I have a full walk through unheated attic with 3 1/2 " of insulation in the rafters.
I intend to remove the insulation and install approx. R-30 in the floor and cover with
Plywood to use as storage. In the future I will be heating the space and converting it
Into a bedroom.
My question is can I then insulate the rafters and what about the floor insulation MannyR

As long as you leave breathable space in the rafters,the roofline must have airflow.

A product sold at Menards or Home Depot is Styrofoam,conforms to the rafter allowing the insulation to rest against it,but not the roof decking.

The roof would have condensation with insulation against it,superheated in the summer,and the condensation would begin the rotting of the decking and eventually the rafters.(to say nothing of the shingles)

Make sure the eaves have open vents and allow the air to pass uninhibited to the peaks.

Hope this answers your question.

KBC

How is the headspace in the attic. Many places will not allow you to have bedrooms in attics. Another thing that you need to check on is the availability of an egress window. Meaning if there is a fire can someone in the attic get out?

Aaron

You may have to fur down the rafters to allow enough space for r-30 abot 10" will do

This is a complicated issue and you don't really provide enough information for a full answer.
I would disregard much of the previous answers Manny.

First, how are you going to be able to get R-30 into a typical joist channel? You would need to have 2x12 ceiling joists with most insulation materials to achieve that. You want to make sure that you don't over-compress the fiberglass or cellulose insulation in an attempt to reach R-30 since you would be squeezing out the air spaces that make insulation work.

The possible concern you have with redundant insulation (ceiling and rafters) is that of creating a condensing surface. I don't think that would be an issue in your case as long as you followed good practices in the installation of both insulation materials.

If you use insulation with a vapor retarder than the vapor retarder should face the living space in a heating climate and should probably face the exterior in a humid cooling climate. Unless you live in a rather extreme heating or cooling climate you may want to avoid using a vapor barrier.

If you post some photos I will try to be more specific. As a previous replier mentioned you may (but not always) need to keep an air space at the underside of the roof sheathing in the rafter channel. Unvented attics are now allowed by code and can work quite well. Medium or high density spray foam insulation applied to the rafter channels is usually the way to make an unvented attic work without creating a condensing surface.

Certainteed has a terrific free tutorial on their website that will walk you through correct insulating and ventilating of attic spaces. Try it!