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Insulation may not be the only thing you need. It is also important to have air flow through the attic. this is best accomplished by eave or soffit venting and ridgevent.
According to most building codes, you need one square foot of vent area for each 150 square feet of attic floor space. The minimum is one square foot for every 300 square feet of attic floor space if there is a vapor retarder or the space is balanced between the ridge and intake vents. A balanced ventilation system means about 50 percent of the required ventilating area should be provided by exhaust vents in the upper portion of your attic with the remaining 50 percent provided by intake vents.
Spray foam insulation is by far the best for an attic. It is sprayed in the rafters areas and basically seals off the entire attic and stops the heat as soon as it enters from the roof. It can be very expensive though compared to regular fiberglass insulation or blow in insulation.
Insulation is insulation regardless of type. Whatever the R-Value per inch of the specific material is, times the number of inches, will equal the total R-Value. We are looking for R-49 to be Energy Star.
If the roof is not vented or we are talking about a hot roof deck, that is another consideration.
The floors in your home, while insulation will certainly mitigate this delta, will be different temperatures as the laws of thermodynamics are universal.
Get the insulation in the attic to R-49, seal the ductwork, seal the top plates, etc. and you will be good.
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