What Causes Ice Dams - And How to Prevent Them
In a worst case scenario, an ice dam may breech the underlayment and warp or decay sheathing, cause excessive moisture buildup in the attic, degrade insulation performance, cause interior stains then leaks, and over time cause structural damage to trusses, ceilings and walls.
The causes of ice dams can vary depending on who you may talk to. Many homeowners point to frozen or clogged gutters, dark shingles that absorb heat or low roof pitch. All these explanations might relate to roofing problems, yet they probably don't cause the vast majority of ice dam problems.
Heat loss from the house to the attic tends to be the primary culprit of ice dams. However, ice dams can be tied to two factors in the attic: heat buildup and inadequate ventilation.
Escaping heat from interior rooms enters the attic and forms warm spots on the underside of the roof. These spots warm the outside surface of the roof, melt snow/ice on the outside and cause the ice dam cycle to begin.
Preventing Ice Dams
Sealing bypasses (warm air leaks) from interior rooms to the attic is probably the most effective way to prevent ice dams. Bypasses are typically found around vent pipes, exhaust fans, chimneys, attic hatches, wall/ceiling framing joints, wiring and lighting fixtures. Use silicone caulk, poly plastic as fill material, foam sealant or a combination of all three to stop air flow around these areas.
Having a well insulated attic is an excellent way to increase energy efficiency and may slow down heat loss, but ice dams may still persist because warm air may bypass through insulation.
Not exactly an "ice dam," plugged or overweighted gutters can cause ice to build up and back up onto the roof. This condition can be remedied with electric heating cables. Once the gutter thaws, it should be refastened to slope correctly and should be cleaned out every fall.
A properly ventilated attic prevents moisture buildup by allowing air flow to carry out moisture. Although heat bypasses are the primary cause of ice dams, ventilation helps deter ice dams by keeping the inside surface of the roof cool and dry.
Signs of inadequate ventilation include rusty nails or rust spots on insulation caused by moisture forming and dripping off nails, frost buildup during the winter, or possibly a lingering musty smell.
As a general guideline, half the vents should be at least 3' vertically higher than the draw vents and positioned as close to the eave as possible. A vent to attic space ratio of 1:300 means 1 square foot of attic ventilation for every 300 square feet of space. Vents closer than 3' and no vapor barrier in the attic may require a 1:150 ratio.
Passive venting refers to the unassisted, natural movement of air into and out of the attic. Using mechanical means like fans may be unreliable, waste energy, and at times draw in moisture and warm air.
Typically, passive venting is done by installing vents at the soffits that draw cool air in and vents near or at the ridge that allow warm, moist air to escape. Soffit vents should always be unblocked and insulation should be held back with baffles to permit unrestricted air flow.