Ice Damming a Result of Excessive Attic Heat

If your home has a steep sloped roof, and is more than 20 years old, there is a 98 percent probability that your attic has excessive heat, leading to ice damming. This can result in subsequent water damage, and may have other serious problems identified by that heat, too. Even newly constructed homes may have serious attic heat problems.

Your response may be, "I don't live in a snow climate, so I don't have to be concerned about ice dams, right?" Wrong! The problem of ice damming is a much deeper issue than it may first appear, and may be indicative of construction and health related problems other than just that of a "roof" problem. Ice damming can lead to delamination of the roof sheathing, wood rot of the roof rafters, and failure of the insulation - leading to higher fuel bills, the development of mold growth, and health problems disguised as asthma, allergies, colds and sinus conditions.

Most people attribute the problem of ice damming to the "roofer," saying the roofer caused it. But this may not be the case. Although in some cases, the accuser would be right, in most cases, the roofer is not to blame for ice damming and related problems. He may have done everything correctly when installing a roof, and still there may be ice damming and water leakage - not from unsatisfactory roofer workmanship, but rather as a result of one of several subtle sources of heat, created by:

  1. The Architect
  2. The General Contractor
  3. The Heating, Ventilating and Air Conditioning Contractor
  4. The Insulation Contractor
  5. The Electrician
  6. And yes, let's not forget, the Homeowner, too!

Complex Roof Designs Create Ice Dams

complex architectural roof design, 18.3 Kb Architects and designers are constantly trying to create aesthetically pleasing new homes. However, beautiful but complicated roof designs are not always fully compatible with a variety of climates. In fact, complex roof lines create some distinct and intrinsic problems regarding weather related issues, such as ice and snow. The above, newly constructed property has several incompatible, climate-related design problems, including the channeling of snow and ice from two portions of the large, main, intersecting roof line, down and onto the lower roof area, entrapping snow and ice between the sidewalls of the main house and the sidewalls of the dormer. Compounding this problem is the reduction of adequate ventilation capabilities, reduced by the lack of adequate soffit inlet ventilation air flow. This allows excessive heat build-up, further adding to the creation of ice dams. This roof displays classic snow-melt patterns of unsatisfactory attic ventilation.

flawed dormer design 24.2Kb A closer look at this dormer reveals a very large, steep intersecting roof, emptying its snow load and ice melt down onto the lower roof space, which is then entrapped by the dormer sidewalls. Note the ice dams forming at this dormer.

The snow melt pattern of this roof is a distinct indication of excessive, unwanted heat in the attic space. Note the spike of white snow along the rake, outside the living area. The rule of thumb is that properly ventilated attic air should never be more than 15 degrees hotter than the outside air. If it is hotter, then that is an indication of additional heat sources that should be eliminated, and/or an indication of unsatisfactory attic ventilation that must be corrected immediately. Snow melt of an attic without excessive heat build-up and with functional attic ventilation should be totally and uniformly white. If the snow melt were the result of heat from the sun, all of the roof would be clear, without any white spikes being present. Attic heat is the enemy of all roofs, and poses a problem for the insulation, h.v.a.c. systems and health factors, too.

Excessive Attic Heat: The Primary Culprit

How does heat enter the attic? How does heat exit the attic? These are two very important questions that must be asked for every construction design, whether it is for new construction, additions or renovations. Without heat, there are no ice dams, no moisture condensation problems and no health issues.

The following are pictorial examples of some ways unwanted heat enters the attic space and some reasons why heat can not exit that attic space. Each of these conditions is attributable to additional heat, the ultimate enemy of a healthy home.

firebox heats attic air 17.5 Kb You are standing in the attic, looking down into the open space around the framed box of a living room fireplace. Talk about heat! This is a major heat source, open directly into the attic space due to unsatisfactory construction, and is a major problem for the creation of ice dams. Other heat problems exist, too. Look closely at the gray area in the photograph, which is the back of the drywalled walls. No insulation! Therefore, even when the fireplace is not in use, excessive heat is still entering the attic through the uninsulated walls. This home, located in the northeastern U.S., will have huge utility bills for heating and air conditioning. creating attic heat problems 29.7 Kb When constructing the addition of another home, only limited space was available to install the h.v.a.c. system, so it was installed within the attic space. Big mistake! Aside from the very difficult access for monthly maintenance, the low sloped roof eliminated air spaces above the furnace, further increasing heat build-up in the attic, leading to snow melt and ice damming. The silver image at the top center of this photograph is a full-sized, natural gas-fired, forced hot air furnace. Excessive flex ducts allow additional unwanted heat to enter the attic space. The bottom center of this photograph reveals a "typical" gap in the pink insulation, allowing another unwanted heat source to enter the attic from the living space below.

hvac trap needs servicing 19.5Kb (Tip: If you do have an attic h.v.a.c. system, be sure to periodically clean out the overflow trap for the air conditioning pan. These devices tend to block up, causing the pan to overflow and cause ceiling water damage, mimicking a roof leak.)

