Here in Minnesota we get a lot of snow. This year my roof vents are covered with snow and there is no ventilation in the attic (I have a hip roof). I am thinking of getting a couple of power vents to prevent this from happening again. I need to control the vents manually so I can turn them on when we get a lot of snow. Also, should I replace all the vents (about 7) are just a few? And should I replace all the soffits to enable better air flow with the power vents? The ones I have do not have too many vents in them. Does anyone have a formula for determining the area of the vent intake (and outtake) vs the size for the attic space (or roof area)?