Spray Foam Attic Floor or Rafters: Which is Best to Insulate?
The first big pro to insulating the attic floor over the rafters is that when you measure out the square footage, you're going to have less almost every time when you're looking at the attic floor versus the underside of the roof. Your cost is directly related to how much square footage is being foamed. So obviously the attic floor is going to be the most cost-efficient option, especially if you have a steep attic or a complex roof design with multiple peaks.
The biggest thing to keep in mind is if you're insulating the attic floor then you're going to need ventilation. Now, is that a pro or con? Sometimes that's up for debate, but keep in mind you will need ventilation usually in the form of soffit vents, baffles, and a ridge vent up top. It is also necessary to point out that insulating the attic floor cuts off the attic from the rest of the home, making it an unconditioned space that can experience vast temperature swings depending on the weather.
We do not recommend this method primarily because most homes have HVAC ductwork and sometimes the HVAC system is in the attic. Since the attic is not air-tight and allows air and moisture in, the fluctuations in temperature work against the HVAC system.
Spray Foam Attic Rafters
Insulating the attic rafters with spray foam creates a conditioned space where your attic is the same temperature as the rest of your home. This keeps the external weather and moisture outside of the attic space so you don't have to worry about any moisture problems or air infiltration.
You also do not need roof vents when you insulate the attic rafters. Is that a pro or con? If you ask three contractors, you'll probably get four answers.
Generally, our rule of thumb is to seal everything up as tight as possible and rely on your mechanical ventilation to give your house the air exchanges that it needs. So when you insulate the attic rafters no soffits, baffles, or ridge vents are needed and need to be sprayed over to create the air seal.
Another major difference is that if you insulate the attic rafters your floor is free. You may not have actual flooring on your attic floor, maybe it's just open trusses. This method creates the opportunity for you to lay a floor down and use that space for storage or potentially a living space if it's large enough.
Finally, if there are appliances in the attic such as an HVAC system or even just central air, which is common in southern climates, you almost always want to insulate the attic rafters in this situation. This is because you want those appliances inside the building envelope to help them run more efficiently.