Describing the ice dam process does not need to be complicated; in fact, the definition is quite simple. Ice dams occur when the roof sheathing may have temperature differences from one roof section to another. Historically, roof sheathing at the eaves typically is colder than sheathing located over heated spaces. Ice dams do not occur every time it snows, but they occur when a temperature differential of about 10 degrees Fahrenheit or more exists down the drainage plane of roof sheathing, enabling a thaw-freeze process to occur. Ice dams are most likely to occur when the outside ambient air temperature is 22 F or slightly colder.

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.

Although many people believe sunlight, a form of solar radiation, contributes to ice dam formations, it doesn’t. Proof by contradiction can be used to show why sunlight is not responsible for ice dam formations. Making the assumption solar radiation causes ice dams, you should expect that nearly every roof system exposed to sunlight would form an ice dam, including unheated buildings. You also would expect ice dams to not occur on cloudy days or at nighttime. Experience tells us this assumption is false because all roofs exposed to sunlight do not form ice dams, unheated buildings never form ice dams, and ice dams form and actually are more likely to form on cloudy days and at nighttime.

Impact of Poor Ventilation on Ice Dams

Poor ventilation in itself is not a cause of ice dam formations, and natural (passive) methods of ventilation, such as intake and exhaust vents, will not prevent them. In fact, a building with an efficient thermal boundary, regardless of ventilation, is not a typical candidate to form an ice dam. Unventilated attic assemblies can be constructed to be free from ice dams as long as insulation levels are great enough to maintain consistent sheathing temperatures and air leaks are eliminated.

Inadequate Attic Insulation Causes Ice Dams

The most common causes for ice dam formations result from inefficiencies in a heated structure’s thermal boundary. Inadequate levels of insulation, displaced insulation and air leakage are examples of these inefficiencies. Continuous levels of insulation installed according to the Department of Energy’s recommendations will prevent ice dams from occurring as long as heat sources are not present in the attic space. Ice dams and their resulting leaks usually can be corrected with improvements to the thermal boundary alone.

When snow covers a roof system, it insulates the attic space and prevents heat from escaping. An inch of snow has an R-value of about 1. This means 12 inches of snow have nearly the same amount of thermal resistance as wall insulation and can cause ambient air and roof sheathing temperatures in attic spaces to rise when covered in snow. Building occupants and homeowners have a tendency to crank up the furnace during cold winter days as well, and air leakage becomes problematic. Warm air can leak into attic spaces from areas such as attic hatches or penetrations such as pipes, light fixtures or vents.

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.

A relatively new trend in construction is to locate mechanical equipment, such as furnaces, in attic spaces of steep-slope structures. This can be disastrous for roofing contractors in northern climates. Furnaces can give off tremendous amounts of heat and create temperature differentials in a roof sheathing. Many complex attic assemblies, especially those in multifamily properties, are constructed with firewalls or other types of framing that contain the heat from spreading to other areas. The result often is ice dam formations in places roofing contractors historically are not used to seeing and are extremely difficult to fix.

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.

Historically, ice dam remediation has been focused on issues surrounding thermal inefficiencies, but as attic spaces are constructed in a more complex manner, ice dam formations are more likely to occur. Unfortunately, building codes and manufacturer recommendations do not consider the unintended consequences of placing furnaces, other mechanical equipment and ductwork in attic spaces. Ventilation codes and underlayment specifications are ineffective in their current forms, and ice dam formations no longer are confined to the eaves.

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.