rear ice dam 20.4 Kb Rear Ice Dams

For this home, ice dams were inevitable. Note the classic snow melt pattern, also seen on the home discussed above. Compounding this design problem even further, when the addition was installed, the entire house was re-sided. However, the original wood-covered soffit overhangs were never removed before installing the new perforated soffit vents, which eliminated inlet air flow to the attic, an important and necessary element for all healthy homes.

flex heat duct problem 8.5 Kb The new addition had open, perforated soffit vents installed, but were then covered with insulation, eliminating needed and necessary inlet air flow to the attic. Soffit vent protectors were not utilized and should have been installed to help keep insulation from blocking these vents.

The pink insulation was not installed according to specifications, and was out of position, allowing heat to enter the attic from the living space below.

Twelve inches of insulation, creating an R-38, is optimal. Remember, only have one vapor barrier and install it facing toward the heated surface. If additional layers of insulation are needed to attain 12", use either unfaced insulation or blown-in insulation.

close-up of ice dam 25 Kb This is a close-up of an ice dam. The white vertical lines on both the left and right sides are frozen water that had infiltrated this home and then froze again when exiting through the siding weep holes and soffit vent holes.

Look at what further aggravated this particular ice dam formation: The electrician installed a "standard" recessed lighting fixture in the master bedroom ceiling for use as a reading light over the bed. However, when the light fixture was utilized with snow cover on the roof, additional heat was generated in very close proximity to the low sloped roof, causing pre-mature ice melt and ultimately an ice dam. The roofer did not stand a chance!

heat by electric - bad insulation - blocked vents 16.6 Kb If you design a recessed light near the roof and on an outside wall, install a double-walled insulated lighting fixture that can then be safely insulated over to help reduce unwanted heat from entering the attic. (Note: This installation was also a fire hazard, since the insulation was packed against the unit, eliminating the required 3" clearance!)

Two additional problems identified in this photograph are:

  1. Insulation that is out of position, allowing heat to enter the attic.
  2. Insulation that is extended out over the soffit inlet vents, completely eliminating critical inlet air flow.

Air flow is easily explained using a ketchup bottle as an example. When you open a new bottle of ketchup and turn it over, the ketchup does not immediately come out. In order for the ketchup to exit the bottle, air must enter the bottle and occupy the space just left by the ketchup. The same scenario holds true for your attic. In order for heated attic air to exit the attic, make-up air must be able to enter the attic. Having only inlet vents, or having only outlet vents is as good as having no vents at all. You must have both inlet and outlet vents in a balanced proportion. Click here for more information on proper attic ventilation.

Insulation is designed to keep heat within the living space in winter. All sidewalls facing an unheated surface must be insulated, with the vapor barrier facing the heated surface. Attic air space should be considered unheated space. Thus, all walls adjoining that space must be insulated to help reduce heat entry into that attic space. Doors leading to unheated spaces should be addressed and trimmed as if they were outside doors, complete with saddles and weatherstripping. Shown in this photograph is an attic stairwell, with a large, uninsulated side wall above the entry door, leading to the unheated attic space. This particular newly constructed home had excessive ice dam leakage and water damage, and the owners demanded that the roofer shovel the snow off of the roof, which he did. Remember: heat, not the roofer, is usually the culprit in creating ice dams.

 bathroom vents to soffit - unsatisfactory 13.3 Kb Another common way for heat to enter the attic involves installing the bathroom exhaust vent duct into the soffit vents, rather than correctly installing the duct to an outlet vent. This is a problem because soffit vents are inlet vents, not outlet vents. Everything that exits the house at a soffit vent immediately re-enters the house in the attic space, because that is what soffit vents are designed to do. Not only will this add heat, but it will also add humidity and moisture to the attic, defeating the very reason for installing the bathroom fan in the first place. Even a home with functional attic ventilation can now develop molds on the roof sheathing cavities above these unsatisfactory ducts.

Health Hazards

 attic moisture causes black mold 30.2 Kb One of the most common types of mold that thrives in a moist attic environment is black mold. Black mold spreads by spores, and there is growing evidence of serious health hazards associated with breathing in the black mold spores. This type of mold has been found in 80 percent of the homes I have inspected over the years, both new and old. In fact, my own doctor called me to inspect his "dripping wet attic" of his brand new home. Concerned about low humidity, he had the central humidifiers cranked to the max. Unfortunately, his builder also had minimal ventilation. The consequences - wet roof sheathing covered with heavy black mold that his family members were allergic to.

The black area above the yellow insulation in the photograph is not black due to a lack of light. It is actually blacked by heavy black mold growth that is a serious health and safety issue for mold-susceptible people. This is an unhealthy home syndrome that must be corrected immediately. Remember, just because you are not allergic to molds today does not insure that you will not be allergic to molds tomorrow. If you are experiencing breathing and allergic type reactions, contact your doctor and then have your attic inspected for proper insulation, ventilation and heat sources.

