Another source of ice in the attic is caused by attic bypasses. This is where hot air is allowed to escape the conditioned air envelope of your heated space. The large amount of air cannot be dipersed by the attic venting. Examples of attic bypasses include gaps in the vapor barrier around electricle boxes for ceiling fixtures, gaps around plumbing vents and soffits over cabinets or showers. I had a huge bypass over the soffit in a bathroom shower. The dumbazzes that sheetrocked did not put any rock over the top of the soffit. Then the dumbazzes that insulated merely stretched vapor barrier over the top of the soffit and blew in insulation. Staples and plastic did not support the weight of the insulation. I had an impressive pillar of ice that stretched from bathroom ceiling to the roof. Also, make sure your exhaust fans are vented through the roof and the piping is air tight.
Yes, this is the most current building science, in my opinion. I'd reccommend stapling 6mil plastic sheeting to all ceilings of new construction, then nailing 1x4's over the sheeting to have something to screw the drywall to. Nail the ceiling boxes to horizontal 2x4 blocking and seal the hole for the wire with expanding foam. This helps minimize the "stack effect" in a home.
