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What is an ice dam? Why do icicles form? How To Prevent Ice Dams in 3 Impactful Ways

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Winter 2021 left us with unprecedented icicles and ice dams, including property owners who never had ice dams before. Why? Simply put, an icicle is a MEASURING STICK OF HEAT LOSS FROM THE LIVING SPACE. We can prevent ice dams from forming and prevent ice dam removal. Ice dams form when snow melts on the top, warmer part of a roof, water flows down to the colder eave overhang, where it refreezes. As the ice accumulates, it forms a blockage that prevents additional snowmelt from flowing off the roof.

The ice backs up under the roof shingles, where it melts again, soaking the roof sheathing and leaking into the attic. It can leak through the ceiling drywall below and into your living space. In addition to the interior water damage caused, large ice dams can be very heavy and can damage gutters and even present a safety hazard to people below.

what causes ice dams and how to prevent ice dams with Green Attic Insulation
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Ice dams form when the space inside an attic along the underside of the roof deck is above the freezing point. Notice this is the UNDERSIDE.  As the warm air beneath the roof heats the shingles and melts the snow on the roof, the water flows down the roofline until it reaches the overhanging eave structure and refreezes. Ice dams usually start or worsen after a heavy snow because of snow’s insulating properties. The snow layer traps warm air beneath the snow, which causes it to melt.  

 Ice dam prevention  is a matter of thermal physics ( Warm Air Rises) . If the air in the attic or against the bottom of the roof deck remains cold, it can will not melt the snow lying on top of the roof, eliminating the water necessary for ice dams.

Ice Dam Prevention can be done through a combination of: 


  • 1. Ventilation under the roof sheating, keeping colder outside air circulating through the attic and preventing it from warming above the freezing point so it can melt snow on the roof.
  • 2. Insulation in the ceiling below the attic, which will prevent warm air from rising up into the attic space to melt snow on the roof. Green Attic installs R-60 on traditional attic floors, optimal summer and winter thermal diffusion ( preventing heat escape in winter and preventing heat intrusion in summer.)
  • 3. Air Sealing heat sources such plumbing, hvac, electrical penetrations through the attic floor pressure barrier.
prevent ice dams ventilation

Air Sealing air highways from the living space below the attic is just as important as the insulation itself. Gaps around plumbing pipes and around chimneys can be a significant source of heat flow into the attic from the spaces below. This includes the bypass ( the gap from between the trim and quarter-round shoe supplying the bypass ( The gap between the drywall and top plate).

The benefit of a comprehensive insulation effort is that it not only helps prevent ice dams, but it also reduces energy costs. But be aware that insulation alone rarely is enough to prevent all ice dams. It needs to be done in conjunction with improved ventilation.

Note : Keep gutters clean.  Get rid of all those fall leaves before the snow comes. Also, make sure your downspouts are functioning properly. Melted snow has nowhere to go if the gutters are clogged.

Note: Insulation will only stop ice dams caused by heat loss from the attic; it will not stop icicles due to natural melting conditions relevant to the outside melting and refreezing temperature in a short period of time. There is a difference between icicles and ice dams. Each home has unique characteristics built by different builders using different materials and methods. Results may vary.

Insulation Institute/ Why Insulate/ Preventing Ice Dams

Green Attic Insulation is a family owned and operated local business with 11 years of satisfied clients. We focus on residential retrofitting of existing homes, with the goal of making homes more comfortable in the summer and winter, while lowering monthly heating and cooling costs. Give us a call today at 847-777-4777 or fill out our contact us form on the main page of

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How To Prevent Ice Dams in 3 Impactful Ways

An Icicle is simply a measuring stick of heat loss from the attic. Ice dams form when snow melts on the top, warmer part of a roof, water flows down to the colder eave overhang, where it refreezes. As the ice accumulates, it forms a blockage that prevents additional snowmelt from flowing off the roof. It can be prevented by adding insulation, proper air sealing and ventilation of the attic.

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