Originally Posted by Perry525

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At first glance, this is a good idea.
However, you must take water vapor into account!
As you probably know, water vapor is created in the home by cooking, washing, breathing, sweating.
Water vapor once created is held in the air until it meets a cold surface, or if outside meets a patch of colder air, when it turns to rain, etc.
Indoors it heads for the nearest cold surface, which often means straight for the attic where it condenses on the underneath of the roof, possibly leading to mold and wood rot.
If you are going to insulate the roof, you must use at least 3 inch thick polystyrene, or another closed cell insulation, and you must ensure there are no holes or cracks in the polystyrene where the water vapor can gain access to the roof.
Fiberglass and cellulose and other open cell, so called insulation's should not be used as water vapor moves through these with ease and it is impossible to make an air tight surface.
After you have filled the spaces between the rafters you should then add another layer of polystyrene below the rafters to stop the water vapor from condensing on them.