How various rafter can you overrun from a 40 square foot roll of insulation? ...

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Answers:    You need to multiply the rafter length by 2 (for 24" OC) or 1.33 (for 16" OC). Divide this into 40 and to be exact how many rafter you can cover if you assume no waste. In truth a waste factor of just about 5% is what one usually sees. This can be far difficult if the roll of insulation does not divide evenly or match you rafter distance.
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Its just math...first are the rafter 24 or 16 " on center? for instance if thy are 24 then you can crowd 20 ' of space with 40 sf.