A lot of people, including many insulation contractors, will tell you that you can’t add insulation to a closed mobile-home roof cavity. I can tell you from personal experience that they’re wrong. The best ways to access the roof cavity are either through a hole in the rooftop or by disconnecting the long edge of the roof where it fastens to the wall. Insulating the roof cavity will typically save between 10 and 15 percent of heating and cooling costs and cost around a dollar a square foot, assuming you can find an insulator who will do it..
The insulator should use fiberglass insulation blown with an insulation-blowing machine. Cellulose insulation is too heavy and can corrode the metal roof from underneath. Getting thorough insulation coverage is the most critical part of mobile home roof insulation, and it requires inserting a hose or pipe into the roof cavity. This hose or pipe is attached to the blower hose, which is connected to the blower. Its purpose is to ensure that every cubic foot of roof cavity gets filled with insulation. See Insulation Comparison: Fiberglass and Cellulose for more information.
Cutting a hole in the top requires careful patching, and I always install two patches – the first overlaps the hole and the second patch overlaps the first patch. The first patch is stiff galvanized sheet metal, sealed to the metal roof with roof cement and fastened. The second patch is foil-faced butyl roof patch – one variety goes by the name of “Peel and Seal.” Disconnecting the long edge of a metal roof also involves careful refastening with staples and screws and careful resealing with putty tape or caulking.
Many weatherization agencies working under the Department of Energy’s Weatherization Assistance Program have used these methods for years with great success. You can learn more about the details of these roof insulation techniques in my book, Your Mobile Home: Energy and Repair Guide for Manufactured Housing.