attic insulation question right or wrong and why?

a friend who happens to do home improvements recommends installing insulation on the underside of the roof. From within the attic of course. I haven't been able to find a thing that suggests doin so or not. All I've been able to assess is that by doing so may risk rotting the wood perhaps of the roof. the insulation has vapor barrier paper or batting with paper on the bottom towards living space.
His thinking is that keep the heat from entering the attic. Id have to aggree, but why is it that I don't see this practiced in any other houses. I know that installing the insulation on the underside of the roof is an incredibly tedious and difficult thing ? anyone have comments
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on 9/5/2007 3:27 PM cmply said the following:

The common thought was to insulate the floor of the attic, or the rafters of the attic, but not both. This was the theory before Central AC. Now, that central AC is a common feature, the theory is to have a channel between the soffit vents and a ridge vent, which is a passive vent, and insulate the rafters, including that passive vent. Do you have a Central AC in the home and it is mounted in the attic? Then you need both. One to keep the cold from the living area ceiling in the winter, and one to keep the hot air from the AC unit in the summer. Others may disagree, but the dual insulation makes sense.
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Bill
In Hamptonburgh, NY
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If insulation is installed on the underside of the roof then that suggests a living space or storage area in the attic. My parents did that so they could use the attic space to protect apples from freezing. It is not a difficult job if the temperature is cool enough to work.
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Look at The Building Science Corporation web site. This is run by building science reaserchers. T
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In addition to the comments made I will add that in general it is better to insulate the floor and vent the attic to help keep the roof cooler. If you insulate right under the roof, the roof will get hotter and the extra heat will shorten its life. Also the roof is a larger area so for the same money you can insulate the floor to a higher R value and have a smaller exposed area of transfer.
With HVAC equipment in the attic, I suggest it is best to spend your insulation budget on insulating the equipment and air handling equipment rather than the whole attic.
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Joseph Meehan

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Normal practice in an attic not used as living space is to insulate the attic floor, between the joists to insulate the living space from the unconditioned attic space.
If you want additional help in keeping heat out, the appropriate technique for the underside of the roof sheathing is a radiant barrier. Perhaps that is what your friend was talking about. But this is a foil type product, not traditonal insulation.

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