it has been proven that a hot attic adds about 10 degrees to your shingles. This will take 2 to 3 years away from the life of your shingles.
This will make your attic part of the envelope. This will put you air handling unit and duct work in the envelope. Duct work leaks, it just does. Now it leaks into the envelope. This has proven to increase the performance of this part of the HVCA system 25% to 30% because it is no longer in a hot environment.
Attic access stairs and recess lighting are hard to insulate. They are a weak link in insulating you attic floor. Now that is not a problem.
I will add storage space to my home that is conditioned and better for the items I store in the attic.
There are 2 kinds of spray foams. Icynene and Urethane.
Icynene is softer so it moves with the stucture (wood swelling and shrinking) and contains no VOC's. It can be damaged easier because it is soft.
Urethane harderns adding strength to structures it is applies to. Has a higher R value, does have VOC's and is not damaged as easy.
Both have to have a fire retarder added (ie drywall or the same paper that is used on batt insulation). The same paper that is used on batt insulation call be obtained from the insulator and 9 cents a sqare foot.
You do not have a drywall a "limited access space" (ie attic and crawl space) in my area.
Neither of these insulations absorb moisture, permit convection, sag or settle, contain formaldehyde, require a drying time or supply food for termites.
NOW, since you have just sealed your attic up tight. Your house no longer breathes as well. Warm moist air rises. It usually, easily goes right out of the vents in your attic. So does your conditioned air. Anyways, this warm moist air will be traps in your attic. Not good. Now you must have a return duct put in your attic or another way to get the moist air out of yor air and supply it with conditioned air. This can be done by your HVAC guy.
Since I am building a very tight home, I plan on installing an ERV. This introduces fresh are into the HVAC system. The air is conditioned and circulated throughout the home via your supply ducts. Then some air is taken from the home and exspelled via the returns.
Sorry to get in the HVAC system but I have yet to figure out why insulation and HVAC systems are not treated as ONE SYSTEM.
Anyways, "build it tight and ventilate it right." Do it right or you will end up with moisture problems that will do more harm than good.
William Rose (from the Unv of Ill) has done a lot of research on hot attics along with some research group our of Fl.
Also, if you have not put the roof sheathing on your new home yet, try a sheathing that has aluminum foil one one side. This will help keep your attic cooler. The foil goes on the inside of the attic and is only about $2 more a sheet.
No you do not need an ERV for a hot roof design.
You need an ERV for the living space of house that is tightly built. You have have tightly built house with or with out a hot roof. You can have leaky houses with or with a hot roof system.
A hot roof system can be an easier way to get a tight house specially when you have HVAC equipment or ducts in the attic or where you have can lights in the ceiling of the top floor.
With a tight house you can control the amount of ventaliation and you can recover some the heat that is lost during ventalation.
With a leaky house you don't have any control and you lose all of the heat that goes out the leaks.