Insulating a Flat Roof
Flat roofs have been a daunting task for contractors and homeowners alike since they were invented. They are hard to waterproof, hard to maintain, and hard to insulate. For today's purposes, we will stick with the insulation issue.
In order to properly insulate a flat roof area, you need to be very carefull on the material and the methods used. I will break these into sections for organizational purposes:
Insulation Between Ceiling Rafters
There are only 2 types of insulation that should be used for this method. Spray foam (either closed or open cell) insulation or dense pack cellulose insulation. The reason is because it is very difficult to properly ventilate a flat space because air does not generally move in a straight line. This invites moisture into an area that if trapped, can do a lot of damage and is hard to get rid of. It is much easier and more efficient to have a non-ventilated area and fill the entire cavity full with dense pack cellulose, or seal the area with 4 to 5 inches of spray foam.
One consideration is rafter height. If you only have 2x6 rafters existing, you will only be able to use closed cell spray foam insulation to insulate with because of the R-value restrictions (closed cell foam is R-6 per inch, open cell foam is R-3.5 per inch, and cellulose is R-3.8 per inch). If you have at least 2x8's for rafters, then you can use dense pack cellulose or open cell spray foam.
The closed cell spray foam option costs about $4/square foot. Open cell foam costs about $2.75/square foot. Cellulose insulation costs about $1/square foot. FIBERGLASS INSULATION SHOULD NEVER BE USED IN AN AREA LIKE THIS.
Insulating On The Exterior Roof Surface
This is probably the most popular method for commercial roofs, but can be utilized for residential flat roofs also. This is done by applying a strong adhesive to the foam board, laying it down on the roof surface, and securing with washer head nails. Then the rubber waterproof membrane is adhered to the surface of the foam board. Two or more layers of foam board are normally used in this application to achieve the needed R-value. Again, no ventilation needs to be present. In this application, the ceiling joist spaces may remain empty. Always remember that if this method it used, the rim joists still need to be insulated to keep the insulation continuous from roof to wall to avoid moisture and air infiltration issues.
Insulating The Ceiling
This method is not recommended because the ceiling height would have to drop at least 4 inches to meet code requirements. It also makes future addition of lighting or hardware very difficult.
As you can see, there are limited options available that are truly correct ways to insulate and air seal without having moisture or air infiltration issues. This is why flat ceilings are not a popular construction method for residential use. However, if the proper insulation methods and materials are used, you can have a wonderfully insulated roof.
If you are unsure of what energy improvements to make, it is always a good idea to hire an energy consultant. They will walk through your building with you and show you the areas that can benefit from energy efficiency improvements.