Gene Ash snip - basically heat and moisture from the bottom side of the enclosure rises and then forms ice on the inside of the structure due to the large differential in temperature between the inside and the outside of the house

Gene, thanks for joining the conversation. Most of the time I feel unqualified to contribute but when topics relating to winter and weather and effects on maintaining houses, bee or otherwise, I get stimulated.

Your definition of an ice dam is not what would be accepted in Ontario. You are describing frost build up on the underside of the roof due to lack of ventilation in the attic and or vapour barrier in the ceiling. A real problem but different from an ice dam.

An ice dam describes a build up of ice on the outside of the roof. It occurs under a layer of snow. The roof itself gets heated by the transfer of heat from inside the house resulting in snow melt followed by freezing. The freeze thaw cycle repeats and ice builds up. The ice forms a dam which holds back melted water from higher up the roof. If a significant melt occurs the water is forced back under the roofing material and the roof leaks.

It is interesting to note that the solution to both ice dams and frost build up on the inside is - are you ready - ventilation. A properly operating roofing system in Ontario includes a ventilated air space between the roof and the ceiling (a roof is not a ceiling). The ceiling has a waterproof membrane on the heated side and insulation on the cold side. This reduces moisture in the cold space. The ventilation ensures the roof stays cold.

I speak from experience. My roof will ice dam every year around a skylight. The skylight is close to the lower edge of the roof over a bay window. The attic space between the skylight and exterior wall and roof was filled with insulation ceiling to roof. The house heats the ceiling, which heats the insulation, which heats the roof, which melts the snow, which causes the ice to dam. I run electric heating cord on the roof in late winter when temperatures are likely to cause a major melt. 

Regards Peter

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