Causes of Ice Dams
Temperatures warmer than 22 F will generally not cool roof sheathing enough to cause melted snow to refreeze. Also, when outside temperatures are significantly colder than 22 F, ice dams are less likely to occur because the roof sheathing usually is below freezing in all areas. It’s important to note ambient attic air temperatures and roof sheathing temperatures will differ. For the sake of analyzing ice dam formations, focusing on the roof sheathing temperatures is more important. Infrared cameras and thermometers are excellent tools to measure roof sheathing surface temperatures.
In the absence of an infrared camera, analyzing snow-thaw patterns can be a great visual of roof sheathing temperature differentials. Photo 1 depicts an unheated detached garage with snow on the roof and an inefficiently heated house. Only the snow on the heated house has melted because the roof sheathing temperature is obviously above freezing while the garage sheathing temperature is close to the ambient outdoor air temperature. Refreezing will not occur and cause an ice dam on any section of an unheated structure mainly because the roof sheathing temperatures will be consistent.
In the past, industry professionals have identified several causes for ice dam formations, including solar radiation, poor ventilation, inadequate insulation, air leakage and mechanical equipment in attic spaces. In reality, some of these items are contributing causes while others are essentially nonfactors. To address ice dam root causes, instead of merely trying to repair a leak from a roof’s surface, let’s analyze these items individually to determine their effects on ice dams.
Adequately ventilating an attic space does not guarantee roof sheathing temperatures will remain consistent. Ventilation will reduce ambient attic air temperatures but is less effective at reducing roof sheathing temperatures. According to Building Science Corp., Westford, Mass., proper passive ventilation systems can reduce the temperature of the roof sheathing between a few degrees to no more than 10 degrees than a poorly ventilated attic space. The color of the roofing materials and the sun’s orientation have greater effects on the roof sheathing temperature than the amount of passive ventilation. Thermal inefficiencies, including heat sources, in attic spaces are the only root causes for ice dams and when eliminated, they will prevent them from forming.
Power vents can protect against, and in some situations eliminate, ice dam formations but are not efficient for eliminating their root causes. Mechanical ventilation, typically far in excess of minimum building code requirements, can uniformly cool roof sheathing temperatures to a level below freezing and eliminate the risk of ice dam formations. However, a more efficient approach is to eliminate the heat source in the attic, eliminating the need for mechanical ventilation.
Inadequate levels of insulation allow heat to escape from conditioned living spaces and warm the roof sheathing. For example, an ice dam that has a history of occurring on a house in Climate Zone 5 with an attic insulation level of R-19 can be prevented by increasing the level of insulation to the DOE’s recommendation of R-49. This improvement to the thermal boundary will prevent heat loss and ensure the roof sheathing temperature is below freezing when it is covered in snow. Low insulation levels are relatively easy to fix, but ice dam remediation often is not this simple.
Ductwork is a source of heat within an attic space and any air leakage that occurs from ductwork located above the thermal boundary can cause ice dams. Even ductwork installed efficiently typically is insulated to no more than R-6, radiating heat up to 50 F into an attic space. If leaking, it can exhaust air into an attic at temperatures as high as 80 F. Although not always possible, ductwork located within the thermal building envelope reduces the risk of ice dam formation and increases the thermal performance of mechanical equipment. When it is determined a source of air leakage or improperly installed ductwork is the root cause of an ice dam formation, the scope of work to remedy the problem may be outside the norm for a typical roofing contractor.
Mechanical contractors, design professionals and property owners often are unaware of the burden redirected to roofing contractors when these inefficient systems are installed. If the decisions about mechanical equipment locations were based solely upon roofing considerations, it would be more desirable to place ductwork within the thermal envelope instead of above it.
Most roofing manufacturers and building codes in northern climates require ice and water underlayment to project 24 inches inside the interior walls at the eaves. Depending on the locations of the furnaces and the complexity of attic spaces, ice dams can form well inside the 24-inch area at the eaves that is protected by ice and water underlayment. When a leak occurs from this furnace configuration type, it is no easy task for contractors to explain the cause to property owners nor is it an easy task to correct the problem. HVAC professionals are not likely to provide solutions, and roofing contractors have limited prevention or repair options.
Ventilation codes were drafted in the 1940s, long before the widespread practice of installing furnaces in attic spaces. When a vapor retarder is not present, many building codes and shingle manufacturers require the 1:150 ventilation ratio of 1 square foot of net free vent area per 150 square feet of attic floor space. Unfortunately, when ductwork and mechanical equipment are located within the attic space, ventilation codes are insufficient. In reality, natural ventilation will not cool the roof sheathing near the furnace more than a few degrees Fahrenheit. Mechanical roof ventilation could provide a solution but often is impractical because it would require industrial-type ventilators to be effective.
Other design criteria not addressed can increase the severity of ice dam formations. These items include low roof slopes, poor drainage, large tributary areas, length of eaves, gutter sizes and lengths, and downspout placement.
To further complicate the matter, the architectural community and HVAC contractors are relatively unaware of the problems associated with their design and installation practices, leaving roofing contractors to correct the associated problems. These problems are costly to correct, outside the scope of typical roofing operations and can be difficult to pinpoint. The relocation of mechanical equipment usually is not an option either.
Ice dams can be prevented when attic insulation is properly installed, air leakage does not exist, and all sources of heat are located within the thermal boundary. Inspecting an attic space for heat sources and inefficiencies before new roof system installations can help predict the likelihood of an ice dam formation.
- Attic Insulation can be added to minimize the amount of heat that escapes into the attic. Bathroom vents, electrical outlets, plumbing vents, furnace stacks, and access doors can enable significant amounts of heat to escape a house's living quarters. The temperature of the roof sheathing in an ideal attic space should be close to the same temperature as the outside air. (The ambient attic air temperatures will generally be higher than outside air temperatures).
- Mechanical Ventilation cools the roof sheathing and attic space during winter months and can prevents the roof deck from warming. A freeze-thaw process isn't likely to occur on a roof deck that is the same temperature as the outside air. Passive ventilation, or non-mechanical roof vents allow moisture to escape the attic space in the winter but are ineffective in preventing ice dams. They usually cools the roof sheathing no more than a few degrees in the winter.
- Underlayments, including Owens Corning WeatherLock ice and water guard, adhere directly to the roof deck, creating a water tight barrier between the roof deck and the shingles. The ice and water shield is installed around the perimeter where the overhangs exist and at roof penetrations. The use of ice and water guard will protect the roof deck from ice dams but proper attic insulation and/or ventilation will prevent an ice dam from occurring.
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