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How Ice Dams Form, and How To Stop Them

February 06, 2017

Four ways to prevent ice dams in three types of roof, for two climates in one video. Boom.


Homes in snowy climates can sustain moisture damage from ice dams in the winter.

An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow from draining off the roof. Icicles are an indication that ice dams are forming.

Ice dams are caused by nonuniform temperatures on the surface of the roof. Snow melts over the warmer areas‚ perhaps where the insulation is thin, and heat is escaping from the home.

The melted snow runs down to the cold edge of the roof and freezes, forming icicles and ice dams. The ice dam on the edge of the roof can cause water from snow melt to back up and form pools on your roof under the snow.

Eventually this water can seep through your roof and cause damage to your insulation, ceilings, and walls.

Ice dam prevention begins at the ceiling

The best approach is to prevent heat from the interior of your home from escaping into the attic and warming areas of the roof deck.

To accomplish this, the first thing you need to do is to make sure the ceiling beneath the attic is air tight. Air seal every seam and hole in the ceiling from either the interior or the attic side.

Try to limit the number of holes you make in your ceiling. For example, you can install track light or ceiling-mounted fixtures instead of recessed can lighting to avoid cutting larger holes in your ceiling.

Another reason to avoid using recessed can lighting fixtures is that they can introduce heat directly into the attic. Especially if they are not airtight, insulation-contact rated, and covered with insulation.

If possible, do not install heat ducts and furnaces in the attic, particularly if you live in an area with significant snow accumulation. It is nearly impossible to air seal and insulate these systems well-enough to prevent heat loss from warming the roof deck, causing snow melt and ice dam formation.

Vented attic needs an air tight floor and smooth air flow

A well-sealed and properly insulated attic is essential to preventing ice dam formation. However, the type and level of insulation you use will depend on whether your attic is vented or unvented.

  • If your attic is vented, make sure the attic floor is well insulated, especially over the top plates.
  • If your home is not designed with raised heel trusses, as shown, you can spray with foam from under the baffle to the attic floor to get full insulation coverage.
  • Leave a minimum of a 2 inch space between the roof deck and the wind baffle to vent the underside of the roof deck. Install vent screens at every rafter bay to provide exterior air to flush away any heat that gets to the roof deck.

If your house has a compact roof assembly with an unvented attic, there are two options for insulating the attic to prevent the formation of ice dams. These options depend on the climate where you live and the weight of the typical snow load on your roof.

Unvented roof with moderate snow (less than 50 pounds/sq.ft.)

If you live in a moderately snowy area, with snow loads less than fifty pounds per square foot, you can choose to leave your attic unvented. However, it will be necessary for you to:

  • Install a fully-adhered air barrier membrane to the top of the roof sheathing that covers your fully-insulated attic.
  • Apply rigid foam insulation equivalent to R-50 on top of the air barrier membrane in two or more layers with horizontal and vertical joints staggered.
  • Install plywood or OSB sheathing over the rigid insulation as a nail base for finishing the roof, and screw down through the insulation into the rafters.

Unvented roof with heavy snow (greater than 50 pounds/sq.ft.)

If you live in a region with snow loads greater than fifty pounds per square foot, the snow itself creates an insulating blanket that can elevate the temperature of the roof deck above freezing, which can cause snow melt and ice dams.

To counteract the thermal effect of the heavier snow load, you will need to add ventilation, by constructing a vent over roof on top of your unvented compact roof.

  • As with the unvented roof procedure, you begin by applying an air barrier membrane to the top of the roof sheathing.
  • Cover with rigid foam insulation, but increase the layers to a minimum R-60, staggering the horizontal and vertical joints as before. Apply roof sheathing and roof membrane over the rigid foam insulation.
  • Leave an inch or so of space for venting between the roofing membrane and the last layer of roof sheathing before finishing the roof.
  • Be sure to vent the fascia to allow exterior air to flow under the over roof to keep it cool.

Upgrading your ceilings, attic, and roof to prevent ice dams will make your home safer and more durable. It also may pay for itself in avoided repair costs for water, mold, and structural damage.

—This video is from the Pacific Northwest National Lab through the Building America Solutions Center. We'd like to thank all of the scientists, builders, and manufacturers of Building America program for doing mind-numbing work sometimes to help us build the best houses we can.

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