My 1890 home has severe ice dams every year which I understand are caused by poor attic insulation, ventilation and air leaks from the floor below. I have already installed soffit (sp?) vents & ridge vents and I'm ready to start on insulating, but I face one really big problem. There are 4 large roof valleys (the house is shaped like a cross), and in these areas the are no vents and the rafters are such that they block air circulation. Ice dams are most severe here. So I need some ideas: is it best to air-seal and insulate the floor, insulate directly on the roof, or insulate the floor and also the valley areas directly leaving the rest of the roof alone? I have already identified many air leaks including the stairway up, but probably not all yet. I haven't found much on-line about roof valley problems in older homes. I also though about putting in fans but eventually we want to put 2 rooms in the attic so insulating has to be compatible with this.
Ice dams are caused by heat from your home melting the snow on the roof.
Where you chose to place your insulation is your choice.
You may choose to fix insulation between the rafters and below the rafters, this is a difficult task and it will require a lot of insulation. But if you do intend to have rooms in the attic, this is the only way to go. You must have insulation below the rafters, followed by a water vapor proof plastic sheet, finished with drywall. Are the attic joists strong enough to make the floor of a bedroom? HAVE YOU ENOUGH HEAD ROOM?
Adding steel beams etc will make this a very expensive project.
The alternative, insulate between the joists and below or above the joists, this uses less insulation.
From the points of comfort and cost, keeping the insulation as close to the present comfort zone as possible, not only uses less insulation it also saves money on heating.
You fully understand that a lot of the heat that escapes from your home is due to the stack effect and the passing wind that pulls the heat through your roof.
Ensuring that all holes into the roof are blocked is a priority, then insulate the access to the roof and make sure the access is water vapor tight.
A desirable feature is sheets of polystyrene fitted below the joists with a water vapor proof plastic sheet fitted below the polystyrene followed by drywall to finish.
This will solve you problem, but it is messy and takes time, it also requires an empty room below to work in.
If you do get round to adding rooms in the attic, then this will stop them from overheating.