We have had some leaking this past winter due to ice dams. After we get everything dried out, we would like to put up some insulation to help prevent this in the future. Is insulating the ceiling enough or do we need to insulate along the actual beams of the house (we have easy access to a crawl space along part of the roof but not for an addition to part of the house which was where the main problem was)?
Ice damns are caused by heat rising to the underside of the roof, heated it and melting the snow, where it refreezes as ice.
You need to make sure no hot air is leaving your house - insulate thoroughly.
Airflow is also crucial - make sure you leave baffles to provide airflow from the eaves to every part of the attic. If you have a "one and a half" or similar you need to provide that channel of air between the corner (of roof) crawl spaces and the proper attic.
A well ventilated attic, with no heat bleeding into it, will not ice dam!
Just to point out, if your roof is un-penetrable by water, for example if it's an epdm rubber roof, water can pool against the roof behind ice dams and will not enter the house.
Ice dams are nearly impossible to prevent in certain weather. Here in Massachusetts, I'm hard pressed to find a house without some ice dams, even brand new houses.
Clearing the snow from roof and insulating the attic are the surest way to prevent ice dams.
The difference between those that leak and those that don't is often whether the water that inevitably builds up behind the dam can find a way into the house.
I'm not saying that reducing ice dams by making sure warm air doesn't filter up against the roof isn't a good thing to do, but just that the roofing materials and roofing job are equally if not more important.
Ok, since you know that thermal insulation is crucial in order to avoid ice damps I will make the long story short: two places are crucial when it comes to insulating roofs. Doesn’t just put thermal insulation horizontally, on the floor of your attic; put it between roof rafters and if temperatures are REALLY low you can put it even around rafters. I would suggest you to use rock wool. Because of condensation you can wrap it with PVC foil, or put foil as an extra layer under the wool. Also, eaves must be protected same way. In this case insulation should go over facade wall 10-20cm (4-8 inches). Wool should be 8-10 cm (3-4 inches) just to be on the safe side. One other thing I forgot to mention: everything that is going through your roof (ventilation etc.) should also be insulated. In this case you can wrap it with wool which could be thinner, because thick wool can't be folded.