Further compounding the very unsatisfactory conditions of this attic is the use of loose, blown-in, fiberglass insulation, lacking a vapor barrier, which now doubles the amount of attic ventilation needed. Normally, the ratio of net free-air vent space to attic floor space being ventilated is 1/300, meaning that for every 300 square feet of attic floor space, one square foot of net free-air vent space must be provided. However, that ratio of required vent space now doubles to 1/150 for this type of insulation. Let us not overlook the heating duct near the roof line, too, which also adds additional heat - there is that word again, heat!

Proper Attic Ventilation

unsatisfactory attic ventilation 30.7 Kb You now know what not to do to create attic heat. Let us now discuss things to do to get rid of unwanted and damaging attic heat.

First, to avoid ice damming and other problems, make sure that every steep sloped roof has continuous soffit inlet vents. Many homes designed and built in the 1950s were constructed without any soffit overhangs, making the installation of conventional soffit inlet vents impossible. Fortunately, the importance of attic air flow, even for homes built without soffit overhangs, is now becoming better understood. There are many new products on the market that will provide inlet air at the eave of the roof by bringing in the air from behind the gutters, through the roof sheathing, and under the shingle materials. Both roofing and carpentry work are required to install these new inlet vents, which are available wherever most roofing materials are sold.

Second, you must have ample outlet vents in the form of 9" roof vents, gable end vents, or ridge vents. Since heat rises to the peak of the roof, the logical outlet vent should be located at the peak of the roof, were an opening to the outside would then let unwanted heat exit quickly and efficiently. However, a closer look at the interior attic space of this newly constructed home reveals absolutely no outlet vents. This is unsatisfactory, and a disaster waiting to happen.

 classic snow melt for ice dams 23.7 Kb This new home had no ridge vents on this portion of the attic, with only a very, very small gable end vent installed. It was an ice dam waiting to happen. If your home does have gable end vents or standard roof vents, do not cover them with plastic during colder weather, thinking that you are saving heat. Doing that is the equivalent of "smothering" your home's breathing capability.

unsatisfactory ridge vent 12.2 Kb Be aware that just because you see a ridge vent on the outside of your roof, it does not mean that it has been properly installed. Most manufacturers of ridge vents, including all of the major shingle manufacturers, require almost a 2" cut away on each side of the ridge board (the reason it is called a ridge vent) to allow for the proper and needed amount of net free-air vent space to work properly. Unfortunately, many homes do not have their vents properly and correctly installed.

This new home's ridge vent installation was unsatisfactory. Note that the roof sheathing was not properly cut away and removed. Other portions of this roof's ridge vent had both felt paper and shingle materials covering the opening, reducing exiting net free-air flow. Without exits for attic air, heat builds up, melting snow, creating ice dams and causing many other problems.

A note about power vents - many times they won't do the trick. They are thermostatically controlled and thus do not work in the winter, when it is actually more important to ventilate your attic. When insulation gets wet, it loses some of its "R" value, or resistance to thermal exchange, thus increasing the work required by heating and air conditioning systems. I have seen many a home look like a scene right out of "Dr. Zhivago's" Ice Palace!

no snow melt - no heat! 11.8 Kb This porch roof still has almost a foot of snow on it because there was no heat source under the "open" porch to cause snow melt. The main house roof should have looked the same. If your home has the snow melt patterns discussed above, then you have at least one of the above problems that must be addressed immediately to maintain a healthy home and a healthy living environment.

Winter is just around the corner. Correcting the above problems is easy during warm weather, but almost impossible to correct in cold weather, so act now. If your home has experienced any of the above scenarios, start correcting these problems before they become bigger problems.

After reading all of this data, you may say, "All I need is one of the many self-sticking ice and water barriers installed with my roof to eliminate ice damming." You would be right - to an extent. This material eliminates water infiltration through the roof in most cases. However, it would not solve other conditions related to the "inside" of the attic space, and other problems which would still occur even with the installation of expensive self-sticking barriers. It's a cold hard fact: If you do not have excessive heat in the attic, you do not need this expensive material. Unfortunately, most homes have excessive heat and now, most roofing manufacturers are insisting on the installation of self-sticking barriers.

In 1992, while still doing general contracting, I renovated an older, plank-constructed home for a client, including the installation of soffit vents, ridge vents and 12" of fiberglass attic insulation. In 1993, the northeastern U.S. had record snow fall, record cold weather and winter rains. My client was the only one in her neighborhood who had absolutely no ice dam leakage and no water damage that winter.


  • Avoid complicated roof designs in snow climates.
  • Reduce unwanted attic heat sources.
  • Remove attic heat with proper ventilation.
  • Survey your home for molds and mildews.

This helpful article was provided by community member Ron Hungarter.