If u have combustible gas furnace, h20 heater in attic n u make it a sealed system by spraying foam on rafters make sure u know what u are doing for make up air damper and have someone engineer it . I suggest not doing the rafters but doing the ceiling joists and walls or do walls with rock wool and ceiling joist with foam if u wanna get close results for 1/2 the price. I build high end homes for a living and that is how I'm doing my personal home I'm currently building. Walls rockwool, between floors 12" batts, ceiling / attic floor open cell foam . Also if you foam rafters is a pain in the butt if u ever have roof leak
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