What is Ice Damming?
Ice damming is a phenomenon that occurs in climates where snow accumulates on the roof surface. Most frequently it forms at the lower edge of a roof but can also occur near party walls (i.e. parapet walls that are shared between separate residences sharing the same roof), at the bottom of roof valleys, and below skylights.
Ice damming is a result of snow melting, flowing down the sloped roof, and refreezing at a colder area of the roof surface. Icicles hanging off gutters, between siding boards, from the soffit are a sure sign that ice damming is taking place. Another sign is discolored icicles. Ice damming can be worsened by gutter systems. In some heavy snow areas gutters and downspouts in installed seasonally as the weight of the ice can pull the system away from the building.
Damage from ice damming can be seen along the wall/ceiling intersection line on the interior as stains, dampness, or dripping. Damaged areas may not be located directly in line with the ice damming problem. Water can travel along rafters and joists and manifest itself fairly far from the cause. If you find evidence of water only when it is cold and there are icicles present and not in rainy seasons or drier, warmer weather, you can be confident that ice damming is the cause of the problem.
Causes of Ice Damming
In cold climates the ceiling surface in the living space below the roof/attic needs to be insulated from the cold interior of the space between the ceiling and the roof. This is not just a cost issue (wasted heat) but necessary to keep the roof surface cool. The space below a roof needs to be well ventilated with outside air from the peak to the eave. If heat escapes into the attic and the attic is not sufficiently ventilated, the warm air will cause the snow on the roof to melt, flow down to the colder overhang area of the roof, refreeze, and begin to build up forming a dam. As this process continues, the water “backs up” the roof higher and higher and eventually under the surface of the roof covering. The water finds an entrance point such as the joint between the structural wood surface of the roof, and travels to an interior wall or ceiling.
Tech Tip
Total vent area should be 1/300 of the floor space of the attic. Total vent are equals the sum of the actual vent area provided by the soffit, ridge, roof, and gable vents. At least 50% of the total vent area should be provided by the soffit vents. If the soffit vents provide less than 50% of the total vent area, a negative pressure can occur in the space below the roof and warm, moist air can be drawn in exacerbating the ice damming problem, increasing heating costs, an damaging the underside of the roof structure from moisture.
The floor space of the attic is measured to be 1200 sq. ft.
Total Required Vent Area: 1/300 X 1200 = 4 sq. ft.
Soffit Vent Area: .5 X 4 = 2 sq. ft.
Combined gable, ridge, and roof Vent Area: .5 X 4 = 2 sq. ft.
Ways to Prevent Ice Damming
- Avoid installing recessed ceiling lighting in any ceiling under an attic.
- Don’t install skylights.
What About Heat Tape?
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Here is a related article:
Got Attic Mold? Lets Talk Energy Conservation!
It happens to countless homeowners around the end of the year – you make the annual visit to your attic to collect the holiday decorations and what do you find? Spots and blotches covering the bottom of the roof sheathing. Worse yet – it turns out to be attic mold!
What does energy conservation have to do with mold in the attic? Well if you take a step back and consider how the house behaves as system, they are often directly related.
Building science experts have long been using the “house as a system” approach to diagnose the cause and origin of building defects.
For example, ice dams. These are often caused by warm air seeping into the attic which causes the snow and ice on the roof to melt. The water drains to the edge of the roof (which is colder than the rest of the roof because it is an overhang and not warmed by the attic), freezes and creates an ice dam. As this process is repeated daily, the ice dam grows larger. Eventually water is forced under a shingle where it can seep into the house.
Understanding how the house behaves as a system and the various causes and effects is necessary to diagnose most building related problems.
But how about that attic mold? How did it get there?
Mold requires chronic moisture to form and to thrive, so source(s) of moisture must be present. Possibly the moisture came from outdoors. The roof is newer and a quick check of the roof shows no obvious damage or leaks.
Possibly the moisture came from indoors. During the heating season, the interior of the house frequently has high moisture levels, especially bathrooms and kitchens. A quick check shows that all bathroom fans, kitchen vents, etc. are properly ducted completely outdoors and not into the attic. The amount of insulation looks good and the attic is well ventilated.
Don’t give up – you are almost there! Remember the house as a system? You know that warm, moist air is in the house, but how is it getting into the attic?
By air leaks! Air leaks are the leading source of energy loss in most houses, and a frequent source of chronic moisture that can cause attic mold. Most homeowners are well aware of air leaks around windows and doors (especially old ones), but many overlook the numerous gaps leading directly into the attic!
Have a look around the attic and you may find large gaps around recessed lights and fans, holes where wires or pipes are installed, even large gaps around the chimney. And don’t overlook the whole house fan and especially the folding attic stair - a big, uninsulated hole in your ceiling that is often overlooked!
These gaps can add up to a large hole that allows warm, moist air from the house to flow right into the cold attic. The warm moist air condenses on the cold roof sheathing, creating chronically damp conditions that can lead to attic mold growth. And the energy loss – it can be like leaving a window open all winter long!
Seal these air leaks and you stop a significant moisture source. And just think of all the energy you can save and the cold drafts you can stop!
Mark D. Tyrol is a Professional Engineer specializing in cause and origin of construction defects. He developed several residential energy conservation products including an attic stair cover, an attic access door, and is the U.S. distributor of the fireplace draftstopper. To learn more visit www.batticdoor.com
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