent coatings are used with spray foaminsulation, they are specific to each insulation brand. You cannot use one intumescent coating with a brand that it was not intended for. If you do not use the properspray foaminsulation brand and intumescent coating combination, then your work will not technically be up to code. Additionally, it is important to keep in mind that intumescent coating can only be used as an ignition barrier alternative if it is being used in an attic or crawl space. And, even then, it can only be used if space is not used for storage and is accessible through a small hatch or door. Spray Coating
What do you mean by "our AC units require fresh air ventilation"? Fresh air doesn't come from the attic. If there's an atmospheric combustion appliance in the attic, such as an 80 AFUE furnace, then you shouldn't be encapsulating the attic anyway. If that's the case, you don't want spray foam on the roofline at all. You can either change out the furnace to a sealed combustion unit or do your insulating and air-sealing at the flat-ceiling level. Spray Coating Services

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Charles: That photo is closed-cell SPF, and it's not sprayed perfectly uniformly. As long as it has the minimum thickness required (3" on a ceiling here in the Atlanta area), it should be fine. I've discussed some problems with SPF installations in the articles listed in the Related Articles section above. This photo is from about 10 years ago, though, and has no ignition barrier.   Insulation Spray Coating

One of the high-tech insulation products being utilized these days is a foam-spray product called "Icynene," which is a foam that starts as two liquid components (a and b, if you will) that are heated up and sent through two hoses and meet at a tip of a gun where the magic begins. This is a thin film that expands to 100 times its volume in eight seconds, when it's permanently set up. Spray Coating Services

As this example illustrates, it's important to seal the envelope completely. One of spray foam's biggest selling points is its air-sealing ability, but it can't seal places where it's not sprayed. One of the nice things about using spray foam in new construction is that you can do a Blower Door test before the drywall goes in. Even better, you can test for leaks with a fog machine. Spray Coating

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