Jennifer, I'm not an expert on the health effects of closed cell foam. A lot of people live in houses with closed cell foam and have no health problems from it, at least not short-term, acute problems. I do know of one person who had it removed from her crawl space because she was convinced it caused her dog to get sick, but I know only what she told me.  
I am building a house in Central Texas (Caldwell). Several builders are cautious about using foam insulation and/or a closed attic. I would like to use both. Here in Texas, heat and humidity (except for the past few years of drought) are a continuing problem. Which type of foam would be the best to use in our home, where should the vapor barrier be or should be use one at all, if we are using fans in the exterior walls to supply fresh air to the house, do we need a vented attic or will it cause more problems than it solve? I have printed out your article and the blogs to give to my contractors and architect, but I would really appreciate your comments on the products being used in my part of the US. 
Warm spraying is a novel modification of high velocity oxy-fuel spraying, in which the temperature of combustion gas is lowered by mixing nitrogen with the combustion gas, thus bringing the process closer to the cold spraying. The resulting gas contains much water vapor, unreacted hydrocarbons and oxygen, and thus is dirtier than the cold spraying. However, the coating efficiency is higher. On the other hand, lower temperatures of warm spraying reduce melting and chemical reactions of the feed powder, as compared to HVOF. These advantages are especially important for such coating materials as Ti, plastics, and metallic glasses, which rapidly oxidize or deteriorate at high temperatures.[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]
Vacuum plasma spraying (VPS) is a technology for etching and surface modification to create porous layers with high reproducibility and for cleaning and surface engineering of plastics, rubbers and natural fibers as well as for replacing CFCs for cleaning metal components. This surface engineering can improve properties such as frictional behavior, heat resistance, surface electrical conductivity, lubricity, cohesive strength of films, or dielectric constant, or it can make materials hydrophilic or hydrophobic. Spray Coating
Most electric powered airless pumps have an electric motor connected through a gear train to the paint piston pump. Pressure is achieved by stopping and starting the motor via a pressure sensor (also called a transducer); in more advanced units, this is done by digital control in which the speed of the motor varies with the demand and the difference from the pressure set-point, resulting in a very good pressure control. Some direct drive piston pumps are driven by a gasoline engine with pressure control via an electric clutch. In electric diaphragm pumps, the motor drives a hydraulic piston pump that transmits the oil displaced by the piston, to move the diaphragm. Coating Services
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
The CWST Thermal Spraying, FW Gartner and Bolts Metallizing business units in AZ, CT, MA, SC and TX apply HVOF, HVAF and plasma thermal spray coatings for flight and industrial gas turbine applications as well as tungsten and chrome carbide wear coatings utilized in Oil & Gas, Mining, Steel Mill and other demanding industrial markets. FW Gartner also provides Laser Cladding and PTA welding services on new and service run parts for components operating in severe service environments. Spray Coating
Thank you, Allison. We have a split system unit (actually 5) with the heat being propane. The foam guy said that it needed ventilation, so completely sealing the attic wouldn't be a good idea. It sounds like you are saying that the systems need to be vented out of the attic or replaced with some type of closed system. But I shouldn't bother spraying if the plan is to leave the soffet vents open. Is that correct?
Allison--would be very interested in your take on the GHG issues of foams, which have received lots of smart commentary recently. To highlight a few: architect Jesse Thompson's comments on Tom's Good vs Bad post on Energy Circle, the very thorough piece by Alex Wilson on Green Building Advisor and Michael Anschel's cautionary diatribe on Remodeling.  Coating Services

High volume low pressure (HVLP) is similar to a conventional spray gun using a compressor to supply the air, but the spray gun itself requires a lower pressure (LP). A higher volume (HV) of air is used to aerosolise and propel the paint at lower air pressure. The result is a higher proportion of paint reaching the target surface with reduced overspray, materials consumption, and air pollution. A regulator is often required so that the air pressure from a conventional compressor can be lowered for the HVLP spray gun. Alternatively a turbine unit (commonly containing a vacuum cleaner derived motor) can be used to propel the air without the need for an air line running to the compressor.

Open cell spray foams are less common and have a lower resistance to heat flow than closed cell spray does. Open cell spray foaminsulation expands when it is applied, which allows it to be installed in hard to reach areas, such as small wall and ceiling cavities. Open-cell spray foam is typically cheaper than closed cell foam is, but it is more permeable and vulnerable to fire hazards. Coating Services
When you want to take any metal surface and make it more visually appealing, pay a visit to Xtreme Power Coating in Grand Prairie TX. Since 2003 we have provided powder coating services in Grand Prairie, TX that bring our customers’ furniture, iron fences and decorative metal pieces to life. Powder coat finishing transforms dull, worn out metal and gives it a bold, beautiful sheen that passersby won’t be able to help but notice. Prepare for compliments once your metal décor receives a powder coat finish make over. When it comes to powder coating in Grand Prairie, TX, no one else comes close to Xtreme Powder Coating.
The problem was that the installer was doing his first spray foam job ever, and the thickness of the insulation varied from zero (visible roof deck) to about 9". Unfortunately, good average thickness doesn't cut it. The coverage needs to be uniform because a lot of heat will go through the under-insulated areas. (See my article on flat or lumpy insulation performance.) Spray Coating
Weather & Drying Guidelines: Ames® Super Primer™, Maximum-Stretch® and Iron Coat® is best applied between 50° to 90° F (10° to 32° C) on warm dry surfaces. Apply when the streets are dry, the sun is in the sky and no inclement weather is forecast. Starts to dry in about 2-8 hours, depending on thickness of application and weather. It is recommended to allow a minimum of 24 hours between coats with no rain in the forecast and no freezing temperatures. Low temperatures, high humidity and evening and morning dew will require increased drying/curing time. Do not apply after 2pm for best drying. The earlier you can put it on, the better. Make sure any morning dew is completely dried off before proceeding. Spray Coating
Thermal barriers come in the form of 1/2-inch gypsum drywall or any material that is proven to have an equal fire resistance to gypsum wallboard. Additionally, alternate assemblies can be used if they have been tested in a laboratory and approved. Ignition barriers are a lower standard to meet, and can be satisfied with a number of different materials. Like thermal barriers, they can be substituted with equivalent fire resistant materials and alternate assemblies, such as an intumescent coating. Spray Coating
I applied this product on a rolled asphalt flat patio roof. It was over 90 degrees and I could not touch the roofing at all on the first coat. I was laying on layers of cardboard to apply the silicone under the gutters and then rolled the first coat. The next day I brought all the cardboard to apply the coating under the gutters. I accidentally touched the roof and it was cool‼️  I threw the cardboard off the roof, laid down to do the gutters and roll the second coat. I was amazed how well this sealed and cooled the patio. I am going to apply this to my other patio and covered parking which are metal. I can’t wait to apply for sealing and adding cooling to these areas. Roof Coating Supply answered all my question and they ship promptly - I had my order in 3 days. Thank you. Spray Coating