Ice dams

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Ice Dams are caused by hot air entering the attic from living space below which melts snow/ice on a roof. The meltwater then flows toward the colder eaves where it refreezes.

The two key elements in preventing ice dams becomes to keep heat from entering the attic in the first place and to maximize ventilation to allow heat to escape and keep the attic cool.

It seems you have solved the ventilation portion of the equation by installing proper vent SO LONG AS your entire roof line also has a RIDGE VENT to allow hot air to escape. Since you haven't mentioned a ridge vent above the 'attic' potion of the 2nd floor, I need to point that out.

But as for controlling heat transfer into the attic from the living space from the living space....this is usually controlled with insulation.

And as a side note you DO NOT NEED any type of plastic 'vapor barrier' anywhere in conjunction with this resolving this problem...so just forget it altogether.

The key in insulating a gambrel, like a cape cod roof...is to know how and where to place insulation.

In your case, your living space is essentially a square box inside the Ganbrel second story.

This also means that you have "3" attics:

1 above the ceiling and 1 on either side of your side walls.

Insulation should be installed from eave-to-ridge in the following manner:

-R-30 or higher in the FLOOR of the side attic.
-R-11 or R-19 or higher in the side "kneewalls".
-R-19 or R-30 or higher in any sloping roof sections above knee walls....
-R-30 or higher in the small attic above the ceiling.
-R-11 or R-19 between stud spaces in gable end walls

While this proper insulation method will eliminate MOST heat loss into an attic space, it does not account for ALL heat loss.

A very significant amount of heat loss will STILL occur into a attic from any chimneys, chases, pipes, vents, electrical cables, electrical outlets, wires, ducts and everything else that is installed bewteen the living space and "attics".

You need to seal all these areas with expandable foam insulation or caulk to insure no warm air is escaping into the attic.

Insulating without also sealing an attic from air leaks below can be essentially useless in combatting ice dams.

You can also install de-icing cables above special problem areas such as your windows.
Adding dormers may do little to correct the problem if all the other factors are not also addressed and will be a waste of money if addressing this common correction methods help.

In addition to using a snow-rake to remove snow as it falls, there is little else to do.

If you need additional help, write me:

homebild@praize.com

I work New Hampshire on occaision.