Depending on the weather and usage of the roof during its lifespan the insulation board may still be in a good enough condition to re use when the time arrives to replace the roof itself.
Insulating a flat roof without ventilation.
The answer depends on the type of roof you have.
Ventilation is necessary to allow moisture accumulated inside to escape.
A cold roof or hot roof.
This misconception is so strong that some people actually choose to not get flat roofs because they are worried about the roofs not being properly insulated.
Since you currently do not have ventilation and it sounds like you have access to the attic the most cost effective insulation technique you can use is to spray closed cell foam using spray foam kits around the perimeter of the roof between the roof surface and the ceiling to a nominal depth of at least 2 inches to prevent air infiltration.
Proper ventilation in flat roofs.
The old roof was tar and gravel so the switch to tpo should show significant change.
To produce a hot roof rigid insulation is fitted outside above the weatherproofing.
Unvented flat roofs a technical discussion flat roofs have often been built with the insulation all on top of the vapor closed drainage plane as this avoids dew point condensation issues inboard of the roofing.
Hot roofs have the roof deck then insulation then the roofing material.
It depends on where the insulation is installed.
Should i leave out the soffit vent.
Any type of roof from tpo to epdm can be either hot or cold.
Also there is a soffit vent but none at roof peak.
I have an insulation factor of about 8 10 plus the reflectivity of the white tpo.
This is then covered with a further weatherproofing layer.
Cold roof systems are highly inadvisable as insulation from the inside will inevitably lead to the formation of condensation.
It also lets hot air escape in the summer.
However in many situations the insulation may need to be primarily or entirely below the flat roof membrane and between the roof joists if so what are the best options.
Is there any advantage to trying to get any insulation in between the 2 8 rafters.
The material that works best depends largely upon preference in a given situation.
Like other roofs insulating a flat one requires ample ventilation a vapor barrier and sealed bypasses.
Cold roof a very bad idea.
How much ventilation does a flat roof need.
Insulating a flat roof requires ventilation above the insulation.